DotNetCode

How to use Python in Visual studio Code IDE

2020-04-11T06:22:00.001-07:00

Install SSDT in visual studio 2015-Error 0x80070002: Failed to send request to URL: https://go.microsoft.com/fwlink/?LinkId=817280&clcid=0x409

2020-04-10T19:08:00.000-07:00

Hi All,

Here i have tried to install SSDT in Visual studio 2015 through online.But am not able to make it success.Everytime it has thrown below error at the some point, then i have tried various method to fix this issue but nothing get helped.

Finally i have found solution form the below link.Here you can download standalone installer for SSDT.After i tried this SSDT has installed Successfully.

https://docs.microsoft.com/en-us/previous-versions/mt186501(v=msdn.10)?redirectedfrom=MSDN

Direct Link to Download the file.

https://go.microsoft.com/fwlink/?linkid=832313&clcid=0x409

AttributeError: module 'tensorflow' has no attribute 'app'

2020-04-08T01:04:00.003-07:00

Hi All,

i have got this error while doing custom object detection in tensorflow.

Here the solution which i found
It occurs due to Tensorflow version
If it is TF2.0 then you need to replace with tf.compat.v1.flags

Change the import tensorflow keyword

Correct One:

import tensorflow.compat.v1 as tf

ImportError: No module named 'nets'

2020-04-07T09:17:00.000-07:00

Hi All,

i have got this error while doing custom object detection in tensorflow.

Here the solution which i found

step 1:
First you need to go to this path in the command prompt..

....(local path)/tensorflow_models/models/slim

Step 2:
if you have Build file in the folder then delete the file

Step 3:

Type Setup.py and press enter

Now its start the installation.Issue will be solved.

Getting started with Jupyter Notebook and machine learning in python

2020-04-07T06:50:00.001-07:00

Introduction about CountVectorizer with an example in Machine Learning

2020-04-07T06:49:00.001-07:00

How to use Table-Valued Parameters in sql?

2017-11-01T03:50:00.002-07:00

Table-valued parameters are declared by using user-defined table types. You can use table-valued parameters to send multiple rows of data to a Transact-SQL statement or a routine, such as a stored procedure or function, without creating a temporary table or many parameters.

Advantage:

Do not acquire locks for the initial population of data from a client.
Provide a simple programming model.
Enable you to include complex business logic in a single routine.
Reduce round trips to the server.
Can have a table structure of different cardinality.
Are strongly typed.
Enable the client to specify sort order and unique keys.
Are cached like a temp table when used in a stored procedure. Starting with SQL Server 2012, table-valued parameters are also cached for parameterized queries.

Disadvantage:

SQL Server does not maintain statistics on columns of table-valued parameters.
Table-valued parameters must be passed as input READONLY parameters to Transact-SQL routines. You cannot perform DML operations such as UPDATE, DELETE, or INSERT on a table-valued parameter in the body of a routine.
You cannot use a table-valued parameter as target of a SELECT INTO or INSERT EXEC statement. A table-valued parameter can be in the FROM clause of SELECT INTO or in the INSERT EXEC string or stored procedure.
Compared to bulk operations that have a greater startup cost than table-valued parameters, table-valued parameters perform well for inserting less than 1000 rows.

Code:

/* Create a table type. */ CREATE TYPE LocationTableType AS TABLE ( LocationName VARCHAR(50) , CostRate INT ); GO /* Create a procedure to receive data for the table-valued parameter. */ CREATE PROCEDURE dbo. usp_InsertProductionLocation @TVP LocationTableType READONLY AS SET NOCOUNT ON INSERT INTO AdventureWorks2012.Production.Location (Name ,CostRate ,Availability ,ModifiedDate) SELECT *, 0, GETDATE() FROM @TVP; GO /* Declare a variable that references the type. */ DECLARE @LocationTVP AS LocationTableType; /* Add data to the table variable. */ INSERT INTO @LocationTVP (LocationName, CostRate) SELECT Name, 0.00 FROM AdventureWorks2012.Person.StateProvince; /* Pass the table variable data to a stored procedure. */ EXEC usp_InsertProductionLocation @LocationTVP; GO

How to improve SQL Performance when searching by date

2017-09-29T04:32:00.000-07:00

If you have a datetime column in a WHERE clause, and you need to convert it or use a data function, try to push the function to the literal expression.

For the below two query , the First one take more query cost rather than the Second one,

SELECT OrderID FROM dbo.Orders WHERE DATEADD(day, 15,
OrderDate) = '07/23/1996'

SELECT OrderID FROM Orders WHERE OrderDate = DATEADD(day,

-15, '07/23/1996')

In below Figure you can check the query cost

How to create Dynamic Pivot Table in SQL?

2017-09-29T01:29:00.002-07:00

Here below code, which describes to create Dynamic Pivot view from the Table.

Create table yourtable (itemID INT, part CHAR(1))

INSERT INTO yourtable VALUES(1,'A'),(1,'B'),(2,'A'),(2,'A'),(2,'A'),(3,'C')

DECLARE @colsSorted AS NVARCHAR(2000), @sql AS NVARCHAR(4000)

select @colsSorted = STUFF((select DISTINCT ', '
+ quotename( Cast(ROW_NUMBER() OVER(PARTITION BY itemID ORDER BY part) as varchar(3)) ,']')
FROM yourtable
FOR XML PATH (''),type).value('.','varchar(max)'), 1, 2, '')
--Print @colsSorted

Set @sql=N' if object_id(''anewtable'',''U'') is not null drop table anewtable ; with mycte as (SELECT ItemID, '+ @colsSorted + ' FROM (
Select ItemID,Part, Cast(ROW_NUMBER() OVER(PARTITION BY itemID ORDER BY part) as varchar(3)) as Cols

FROM yourtable
) src
PIVOT (Max(part) for Cols IN ('+ @colsSorted +')) pvt )
Select * into aNewtable
from mycte;'
--print @sql
exec sp_executesql @sql;

select * from aNewtable

select * from yourtable

drop table yourtable

Happy coding!!!!!!!!!!!!!!!!!!

Office has detected a problem with this file. To help protect your computer this file cannot be opened.

2017-07-24T03:03:00.001-07:00

Hi,

If you got this below error on your code,

Office has detected a problem with this file. To help protect your computer this file cannot be opened.

Here you can find the solution to avoid such errors.

Just add this below line when you are initializing excel application.

if you set it to msoFileValidationSkip before the Open statement, it should bypass the file protection check.

excelApp.FileValidation = MsoFileValidationMode.msoFileValidationSkip;

How to get the value from HTML control to code behind?

2017-07-10T04:13:00.003-07:00

Here you can find the value from HTML control to code behind using ASP.net

if you still want to get or set values to HTML controls without runat="server" then you can use Request.Form collection to get the value. You can use public property and embedded code blocks to set the value from server. Refer the code below,

ASPX
<input id="txt1" name="txt1" type="text" value="Set in Client Side" />

<input id="txt2" name="txt2" type="text" value="<% =ServerValue %>" />

CodeBehind

public string ServerValue = String.Empty;

protected void btnSave_Click(object sender, EventArgs e)

{
string ClientValue = Request.Form["txt1"];
ServerValue = "Set in Server";
}
}

Please like .....

How to Zip and Unzip a file from folder without using opensource in c#

2017-02-19T06:47:00.000-08:00

i would like to share the code to zip and unzip a file without using any opensource which is directly used reference file in dot net framework.

Please add the reference file to ZipFile is contained in the assembly System.IO.Compression.FileSystem.

To zip a file

 System.IO.Compression.ZipFile.CreateFromDirectory(startPath, zipPath);

To UnZip a File

 System.IO.Compression.ZipFile.ExtractToDirectory(startPath, extractPath);he reference file to ZipFile is contained in the assembly System.IO.Compression.FileSystem.

What is needed to configure a new extension for use in ASP.NET?

2016-07-03T22:34:00.001-07:00

Got a question a while back from a customer asking how to enable ASP.NET pages with custom file extensions, so I played around a bit to see how to do it, and here’s what I came up with:

1. Open up the IIS 5.1 or 6 management console, and navigate to the virtual directory (or web site) that you want to configure. Right-click the folder and select Properties.

2. On the Directory (or Web Site) tab, click the Configuration button.

3. On the Mappings tab, click Add, and enter aspnet_isapi.dll (the path is under Windows\Microsoft.net\Framework\<version>\…check the existing mapping for .aspx if you need the exact path) as the Executable, and your desired extension under Extension (I used .foo). Clear the “Check that file exists” box. Click OK (if OK is greyed out, tab around in the textboxes a bit, that usually seems to clear the issue that prevents it from being active). Click OK.

4. In your web.config file (or the main web.config for the machine, if you want this to apply to all sites), add the following HttpHandler mapping (inside the <system.web> tags:

</httpHandlers>

5. Also in your web.config, add the following Build Provider mapping (goes between the <compilation> tags, you may need to edit the default tag, since in ASP.NET 2.0 it defaults to a self-closing tag):

</buildProviders>

The complete <compilation tag should look similar to the following:

</buildProviders>

</compilation>

6. Add a new web form to the page (probably easiest to stick with single file pages for this), add controls, etc., and when you’re finished with the page, rename the extension to the one you configured in IIS.

7. Browse with IIS to test.

Create a SQL Connection Stringusing text file

2016-05-30T21:53:00.000-07:00

On the computer that requires the connection string, create a new file with a file extension of .udl.
1. To perform this task, you will have to be viewing file extensions. If you are unsure how to do that, seeHow To: View File Name Extensions.
2. Create a new text file and then rename the three letter file extension .txt to .udl.
  - For example, if you create a file named ConnectionString.txt, just rename it to ConnectionString.udl.
  - The rest of these steps will assume that the file is actually named ConnectionString.udl. If you created another file name with a .udl file extension, that is fine, just substitute the appropriate name as needed in the following instructions.
Right-click ConnectionString.udl that you just created and then click Properties.
On the Connection tab, fill out the connection properties according to the server, authentication type, and database name that you need. This is typically something you would already know, but if you do not, you may have to contact the database administrator or go look at the database connection properties yourself. If you need help with that, check out the Server Management How To Pages .
Click Test Connection. Hopefully it will succeed. If not, check the credentials, authentication type, and any firewalls (How to: Configure a Windows Firewall for Database Engine Access ) between servers.
Click OK.
Open the url file and inside you will find the connection string. For example, Provider=SQLOLEDB.1;Integrated Security=SSPI;Persist Security Info=False;InitialCatalog=FIMCertificateManagement;Data Source=FIMDC1 (as shown in the following figure).
Remove Provider portion of the string. Using the example above, the connection string for the application would be: Integrated Security=SSPI;Persist Security Info=False;InitialCatalog=FIMCertificateManagement;Data Source=FIMDC1

Difference between non unicode and unicode in sql server

2016-05-29T03:41:00.003-07:00

Non-Unicode	Unicode
(char, varchar, text)	(nchar, nvarchar, ntext)
Stores data in fixed or variable length	Same as non-Unicode
char: data is padded with blanks to fill the field size. For example, if a char(10) field contains 5 characters the system will pad it with 5 blanks	nchar: same as char
varchar: stores actual value and does not pad with blanks	nvarchar: same as varchar
requires 1 byte of storage	requires 2 bytes of storage
char and varchar: can store up to 8000 characters	nchar and nvarchar: can store up to 4000 characters
Best suited for US English: "One problem with data types that use 1 byte to encode each character is that the data type can only represent 256 different characters. This forces multiple encoding specifications (or code pages) for different alphabets such as European alphabets, which are relatively small. It is also impossible to handle systems such as the Japanese Kanji or Korean Hangul alphabets that have thousands of characters."¹	Best suited for systems that need to support at least one foreign language: "The Unicode specification defines a single encoding scheme for most characters widely used in businesses around the world. All computers consistently translate the bit patterns in Unicode data into characters using the single Unicode specification. This ensures that the same bit pattern is always converted to the same character on all computers. Data can be freely transferred from one database or computer to another without concern that the receiving system will translate the bit patterns into characters incorrectly.

How to earn money by posting articles and Link in blogger?

2014-08-12T22:38:00.001-07:00

i would like to share the different ways to earn money through online.There are many ways to work through online.Here i share method those who are ready to write content articles.
If you are good writers then don't sit idle,its time to get popularize yourself through tech world.

Recently i found the site by posting content and your own URl links to popularize your blog or site.
Already they are many sites doing certain type of work www.vdsite.com is the Indian site.they ready to popularize your writing skills and you blog site.

Thing you have to do add your article or blog site URl to earn points from this site to improve your user level.
Site address www.vdsite.com

Show popup window to user when leaving from page in asp.net

2014-07-28T03:22:00.000-07:00

Refer the below link............

http://vdsite.com/Articles/241/how-to-show-popup-window-from-user-when-leaving-from-page-in-asp.net

How to use Google map to show multiple location in Asp.net

2014-06-19T22:27:00.002-07:00

Refer the below inks...........

http://vdsite.com/Articles/203/Using-Google-map-to-show-multiple-location-from-Database-in-Asp.net

How AJAX works with Advantage and Disadvantages in Asp.net

2014-06-19T22:25:00.004-07:00

Refer the below link......

http://vdsite.com/Articles/211/How-Ajax-Update-panel-work-in-Asp.net-also-advantages-and-Disadvantages-of-using-Ajax?

Enhance webpage performance by caching images,CSS in asp.net

2014-04-24T01:12:00.003-07:00

Hi.

Refer the below link Improve webpage performance by caching images,CSS in asp.net
http://vdsite.com/Articles/90/Improve-webpage-performance-by-caching-images,CSS-in-asp.net

How to Redirect to non-www-url to www domain with ASP.NET?

2014-01-14T07:35:00.001-08:00

Hi
you have to update to this setting in web.config file........

public class SeoUrls : IHttpModule
{
  #region IHttpModule Members

  public void Init(HttpApplication context)
  {
      context.PreRequestHandlerExecute += OnPreRequestHandlerExecute;
  }

  public void Dispose()
  {
  }

  #endregion

  private void OnPreRequestHandlerExecute(object sender, EventArgs e)
  {
    HttpContext ctx = ((HttpApplication) sender).Context;
    IHttpHandler handler = ctx.Handler;

    // Only worry about redirecting pages at this point
    // static files might be coming from a different domain
    if (handler is Page)
    {
      if (Ctx.Request.Url.Host != WebConfigurationManager.AppSettings["FullHost"])
      {
        UriBuilder uri = new UriBuilder(ctx.Request.Url);

        uri.Host = WebConfigurationManager.AppSettings["FullHost"];

        // Perform a permanent redirect - I've generally implemented this as an 
        // extension method so I can use Response.PermanentRedirect(uri)
        // but expanded here for obviousness:
        response.AddHeader("Location", uri);
        response.StatusCode = 301;
        response.StatusDescription = "Moved Permanently";
        response.End();
      }
    }
  }
}

Then register the class in your web.config:

<httpModules>
  [...]
  <add type="[Namespace.]SeoUrls, [AssemblyName], [Version=x.x.x.x, Culture=neutral, PublicKeyToken=933d439bb833333a]" name="SeoUrls"/>
</httpModules>

Wish You Happy New Year----2014

2013-12-31T20:23:00.001-08:00

Wish you Happy new Year for all my Readers..This year fulfill all your goals get bright future and enjoy your life

Happy New Year to you, your friends, families, peers, and colleagues and here’s to a fantastic 2014 together!

Keep Reading ..................

ASP.Net Interview Questions Part 6

2013-07-21T11:01:00.002-07:00

Globalization -

What is Globalization?
Globalization is the process of creating an application that meets the needs of users from multiple cultures. This process involves translating the user interface elements of an application into multiple languages, using the correct currency, date and time format, calendar, writing direction, sorting rules, and other issues. Accommodating these cultural differences in an application is called localization.

The Microsoft .NET Framework simplifies localization tasks substantially by making its formatting, date/time, sorting, and other classes culturally aware. Using classes from the System.Globalization namespace, you can set the application’s current culture, and much of the work is done automatically!

What are the 3 different ways to globalize web applications?

Detect and redirect approach : In this approach we create a separate Web application for each supported culture, and then detect the user’s culture and redirect the request to the appropriate application. This approach is best for applications with lots of text content that requires translation and few executable components.

Run-time adjustment approach : In this approach we create a single Web application that detects the user’s culture and adjusts output at run time using format specifiers and other tools. This approach is best for simple applications that present limited amounts of content.

Satellite assemblies approach : In this approach we create a single Web application that stores culture-dependent strings in resource files that are compiled into satellite assemblies. At run time, detect the user’s culture and load strings from the appropriate assembly. This approach is best for applications that generate content at run time or that have large executable components.

In ASP.NET, how do you detect the user's language preference on his/her computer?
Use the Request object’s UserLanguages property to return a list of the user’s language preferences. The first element of the array returned by UserLanguages is the user’s current language on his/her computer.

What are the steps to follow to get user's culture at run time?
To get the user’s culture at run time, follow these steps:
1. Get the Request object’s UserLanguages property.
2. Use the returned value with the CultureInfo class to create an object representing the user’s current culture.

For example, the following code gets the user’s culture and displays the English name and the abbreviated name of the culture in a label the first time the page is displayed:
private void Page_Load(object sender, System.EventArgs e)
{
// Run the first time the page is displayed
if (!IsPostBack)
{
// Get the user's preferred language.
string sLang = Request.UserLanguages[0];
// Create a CultureInfo object from it.
CultureInfo CurrentCulture = new CultureInfo(sLang);
lblCulture.Text = CurrentCulture.EnglishName + ": " +
CurrentCulture.Name;
}
}

What are the advantages of using detect and redirect approach to globalizing web applications?
1. Content is maintained separately, so this approach allows the different applications to present very different information, if needed.
2. Users can be automatically directed to sites that are likely to be geographically close, and so can better meet their needs.
3. Content files (Web forms and HTML pages, for example) can be authored in the appropriate natural language without the complexity of including resource strings.

What are the disadvantages of using detect and redirect approach to globalizing web applications?
1. Using this approach requires that the executable portion of the Web application be compiled and deployed separately to each culture-specific Web site.
2. This approach requires more effort to maintain consistency and to debug problems across Web sites.

What is the use of culture attribute of the globalization element in web.config?
The Web.config file’s globalization element is used to create a culture-specific Web application. The culture attribute of the globalization element specifies how the Web application deals with various culture-dependent issues, such as dates, currency, and number formatting.

Web.config globalization settings in subordinate folders override the globalization settings in the application’s root Web.config file. You can store content for various cultures in subfolders within your application, add Web.config files with the globalization settings for each culture, then direct users to the appropriate folder based on the user’s CurrentCulture.

The text on the webform is usually written from left to right. How do you change the writing direction to "right to left"?
The wrting direction of a webform can be changed using the HTML dir attribute as shown below.

You can use the dir attribute individually in panels, text boxes, or other controls as well. Setting the dir attribute on the body element applies right-to-left formatting to the entire page.

What do you mean by neutral cultures?
Neutral cultures represent general languages, such as English or Spanish, rather than a specific language and region. When you set the culture attribute for a Web application in Web.config, ASP.NET assigns that culture to all the threads running for that Web application. Threads are the basic unit to which the server allocates processor time. ASP.NET maintains multiple threads for a Web application within the aspnet_wp.exe worker process.

What are advantages of setting the culture dynamically at the thread level over creating separate Web applications for each culture?
1. All cultures share the same application code, so the application doesn’t have to be compiled and deployed for each culture.
2. The application resides at a single Web address, you don’t need to redirect users to other Web applications.
3. The user can choose from a full array of available cultures.

For what type of web applications setting the culture dynamically is best suited?
Setting the culture dynamically is best suited for simple Web applications that don’t contain large amounts of text that must be translated into different languages.

DataSet-

What is a DataSet?
DataSet is an in-memory cache of data.

In which namespace is the DataSet class present?
System.Data

Can you add more than one table to a dataset?
Yes

Can you enforce constarints and relations on tables inside a DataSet?
Yes, the DataSet consists of a collection of DataTable objects that you can relate to each other with DataRelation objects. You can also enforce data integrity in the DataSet by using the UniqueConstraint and ForeignKeyConstraint objects.

What happens when you invoke AcceptChanges() method on a DataSet?
Invoking AcceptChanges() method on the DataSet causes AcceptChanges() method to be called on each table within the DataSet.

Both the DataRow and DataTable classes also have AcceptChanges() methods. Calling AcceptChanges() at the DataTable level causes the AcceptChanges method for each DataRow to be called.

When you call AcceptChanges on the DataSet, any DataRow objects still in edit-mode end their edits successfully. The RowState property of each DataRow also changes. Added and Modified rows become Unchanged, and Deleted rows are removed.

If the DataSet contains ForeignKeyConstraint objects, invoking the AcceptChanges method also causes the AcceptRejectRule to be enforced.

Is there a way to clear all the rows from all the tables in a DataSet at once?
Yes, use the DataSet.Clear() method to clear all the rows from all the tables in a DataSet at once.

What is the difference between DataSet.Copy() and DataSet.Clone()?
DataSet.Clone() copies the structure of the DataSet, including all DataTable schemas, relations, and constraints. Does not copy any data.

DataSet.Copy() copies both the structure and data.

How do you get a copy of the DataSet containing all changes made to it since it was last loaded?
Use DataSet.GetChanges() method

What is the use of DataSet.HasChanges() Method?
DataSet.HasChanges method returns a boolean true if there are any changes made to the DataSet, including new, deleted, or modified rows. This method can be used to update a DataSource only if there are any changes.

How do you roll back all the changes made to a DataSet since it was created? Invoke the DataSet.RejectChanges() method to undo or roll back all the changes made to a DataSet since it was created.

What happnes when you invoke RejectChanges method, on a DataSet that contains 3 tables in it?
RejectChanges() method will be automatically invoked on all the 3 tables in the dataset and any changes that were done will be rolled back for all the 3 tables.

When the DataTable.RejectChanges method is called, any rows that are still in edit-mode cancel their edits. New rows are removed. Modified and deleted rows return back to their original state. The DataRowState for all the modified and deleted rows will be flipped back to unchanged.

What is the DataSet.CaseSensitive property used for?
When you set the CaseSensitive property of a DataSet to true, string comparisons for all the DataTables within dataset will be case sensitive. By default the CaseSensitive property is false

ASP.Net Interview Questions Part 5

2013-07-21T10:58:00.001-07:00

Exception Handling-

What are Exceptions?
Exceptions are unusual occurrences that happen within the logic of an application.

What are the 3 approaches to handle exceptions in a Web application?
1. Use exception-handling structures to deal with exceptions within the scope of a procedure. This technique is called structured exception handling (SEH) in the Visual Studio .NET documentation.
try
catch
finally
2. Use error events to deal with exceptions within the scope of an object.
Page_Error
Global_Error
Application_Error
3. Use custom error pages to display informational messages for unhandled exceptions within the scope of a Web application.

Where will the control flow if an exception occurs inside a try block?
If a statement in a try block causes an exception, control flow passes immediately to the next catch statement. When control flow passes to a catch block, the statements contained in the catch block are processed to correct the error or otherwise handle the exception.

Will the finally block gets executed, if an exception occurs?
Yes, a finally block will always be executed irrespective of whether an exception has occured or not.

What is the main use of a finally block in exception handling?
Finally block is mainly used to free resources used within the try block.

How do you raise an exception?
Use the throw keyword to raise an exception. Use this keyword within your exception-handling structure to immediately pass control flow to the catch statement.

Will the following code block compile?
try
{
throw new System.IO.FileNotFoundException();
}
catch (Exception E)
{
Response.Write(E.Message);
}
catch (System.IO.FileNotFoundException FNFE)
{
Response.Write(FNFE.Message);
}

No, a compile time error A previous catch clause already catches all exceptions of this or of a super type ('System.Exception').

Catch blocks are evaluated in the order in which they appear in code. The exception declaration of each catch block determines which type of exception the catch block handles. Always order catch blocks from most specific to most general. So, in the preceding sample, FileNotFoundException should be placed before the general Exception catch block.

What is ApplicationException class used for?
If you are creating a large application or creating components that are used by other applications, you might want to define your own exception classes based on the ApplicationException class. For example, the following code defines a class for the UserLoggedOnException:
public class UserLoggedOnException : System.ApplicationException
{
// Exception constructor (overloaded).
public UserLoggedOnException()
: this("The user is already logged on to the server", null)
{
}
public UserLoggedOnException(string message)
: this(message, null)
{
}
public UserLoggedOnException(string message, Exception inner)
: base(message, inner)
{
}
}
The preceding UserLoggedOnException class inherits its properties and methods from the ApplicationException base class. The new exception class provides only its own constructor to set the default message to display. This is a standard practice.

What are Error Events?
Another way to handle exceptions is through the Web objects’ built-in error events. When an unhandled exception occurs in a Web application, ASP.NET fires the error events shown below.

Page_Error : Occurs when an unhandled exception occurs on the page. This event procedure resides in the Web form.
Global_Error : Occurs when an unhandled exception occurs in the application. This event procedure resides in the Global.asax file.
Application_Error : Occurs when an unhandled exception occurs in the application. This event procedure resides in the Global.asax file.

Error events let you handle exceptions for an entire object in a single, centralized location—the error event procedure. This is different from using exception-handling structures, in which exceptions are handled within the procedure where they occurred. You can use error events in the following ways:

As a substitute for exception-handling structures :
Because error events occur outside the scope of the procedure in which the error occurred, you have less information about the steps leading up to the exception and therefore less ability to correct the exception condition for the user. However, using exception-handling events is fine for tasks where you might not be able to correct the exception in code.
As an adjunct to exception-handling structures :
Error events can provide a centralized “backstop” against exceptions that were not foreseen or handled elsewhere. Using the two exception-handling techniques together lets you catch all exceptions before the user sees them, display a reasonable message, and even record the exception in a log as part of an ongoing effort to improve your application.

Give an example to show how error events can be used to handle exceptions?
To handle an exception using error events, follow these steps:
1. In the Page_Error event procedure, get the exception that occurred using the GetLastError method.
2. Do something with the exception, such as display a message to the user, take steps to correct the problem, or write to an error log.
3. Clear the exception using the ClearError method.
4. Redisplay the page. Web form processing stops immediately when an exception occurs, so server controls and other items on the page might not be displayed after the exception is cleared.
5. Add the following code to Page_Error event procedure on the web page.
private void Page_Error(object sender, System.EventArgs e)
{
// Get the error.
Exception ex = Server.GetLastError();
// Store the message in a session object.
Session["Error"] = ex.Message;
// Clear the error message.
Server.ClearError();
// Redisplay this page.
Server.Transfer("ErrorEvents.aspx");
}
The preceding code stores the exception message as a Session state variable before clearing the exception so that the message can be displayed when the page is reloaded by the Transfer method. The following code displays the saved exception message when the page is redisplayed:

Add the following code to Page_Load event procedure on the web page.
private void Page_Load(object sender, System.EventArgs e)
{
// Display error. if any.
if (Session["Error"] != null)
{
litError.Text = "The following error occurred:
" +
Session["Error"].ToString();
// Clear the Session state variable.
Session["Error"] = null;
}
}

Can you have a try block without a catch or a finally block?
No, you cannot have a try block without a catch or a finally block. A try block cannot exist in isolation. A try block should be followed by either a catch block or a finally block or both.

Is the following code legal?
try
{
Response.Write("Try block executed");
}
finally
{
Response.Write("Finally block executed");
}

Yes, it's legal. A try statement does not have to have a catch statement if it has a finally statement.

What is wrong with using the following type of exception handler?
catch(Exception E)
{
//Some Code
}
This handler catches exceptions of type Exception, therefore, it catches any exception. This can be a poor implementation because you are losing valuable information about the type of exception being thrown and making your code less efficient. As a result, your program may be forced to determine the type of exception before it can decide on the best recovery strategy.

Will the second catch block handle the exception thrown by the first catch block?
try
{
throw new System.IO.FileNotFoundException();
}
catch (System.IO.FileNotFoundException FNFE)
{
Response.Write(FNFE.Message);
throw new Exception();
}
catch(Exception E)
{
Response.Write(E.Message);
}

No. For a catch block to handle the exception, the statement that raised the exception must be inside a try block.

What will happen to the exception raised by the code in the following Button1_Click event procedure?
protected void Button1_Click(object sender, EventArgs e)
{
throw new Exception();
try
{
Response.Write("Hello");
}
catch (Exception E)
{
Response.Write(E.Message);
}
}

The exception will not be handled by the catch block because the statement that raised the exception must be inside a try block.

Managed and Unmanaged Code-

What is Managed Code and Unmanaged Code?
Microsoft ASP.NET Web applications run under the control of the common language runtime (CLR). The CLR controls how the application’s assembly executes, allocates, and recovers memory; therefore, ASP.NET applications are said to use managed code. In contrast, most other Windows executables use unmanaged code because the executable itself determines how memory is used.

Examples of unmanaged code include the Microsoft Win32 API, legacy DLLs and EXEs created for Windows applications prior to the Microsoft .NET Framework, and COM objects.

What is Platform Invoke or pinvoke?
The process of executing native code from within a .NET assembly is called platform invoke, or pinvoke for short. You use platform invoke to call the Win32 API directly, to access existing (legacy) DLLs your company uses, or to access procedures compiled to native code for performance reasons.

What are the steps to follow to use Platform Invoke?
To use platform invoke, follow the following steps:
1. Import the System.Runtime.InteropServices namespace.
2. Declare the unmanaged procedure using the DllImport attribute or the Declare statement.
3. Map the data types of the procedures parameters to the equivalent .NET types.
4. Call the unmanaged procedure and test its return value for success.
5. If the procedure did not succeed, retrieve and handle the exception code using the Marshal object’s GetLastWin32Error method.

What are the limitations of using Unmanaged Code from within a .NET assembly?
Performance : Although native-code DLLs can perform some operations more quickly than equivalent code managed by the CLR, these benefits might be offset by the time it takes to marshal the data to pass between the unmanaged procedure and the .NET assembly.
Type safety : Unlike .NET assemblies, unmanaged procedures might not be type-safe. This can affect the reliability of your .NET application. In general, reliability is a paramount concern with ASP.NET Web applications.
Code security : Unmanaged procedures do not use the .NET Framework’s model for code security.
Versioning:Unmanaged code does not support .NET versioning; therefore, assemblies that call unmanaged procedures might lose the benefit of being able to coexist with other versions of the same assembly.

What are COM objects?
COM objects are another type of unmanaged code that you can use from .NET assemblies. Because COM is widely used, Visual Studio includes built-in tools for importing and using COM objects within .NET assemblies. Visual Studio also includes the option of automatically registering .NET class library assemblies for use from COM.

What happens when you add a reference to a COM object from with in a dot net application?
When you add a reference to a COM object, Visual Studio automatically generates an interop assembly for the object and places it in the project’s /bin folder. The interop assembly is created from the COM object’s type information and contains the metadata that the CLR uses to call the unmanaged code in the COM object. You can then use COM objects from within .NET code the same way that you use .NET classes.

You can view this interop assembly using the Microsoft Intermediate Language Disassembler (Ildasm.exe) included in the .NET Framework.

Can we create a .NET object for use from COM?
Yes, Visual Studio can automatically generate type library information and register a .NET class library assembly for use from COM. These automatic tools do not work for ASP.NET Web applications, so you must isolate the code you want to use from COM in its own Class Library project.

How do you hide Public .NET Classes and other public members from COM?
In some cases, you might want to hide selected .NET classes from COM but keep them public for use from other .NET assemblies. The ComVisible attribute allows you to select which public .NET classes and members are included in the generated type library. This attribute applies hierarchically for the assembly, class, and member levels.
How do you handle exceptions between .NET and COM?
.NET handles errors through exception classes. COM handles errors through 32-bit data types called HRESULTs. All of the .NET exception classes include HResult properties that map to COM HRESULT codes.

If an exception occurs in a .NET object, the exception is automatically mapped to the appropriate HRESULT and returned to COM. Similarly, if an exception occurs in a COM object, the COM HRESULT is mapped to the appropriate exception class, which is returned to .NET, where it can be handled just like any other exception.

If you are creating your own .NET exception classes for use with COM, be sure to set the class’s HResult property so that the exception can be handled within COM.

What are the technical limitations of COM Interop?
The .NET Framework was developed to address the limitations of COM. Because of this evolution, there are limits to the .NET features that you can use from COM. The following list describes these limits:
Static members : COM requires objects to be created before use, so it does not support .NET Static members.
New members : COM flattens the inheritance tree of .NET objects, so members in a derived class that hides members inherited from a base class are not callable.
Constructors with parameters : COM can’t pass parameters to an object’s constructor.

What are the practical limitations of using COM objects?
The following are the practical limitations of using COM objects from .NET:
Shared solutions might not allow COM objects : ASP.NET host service providers that use nondedicated servers can limit or prohibit the installation of COM objects on their servers.
COM objects are prone to memory leaks : COM uses reference counting to determine when to destroy objects and free memory. It is possible for this reference count to become incorrect, leaving objects in memory indefinitely.
Type libraries might be inaccurate : Because COM separates the object’s description from its implementation, it’s possible for this description to not accurately reflect the object. In this case, the generated interop assembly will also include those inaccuracies.
COM is unmanaged code : All the limitations of unmanaged code apply to COM objects as well.

Tracing-

What is an exception log?
An exception log is a list of handled exceptions that occur while your application is running. Reviewing the exception log periodically helps you verify that exceptions are being handled correctly, are not occurring too frequently, and are not preventing users from accomplishing tasks with your application.

What is Tracing and what are the adavantages of using tracing to log exceptions?
Tracing is a technique for recording events, such as exceptions, in an application. There have always been ways to record errors in an application - usually by opening a file and writing error messages to it. But tracing offers the following significant advantages:
Standardization:Building tracing into the .NET Framework ensures that programming techniques are the same across all the applications you develop with the .NET Framework.
Built-in Web support:ASP.NET extends the .NET Framework tools by including information related to the performance and behavior of Web requests.
Configuration:You can turn tracing on and off using settings in your application’s configuration file. You don’t have to recompile your application to enable or disable tracing.
Performance:While disabled, tracing statements do not affect application performance.

How do you turn tracing on and off for an ASP.NET web application?
Tracing can be turned on or off for an entire Web application or for an individual page in the application:
1. To turn tracing on for an entire application, in the application’s Web.config file, set the trace element’s Enabled attribute to True.
Or
2. To turn tracing on for a single page, set the DOCUMENT object’s Trace property to True in the Visual Studio .NET Properties window. This sets the @ Page directive’s Trace attribute to True in the Web form’s HTML.

Where is the trace output displayed by default?
By default, trace output is displayed at the end of each Web page.

While this is fine for debugging purposes, you’ll generally want to write trace output to a log file when you start testing your completed application. To write trace messages to a log file for an entire application, in the application’s Web.config file, set the trace element’s PageOutput attribute to False. ASP.NET then writes trace output to the Trace.axd file in your application’s root folder.

How do you specify, how many page requets should be written to the trace log?
The element's RequestLimit attribute can be used to specify how many page requests to write to the trace log. For example, the following line from a Web.config file turns on tracing for the application and writes the first 10 requests to the Trace.axd file:

How do you write trace messages to a log file for only selected pages in an application?
To write trace messages to a log file for only selected pages in an application, follow these steps:
In the application’s Web.config file, set the trace element’s Enabled attribute to True and PageOutput attribute to False.
For each Web page you want to exclude from tracing, set the @ Page directive’s Trace attribute to False.

What is the difference between Trace.Write() and Trace.Warn() methods of a trace object?
The Trace object provides the Write and Warn methods to allow you to write messages to a request’s trace information. The two methods are identical with one difference: messages written with Trace.Write are displayed in black, whereas messages written with Trace.Warn are displayed in red.

How do you programatically check if tracing is enabled?
The Trace object’s IsEnabled property can be used to programatically check if tracing is enabled.

How do you prevent from trace output being written at the bottom of the web page?
You can prevent from trace output being written at the bottom of the web page by setting the trace element’s PageOutput attribute to False in the Web.config file.

What is the name of the file to which trace log is written?
Trace.axd

Can you view Trace.axd from a remote machine?
No, by default, you can view Trace.axd only from the local server running the application. If you want to view the trace log from a remote machine, set the trace element’s LocalOnly attribute to False in the Web.config file

Session State and Application State-

What is a Session?
A Session is a unique instance of the browser. A single user can have multiple instances of the browser running on his or her machine. If each instance visits your Web application, each instance has a unique session.A session starts when a user accesses a page on a Web site for the first time, at which time they are assigned a unique session ID. The server stores the user's session ID in the Session.SessionID property.

What is the default session timeout period?
20 minutes.

Where do you generally specify the Session Timeout?
You specify the Session Timeout setting in the web.config file.

Can you specify Session Timeout in a code behind file?
Yes, can specify the Session.Timeout property as shown below in a code behind file.
Session.Timeout = 10;

How do you end a user session?
You can call the Session.Abandon() method to end a user session. If a user then tries to access a page the server will assign them a new session ID and it will clear all the previous session variables. You'll typically use Session.Abandon() on log-out pages.

What type of data can you store in Application State and Session State variables?
Application State and Session State variables are used to store data that you want to keep for the lifetime of an application or for the lifetime of a session. You can store any type of data in the Application or Session state, including objects.

Are Application State or Session State variables type safe?
No, Application and Session state variables are created on the fly, without variable name or type checking.

Do maintaining Session state affects performance?
Yes

Can you turn of Session state?
Yes, Session state can be turned off at the application and page levels.

Are Application state variables available throughout the current process?
Yes, Application state variables are available throughout the current process, but not across processes. If an application is scaled to run on multiple servers or on multiple processors within a server, each process has its own Application state.

How do you disable Session state for a Web form?
To turn Session state off for a Web form set EnableSessionState property of the Page to False.

How do you turn Session state off for an entire web application?
In the Web.config file, set the sessionstate tag to False.

What are Application State variables?
Application State variables are global variables that are available from anywhere in the application. All Sessions can access Application State variables.

How to add and remove data to Application State Variables?
//Code to add data to Application State
Application.Add("AppName", "Sample");

//Code to remove data from Application State
Application.Remove("AppName");

How do you remove all Application State Variables data?
//Code to remove all Application State Variables data
Application.RemoveAll();

Transaction-

What is a transaction?
A transaction is a group of commands that change the data stored in a database. The transaction, which is treated as a single unit, assures that the commands are handled in an all-or-nothing fashion. if one of the commands fails, all of the commands fail, and any data that was written to the database by the commands is backed out. In this way, transactions maintain the integrity of data in a database. ADO.NET lets you group database operations into transactions.

What is the main purpose of database transactions?
The main purpose of database transactions is to maintain the integrity of data in a database.

How do you determine which SQL commands are part of a transaction?
You can determine what database commands belong in a transaction by using the ACID test. Commands must be atomic, consistent, isolated, and durable.
Commands belong in a transaction if they are:
Atomic:In other words, they make up a single unit of work. For example, if a customer moves, you want your data entry operator to change all of the customer’s address fields as a single unit, rather than changing street, then city, then state, and so on.
Consistent:All the relationships between data in a database are maintained correctly. For example, if customer information uses a tax rate from a state tax table, the state entered for the customer must exist in the state tax table. Isolated:Changes made by other clients can’t affect the current changes. For example, if two data entry operators try to make a change to the same customer at the same time, one of two things occurs: either one operator’s changes are accepted and the other is notified that the changes weren’t made, or both operators are notified that their changes were not made. In either case, the customer data is not left in an indeterminate state.
Durable:Once a change is made, it is permanent. If a system error or power failure occurs before a set of commands is complete, those commands are undone and the data is restored to its original state once the system begins running again.

Why is transaction processing very important for web applications?
Transaction processing is very important for Web applications that use data access, because Web applications are distributed among many different clients. In a Web application, databases are a shared resource, and having many different clients distributed over a wide area can present the below key problems.
Contention for resources:Several clients might try to change the same record at the same time. This problem gets worse the more clients you have.
Unexpected failures:The Internet is not the most reliable network around, even if your Web application and Web server are 100 percent reliable. Clients can be unexpectedly disconnected by their service providers, by their modems, or by power failures.
Web application life cycle:Web applications don’t follow the same life cycle as Windows applications—Web forms live for only an instant, and a client can leave your application at any point by simply typing a new address in his or her browser.

List the steps in order to process a transaction?
1.Begin a transaction.
2.Process database commands.
3.Check for errors.
4.If errors occurred, restore the database to its state at the beginning of the transaction. If no errors occurred, commit the transaction to the database.

Explain how a DataSet provides transaction processing?
DataSet provide transaction processing through the RejectChanges and Update methods. DataSet also provide an AcceptChanges method that resets the state of records in a data set to Unchanged. Data sets provide implicit transaction processing, because changes to a data set are not made in the database until you invoke the Update method on the data adapter object. This lets you perform a set of commands on the data and then choose a point at which to make the changes permanent in the database.

If an error occurs during the Update method, none of the changes from the data set is made in the database. At that point, you can either attempt to correct the error and try the Update method again or undo the changes pending in the data set using the data set’s RejectChanges method.

Give an example to show how DataSets provide transaction processing?
Let us assume we have a DataGrid that displays employee information. Every row also has a delete button, which when you click will delete that row. On this page we also have a Restore and Commit buttons. When you click the Restore button you should be able to restore the data to its previous state. When you click the Commit button you should be able to update the database with the deletions made in the DataSet.

The code for Commit and Restore buttons is shown below.
private void butRestore_Click(object sender, System.EventArgs e)
{
// Restore the data set to its original state.
dsContacts.RejectChanges();
// Refresh the data grid.
grdContacts.DataBind();
}

private void butCommit_Click(object sender, System.EventArgs e)
{
int intRows;
// Update the database from the data set.
intRows = adptContacts.Update(dsContacts);
// Save changes to state variable.
Session["dsContacts"] = dsContacts;
// Refresh the data grid.
grdContacts.DataBind();
}
The RejectChanges method in the preceding butRestore_Click event procedure returns the data set to its state before the row was deleted. The data set’s AcceptChanges method is the inverse of RejectChanges—it resets the DataRowState property for all the changed rows in a data set to Unchanged and removes any deleted rows.

The AcceptChanges method prevents the Update method from making those changes in the database, however, because Update uses the rows’ DataRowState property to determine which rows to modify in the database. For this reason, the AcceptChanges method is useful only when you do not intend to update a database from the data set.

What are the 3 types of transaction objects available in ADO.NET?
As we have 3 types of database connections in ADO.NET, there are also 3 types of transaction objects:
SqlTransaction
OracleTransaction
OleDbTransaction

What are the steps involved in using a transaction object in ADO.NET?
1.Open a database connection.
2.Create the transaction object using the database connection object’s BeginTransaction method.
3.Create command objects to track with this transaction, assigning the Transaction property of each command object to the name of the transaction object created in step 2.
4.Execute the commands. Because the purpose of transaction processing is to detect and correct errors before data is written to the database, this is usually done as part of an error-handling structure.
5.Commit the changes to the database or restore the database state, depending on the success of the commands.
Close the database connection.

What property of a transaction object determines how concurrent changes to a database are handled?
IsolationLevel property of the transaction object is used to determine how concurrent changes to a database are handled.

What are different isolation levels of a transaction object in ADO.NET?
ReadUncommitted:Does not lock the records being read. This means that an uncommitted change can be read and then rolled back by another client, resulting in a local copy of a record that is not consistent with what is stored in the database. This is called a dirty read because the data is inconsistent.
Chaos:Behaves the same way as ReadUncommitted, but checks the isolation level of other pending transactions during a write operation so that transactions with more restrictive isolation levels are not overwritten.
ReadCommitted:Locks the records being read and immediately frees the lock as soon as the records have been read. This prevents any changes from being read before they are committed, but it does not prevent records from being added, deleted, or changed by other clients during the transaction. This is the default isolation level.
RepeatableRead:Locks the records being read and keeps the lock until the transaction completes. This ensures that the data being read does not change during the transaction.
Serializable:Locks the entire data set being read and keeps the lock until the transaction completes. This ensures that the data and its order within the database do not change during the transaction.

What is the default isolation level in a transaction?
ReadCommitted

What is a Save Point in a transaction in ADO.NET?
SqlConnection object provide one transaction capability that is unavailable for OLE database connections: the ability to create save points within a transaction. Save points let you restore the database state to a specific position within the current transaction. To set a save point within a SQL transaction, use the Save method as shown below.
TransactionObject.Save("FirstStep");

How do you restore a SQL transaction to a specific save point?
To restore a SQL transaction to a save point, specify the name of the save point in the Rollback method as shown below.
TransactionObject.Rollback("FirstStep");

User Controls-

What are ASP.NET Custom controls?
Custom controls extend the tools available to Web developers. Using custom controls, you can encapsulate key aspects of the visual interface and program logic that you want to reuse throughout your application, or throughout your organization.

What are the 3 types of custom controls in ASP.NET?
Microsoft Visual Studio .NET provides three types of custom control for use on Web forms.
1. Web user controls
These combine existing server and HTML controls by using the Visual Studio .NET Designer to create functional units that encapsulate some aspect of the user interface. User controls reside in content files, which must be included in the project in which the controls are used.
2. Composite custom controls
These create new controls from existing server and HTML controls. Although similar to user controls, composite controls are created in code rather than visually, and therefore they can be compiled into an assembly (.dll), which can be shared between multiple applications and used from the Toolbox in Visual Studio .NET.
3. Rendered custom controls
These create entirely new controls by rendering HTML directly rather than using composition. These controls are compiled and can be used from the Toolbox, just like composite controls, but you must write extra code to handle tasks that are performed automatically in composite controls.

What are the limitations of user controls in ASP.NET?
As the user controls are not compiled into assemblies, they have the following limitations:
1. A copy of the control must exist in each Web application project in which the control is used.
2. User controls can’t be loaded in the Visual Studio .NET Toolbox; instead, you must create them by dragging the control from Solution Explorer to the Web form.
3. User control code is initialized after the Web form loads, which means that user control property values are not updated until after the Web form’s Load event.

What are the steps to follow for creating and using a user control in a Web application?
1. Add a Web user control page (.ascx) to your project.
2. Draw the visual interface of the control in the designer.
3. Write code to create the control’s properties, methods, and events.
4. Use the control on a Web form by dragging it from Solution Explorer to the Web form on which you want to include it.
5. Use the control from a Web form’s code by declaring the control at the module level and then using the control’s methods, properties, and events as needed within the Web form.

How do you identify user controls?
User controls are identified by their .ascx file extensions.

What is the base class from which user controls derive?
User controls derive from System.Web.UI.UserControl base class. This base class provides the base set of properties and methods you use to create the control.

What are the steps to follow to create properties and methods for the user control that you can use from a Web form?
To create properties and methods for the user control that you can use from a Web form, follow these steps:
1. Create the public property or method that you want to make available on the containing Web form.
2. Write code to respond to events that occur for the controls contained within the user control. These event procedures do the bulk of the work for the user control.
3. If the property or method needs to retain a setting between page displays, write code to save and restore settings from the control’s ViewState.

What happens when you drag a user control from solution explorer and drop it on a web form?
When you drag a user control from solution explorer and drop it on a web form, Visual Studio .NET generates a @Register directive and HTML tags to create the control on the Web form.

Custom Controls-

What are composite custom controls?
Composite custom controls combine one or more server or HTML controls within a single control class, which can be compiled along with other control classes to create an assembly (.dll) that contains a custom control library. Once created, the custom control library can be loaded into Visual Studio .NET and used in the same way as the standard server and HTML controls.

Composite custom controls are functionally similar to user controls, but they reside in their own assemblies, so you can share the same control among multiple projects without having to copy the control to each project, as you must do with user controls. However, composite controls are somewhat more difficult to create because you can’t draw them visually using the Visual Studio .NET Designer.

What are the steps to follow create and use a custom control in a Web application?
1. Create a solution containing a custom control project.
2. Add a Web application project to the solution, and set it as the startup project. You will use the Web application project to test the custom control during development.
3. Add a project reference from the Web application to the custom control project, and add an HTML @Register directive and control element to use the custom control on a Web form.
4. Create the custom control’s visual interface by adding existing controls to it through the custom control’s CreateChildControls method.
5. Add the properties, methods, and events that the custom control provides.
6. Build and test the custom control.

In general what is the base class for every composite custom control?
System.Web.UI.WebControls.WebControl

Which directive is used to add a custom control to a Web form?
Register directive.

What are the 3 Register directive's attributes?
TagPrefix
This name identifies the group that the user control belongs to. For example, the tag prefix for ASP.NET server controls is “asp”. You use this prefix to create a naming convention to organize your custom controls.
Namespace
This is the project name and namespace within the custom control assembly that contains the controls to register. Microsoft Visual Basic .NET uses the project name as an implicit namespace, so for controls written in Visual Basic .NET, use the project name.
Assembly
This is the name of the assembly (.dll) containing the custom controls. The control assembly must be referenced by the Web application. Referencing the assembly maintains a copy of it in the Web application’s /bin directory.

What are the differences between User Controls and Custom Controls?
1. User Controls are easy to create where as Custom Controls are difficult to create.
2. User Controls cannot be compiled into an assembly, where as Custom Controls can be compiled into an assembly.
3. User Controls cannot be added to tool box, where as Custom controls can be added to the toolbox.
4. You need to have a copy of user control in every project where you want to use it, where as this is not the case with custom controls. You can install a single copy of the Web custom control in the global assembly cache and share it between applications, which makes maintenance easier.
5. User controls are used for reusing existing user interface elements and code, but are not useful for developing reusable components for multiple web applications.

ASP.Net Interview Questions Part4

2013-07-21T10:56:00.003-07:00

Caching-

What is caching?
High-performance Web applications should be designed with caching in mind. Caching is the technique of storing frequently used items in memory so that they can be accessed more quickly. Caching is important to Web applications because each time a Web form is requested, the host server must process the Web form’s HTML and run Web form code to create a response. By caching the response, all that work is bypassed. Instead, the request is served from the reponse already stored in memory.

Caching an item incurs considerable overhead, so it’s important to choose the items to cache wisely. A Web form is a good candidate for caching if it is frequently used and does not contain data that frequently changes. By storing a Web form in memory, you are effectively freezing that form’s server-side content so that changes to that content do not appear until the cache is refreshed.

What directive is used to cache a web form?
The @OutputCache page directive is used to cache a Web form in the server’s memory.

What is the use of duration attribute of @OutputCache page directive?
The @OutputCache directive’s Duration attribute controls how long the page is cached. For example if you set the duration attribute to 60 seconds, the Web form is cached for 60 seconds.

The first time any user requests the Web form, the server loads the response in memory and retains that response for 60 seconds. Any subsequent requests during that time receive the cached response.

After the cache duration has expired, the next request for the Web form generates a new response, which is then cached for another 60 seconds. Thus the server processes the Web form once every 60 seconds at most.

What are the 2 required attributes of the @OutputCache directive?
The @OutputCache directive has two required attributes:
1. Duration
2. VaryByParam. How do you cache multiple responses from a single Web form?
The VaryByParam attribute lets you cache multiple responses from a single Web form based on varying HTTP POST or query string parameters. Setting VaryByParam to None caches only one response for the Web form, regardless of the parameters sent.

You can also cache multiple responses from a single Web form using the VaryByHeaders or VaryByCustom attribute.

The VaryByCustom attribute lets you cache different responses based on a custom string. To use VaryByCustom, override the GetVaryByCustomString method in the Web application’s Global.asax file.

Is it possible to cache a web form without using @OutputCache directive?
Yes, you can cache a web form using the Response object’s Cache property, which returns an HttpCachePolicy object for the response. The HttpCachePolicy object provides members that are similar to the OutputCache directive’s attributes.

Give a simple example to show how to cache a web form without using @OutputCache directive?
For example, the following code caches the Web form’s response for 60 seconds:
private void Page_Load(object sender, System.EventArgs e)
{
// Cache this page
DateTimeLabel.Text = System.DateTime.Now.ToString();
// Set OutputCache Duration. Response.Cache.SetExpires(System.DateTime.Now.AddSeconds(60));
// Set OutputCache VaryByParams.
Response.Cache.VaryByParams["None"] = true;
// Set OutputCache Location.
Response.Cache.SetCacheability(HttpCacheability.Public);
}

The preceding code is equivalent to the following OutputCache directive:
@ OutputCache Duration="5" VaryByParam="None" Location="Any"

What is @OutputCache directive’s Location attribute and the HttpCachePolicy object’s SetCacheability property used for?
The @OutputCache directive’s Location attribute and the HttpCachePolicy object’s SetCacheability property determine where Microsoft ASP.NET stores cached responses. By default, ASP.NET caches responses at any available location that accepts cache items - the client, proxy servers, or the host server. In practice, those locations might or might not allow caching, so you can think of the Location/SetCacheability setting as more of a request than a command.

What is HttpCachePolicy object’s SetAllowResponseInBrowserHistory method used for?
You can override the cache location settings using the HttpCachePolicy object’s SetAllowResponseInBrowserHistory method. Setting that method to True allows the response to be stored in the client’s history folder even if the location setting is None or Server.

Which object can used to store frequently used items in the server’s memory for quick retrieval?
Cache object can be used to store frequently used items in the server’s memory for quick retrieval.

Is the cache object available for all web forms with in a web application?
Yes, the Cache object is global, that is, data stored in the Cache object is available anywhere within a Web application. In this way, the Cache object is very similar to the intrinsic Application object.

What are the 3 different ways to store data in the Cache object?
Use assignment.
Assigning a value to an unused key in the Cache object automatically creates that key and assigns the value to that key. Assigning a value to a key that already exists replaces the cached value with the assigned value.
Use the Insert method.
The Insert method uses parameters rather than assignment to create or change cached data. Insert optionally accepts parameters to establish dependencies and set expiration policy.
Use the Add method.
The Add method is similar to Insert; however, it requires all parameters and returns an object reference to the cached data.

For example, the following Cache statements all add the same item to the cache:

using System.Web.Caching;
private void Page_Load(object sender, System.EventArgs e)
{
if(!IsPostBack)
{
Cache["NewItem"] = "Some string data";
Cache.Add("NewItem", "Some string data", null, Cache.NoAbsoluteExpiration, System.TimeSpan.FromMinutes(1), CacheItemPriority.Default, null);
Cache.Insert("NewItem", "Some string data");
}
}

What are absoluteExpiration and slidingExpiration parmeters of the Insert and Add methods?
absoluteExpiration
A DateTime object that identifies when the data should be removed from the cache. If you’re using sliding expiration, specify Cache.NoAbsoluteExpiration for this parameter.
slidingExpiration
A TimeSpan object that identifies how long the data should remain in the cache after the data was last accessed. If you’re using absolute expiration, specify Cache.NoSlidingExpiration for this parameter.

Which delegate can be used to notify the application when items are removed from the cache?
onRemoveCallback is used to notify the application when items are removed from the cache.

How do you retrieve the value of a cache item stored in the servers memory?
You can retrieve the value of a cache item stored in the servers memory through the item’s key, just as you do with the Application and Session objects. Because cached items might be removed from memory, you should always check for their existence before attempting to retrieve their value, as shown in the following code:

private void Button1_Click(object sender, EventArgs e)
{
if (Cache["ChachedItem"] == null)
{
Lable1.Text = "Cached Item not found.";
}
else
{
Lable1.Text = Cache["ChachedItem"].ToString();
}
}

Which method can be used to remove data from the cache?
Cache object’s Remove method can be used to remove data from the cache as shown in the following code example / sample.

private void RemoveButton_Click(object sender, System.EventArgs e)
{
Cache.Remove("CachedItem");
}

How do you control how long data is cached?
The Cache object’s Add and Insert method parameters allow you to control how long an item is stored in the server’s memory. In practice, these parameter settings provide only indirect control of how long data remains in memory. If your server runs low on available memory, ASP.NET recovers as much memory as possible from expired cache items. If that’s not enough, ASP.NET will unload unexpired items from the cache based on their priority and when they were last accessed.

What is CacheItemPriority enumeration used for?
CacheItemPriority enumeration is used to set the relative importance of cached items. CacheItemPriority.NotRemoveable has the highest priority and CacheItemPriority.Low has the lowest priority.

Which is the only "event” provided by Cache object?
CacheItemRemoved "event” is the only "event” provided by Cache object.

How do you update the Cache object when data changes?
Items stored in the cache are often copies of data that is stored and maintained elsewhere, such as records in a database. Use the Add and Insert methods’ dependency parameter to establish a relationship between a cached data item and an external source, such as a file, a folder, or a group of files.

The dependency parameter accepts a CacheDependency object, which in turn identifies the file, folder, or set of files to watch for changes. ASP.NET checks the time stamp of the items in the CacheDependency object, if one of those time stamps is later than the DateTime entered for the cached item, ASP.NET unloads that item from the cache.

What is fragment caching?
Caching parts of web form is called as fragment caching. Sometimes you want to cache only part of a Web form response. For instance, a Web form might contain many pieces of variable information plus a single large table that almost never changes. In this case, you might place that table in a Web user control and store the response for that control in cache. This technique is called fragment caching.

What are the steps to follow to cache parts of web form?
To cache part of a Web form, follow these steps:
1. Place the controls and content that you want to cache in a Web user control.
2. Set the caching attributes for that Web user control.
3. Create an instance of the Web user control on the Web form.

What is PartialCaching attribute used for?
You can include the PartialCaching attribute in the control’s class declaration to enablefragment caching.

What are the OutputCache directive attributes that apply only to user controls?
Shared
Cache a single response from a user control for use on multiple Web forms. By default, ASP.NET caches a separate response for each Web form that uses a cached user control. This attribute is only available in the .NET Framework version 1.1 or later.

VaryByControl
Cache multiple responses for a single user control based on the value of one or more controls contained in the user control. Can you cache multiple versions of a user control?Yes, You can cache multiple versions of a user control based on the value of controls contained in a user control (VaryByControl) or based on a custom string (VaryByCustom).

If a user control is read from the cache, can you access its members from code?
No, In general, cached controls are used to present data such as queries from a database, rather than as interactive components. However, if you do need to access a cached control from code, you must first check that the control exists. If the control is read from the cache, you can’t access its members from code. Control members are available only when the control is not read from the cache, such as when the control is first instantiated and when it is reloaded after its cache duration has expired.

When caching is set at both the Web form and user control levels, How does the cache settings interact?
The cache location is determined by the Web form setting. Location settings on a user control have no affect.
If the Web form’s cache duration is longer than the user control’s, both the Web form response and the user control response will expire using the Web form setting.

Cookies-

What are Cookies in ASP.NET?
Cookies are small pieces of information stored on the client computer.Use cookies to store small amounts of information on the client’s machine. Web sites often use cookies to store user preferences or other information that is client-specific. Because cookies can be refused, it is important to check whether the browser allows them before you try to create them.They are limited to storing only character data and they are limited to 4K in size.

What are different types of Cookies?
Session Cookies
Persistent Cookies

What are Session Cookies?
Session cookies are stored in-memory during the client browser session. When the browser is closed the session cookies are lost.

How can you create Session Cookies?
You can create session cookies by calling the Add method of the Cookies collection on the Response object. The Cookies collection contains individual cookie objects of type HttpCookie.

//Code to create a UserName cookie containing the name David.
HttpCookie CookieObject = new HttpCookie("UserName", "David");
Response.Cookies.Add(CookieObject);

//Code to read the Cookie created above
Request.Cookies["UserName"].Value;

What is the difference between Session Cookies and Persistent Cookies?
Persistent Cookies are same as Session Cookies except that, persistent cookies have an expiration date. The expiration date indicates to the browser that it should write the cookie to the client's hard drive. Keep in mind that because a user can delete cookies from their machine that there is no guarantee that a cookie you "drop" on a user machine will be there the next time they visit your site.

What are Persistent Cookies used for?
Persistent cookies are generally used to store information that identifies a returning user to a Web site. Typical information found in Persistent Cookies includes user names or user IDs.

How do you create a Persistent Cookie?
You create a persistent cookie the same way as session cookies except that you set the Expires property to a Date in the future which will store the Cookie to the client computer harddrive.

//Code to create a UserName Persistent Cookie that lives for 10 days
HttpCookie CookieObject = new HttpCookie("UserName", "David");
CookieObject.Expires = DateTime.Now.AddDays(10);
Response.Cookies.Add(CookieObject);

//Code to read the Cookie created above
Request.Cookies["UserName"].Value;

What is Cookie Dictionary?
A cookie dictionary is a single cookie object that stores multiple pieces of information. You use the Values property to access and assign new values to the cookie dictionary.

Give an example using Cookie Dictionary?
//Code to create a Cookie Dictionary
HttpCookie CookieObject = new HttpCookie("UserPreference");

//Use the Values property to assign new values to the cookie dictionary
CookieObject.Values.Add("UserName", "David");
CookieObject.Values.Add("Country", "USA");
CookieObject.Values.Add("PreviousVisit", DateTime.Now.ToString());
CookieObject.Expires = DateTime.MaxValue;

//Add the Cookie to the client machine using the Response object
Response.Cookies.Add(CookieObject);

//Code to read the Cookie created above
HttpCookie ObjectCookie = Request.Cookies["UserPreference"];
string UserName = ObjectCookie.Values["UserName"];
string Country = ObjectCookie.Values["Country"];
string PreviousVisit = ObjectCookie.Values["PreviousVisit"];

What are the advantages of Using Cookies?
1. Cookies do not require any server resources since they are stored on the client.
2. Cookies are easy to implement.
3. You can configure cookies to expire when the browser session ends (session cookies) or they can exist for a specified length of time on the client computer (persistent cookies).

What are the disadvantages of Using Cookies?
1. Users can delete a cookies.
2. Users browser can refuse cookies,so your code has to anticipate that possibility.
3. Cookies exist as plain text on the client machine and they may pose a possible security risk as anyone can open and tamper with cookies.

How do you create a Cookie that never expires?
To create a Cookie that never expires set the Expires property of the Cookie object to DateTime.MaxValue.

Are Cookies secure?
No, Cookies are not secure. You must pay attention to the type of data you store in cookies.
1. Cookies are not designed to store critical information so storing passwords in a cookie is a bad idea.
2. Keep the lifetime of a cookie as short as practically possible.
3. Encrypt cookie data to help protect the values stored in the cookie.