Microsoft Dot Net

connection pooling in .net

2012-02-19T21:24:00.000-05:00

Connection Pooling in asp.net

If you are using asp.net to a SQL database, you will most likely use a SQL connection object. If you are using SQL connection object, something called connection pooling is enabled by default. Now the question that naturally arises is what is connection pooling? When your asp.net application connects to SQL server, opening a connection is resource intensive and takes time. So when you call close() on a connection, instead of actually closing the connection it gets returned to a "pool of connections". So this connection is in fact still open. Then next time you call an Open() on a sql connection, this pre existing open connection from the connection pool gets reused. This is called connection pooling. Connection pooling in managed by the ADO.net in the .net framework class library.

Now what if multiple users perform an operation at the exact same time all of which require database connection? In that case new connections to the database will be opened, then all of them will be returned to the connection pool when close() is called on those connections.

In case you don't close connections and your application pool reaches the maximum number of open connections (default is 100 open connections per connection pool), trying to open another connection after that will throw this error

"Timeout expired. The timeout period elapsed prior to obtaining a connection from the pool. This may have occurred because all pooled connections were in use and max pool size was reached.
"

So it is necessary to close a connection after the required operation is complete.

SqlConnection.ClearAllPools() actually closes all the open connections for a particular application. (if you run an EXEC sp_who in SQL server before and after the ClearAllPools(), you can see the connections being closed.

Affect of connection poling on performance

Just for the fun of it, I ran the Query below once with the connection pooling disabled and the second time with connection pooling enabled.

for (int i = 0; i < 100000; i++)

{

SqlConnection conn = new SqlConnection();

string connectionString = System.Configuration.ConfigurationManager.ConnectionStrings["PlayGroundConnectionString"].ConnectionString;

conn.ConnectionString = connectionString;

conn.Open();

conn.Close();

}

These are the results

Connection Pool Enabled?	Time
False	240 seconds
True	2.4 seconds

As you can see with the connection pooling enabled, the performance gains are extra ordinary.

Connection Pooling Quiz

Q>How to disable connection pooling
A> This can be done in the connection string.

<add name="myConnectionString" connectionString="Data Source=MyServerName;Initial Catalog=MyDataBaseName;User ID=mySqlUserName;Password=myPassWord;Pooling=False;""

providerName="System.Data.SqlClient" />

Q>How many application pools can an application have?
A>Many. "Connections are pooled per process, per application domain, per connection string and when integrated security is used, per Windows identity." Even one character difference in two connection strings will result in two different connection pools. Hence it is safer to store the connection string in the web.config file.

Q>If connection pooling is disabled will the connections still be limited to 100 open connections?
A> No.

Q>If you enable windows authentication, what is its significance with respect to connection pooling?
<connectionStrings>

<add name="myConnectionString" connectionString="Data Source=MyServerName;Initial Catalog=MyDataBaseName;Integrated Security=True"

providerName="System.Data.SqlClient" />

</connectionStrings>

A> Every time a new user logs in a new connection must be opened (because a new connection pool is created for every user). That would cause a large number of open connections if the application has a large number of users.

Q>If you reach the maximum number of permissible connections in a connection pool, how long would it wait before the next connection request is timed out?
A>15 seconds (default time out)

Q>If connection pooling is enabled, when you try to connect to the database, will you see the login event on SQL trace?
A> If an open connection was fetched from the application pool you will not see it.

How to read connection string from web.config (asp.net)

2012-02-18T14:32:00.002-05:00

To read connection string from web.config do the following

1>Store connection string in web.config

For using Windows Authentication, your connection string would look like

<add name="myConnectionString" connectionString="Data Source=MyServerName;Initial Catalog=MyDataBaseName;Integrated Security=True"

providerName="System.Data.SqlClient" />

</connectionStrings>

</configuration>

For using SQL Authentication, your connection string would look like

<add name="myConnectionString" connectionString="Data Source=MyServerName;Initial Catalog=MyDataBaseName;User ID=mySqlUserName;Password=myPassWord"

providerName="System.Data.SqlClient" />

</connectionStrings>

</configuration>

2>Read connection string from C# code

string connectionString = System.Configuration.ConfigurationManager.ConnectionStrings["myConnectionString"].ConnectionString;

Some useful tools for .net developers

2012-02-16T15:49:00.000-05:00

Tool	URL	Purpose	Free
fiddler	http://fiddler2.com/fiddler2/	Monitor the traffic between the browser and the websites	Yes
lumzy	http://www.lumzy.com/	Create mock ups for websites for the clients to view and approve before you actually start building the website.	Yes
Beyond Compare	http://www.scootersoftware.com/moreinfo.php	A very useful tool for comparing differences in files and folders.	30 day trial
Collapse all projects within visual studio	· VS 2008 – http://visualstudiogallery.msdn.microsoft.com/DF3F0C30-3D37-4E06-9EF8-3BFF3508BE31 · VS 2010 – http://visualstudiogallery.msdn.microsoft.com/e5f41ad9-4edc-4912-bca3-91147db95b99	An extension for Visual studio that would minimize (collapse) all the projects	Yes
ANTS Memory Profiler	http://www.red-gate.com/products/dotnet-development/ants-memory-profiler/	Find memory leaks in your .net application	14 day trial
SQL formatter	http://www.dpriver.com/pp/sqlformat.htm	An online tool for formatting your SQL code	Yes

Last entered row SQL Server

2012-02-15T17:43:00.002-05:00

These are the various choices you have to get the latest identity entered into a table in SQL server

SELECT @@IDENTITY

SELECT SCOPE_IDENTITY()

SELECT IDENT_CURRENT(‘tablename’)

The most often I personally use is SELECT SCOPE_IDENTITY()

I will defer to an excellent article from Pinal Dave for an explanation on each of these

http://blog.sqlauthority.com/2007/03/25/sql-server-identity-vs-scope_identity-vs-ident_current-retrieve-last-inserted-identity-of-record/

System.CodeDom.Compiler Namespace ( with example)

2012-02-13T15:47:00.004-05:00

System.CodeDom.Compiler: This namespace lets you generate source code files on the fly (at runtime). It also lets you generate assemblies (dlls) from these generated source code at runtime.

Note that it can only generate code in one of the four managed languages (C#, vb, Jscript, C++)

Given below is a simple example that uses CodeDomProvider class in the CodeDom.Compiler namespace.

using System;

using System.CodeDom.Compiler;

namespace ConsoleApplication5

{

class Program

{

static void Main(string[] args)

{

CodeDomProvider provider;

string language = "c#";

provider = CodeDomProvider.CreateProvider(language);

Console.WriteLine(provider.ToString());//Displays Microsoft.CSharp.CSharpCodeProvider

Console.WriteLine(provider.FileExtension);//Displays cs

}

If you add a watch on CodeDomProvider.GetAllCompilerInfo() this is what you see

Compare C# and vb (C# vb difference)( C# vs vb)

2012-02-11T19:20:00.003-05:00

I am not covering and syntax difference here. I am trying to list all the functional differences between the two. Let me know if you can think of any more (in the comments) , I will include it in here.

C#	VB

Was designed ground up specifically for the .net framework.	VB was built as a successor of the pre existing VB. They tried to keep it syntactically as similar as possible to the VB prior to .net.
You can write unmanaged code	NA
NA	isNumeric() can be used to check if the value is numeric
Namespace cannot be imported at the project level and has to be imported into every file using the “using” directive.	Namespaces can be imported at the project level
Multiline comments are possible ( /**/)	NA
For adding asp.net page events, in C# you have to lookup the signature of the page event and type it out (the AutoEventWireup should be set to true)	In asp.net when using vb, page events can be easily added thru the Visual Studio UI.

Besides the fact that vb makes things easier (which could qualify vb as slightly superior) , I have not seen any major functional differences between the two languages. However, most of my C# friends believe C# is superior. Myself being a C# developer, as much as I'd like to believe that, I can't. In my opinion, the choice between C# and vb should purely depend on the in house skill-set. If your associates are more comfortable in one of the languages, it makes more sense to go with that.

bitmask (SQL server and bitmask)

2012-02-10T17:17:00.000-05:00

What is is a bitmask?

Suppose you have a web page where you have a checkbox list (multiple check boxes). The user can select one or many of these check boxes. To keep a track of the checkboxes the user ticked, you can select from two popular options

1> Have a boolean column in the database for each checkbox. (So if you have 10 check boxes, then 10 columns)
or
2>Have just one column that holds a value corresponding to the check boxes ticked.

If you like option 2, then bitmask is your friend.

An example explaining the concept of bitmask

Suppose I have a webpage where the user has to select the fruits he/she likes. For each fruit he/she selects, I have assigned a binary value for that fruit. (Note that every binary position is assigned to one fruit)

Apple: 00001
Orange: 00010
Grapes: 00100
Mango: 01000

If the user selects say apple and mango, then this is what I would do

1>Add the two binary values
00001+01000=01001

2>Convert to integer
binary 01001= decimal 9

3>Save 9 in the database

Note here that the column that saves a bitmask value in database is defined as an integer.

I know at the first sight, it might seem like some information is lost in the process. But its not. Think of it like this, every position in the binary representation is reserved for one check box. So the binary representation of the stored value will have a 1 in the 10s position only if orange was selected. Try it out yourself.

When you actually implement this in an application, you don't need to convert anything into binary. What is explained above is the underlying concept. When you implement, you directly use the integer values.

Binary	Decimal
1	1
10	2
100	4
1000	8
10000	16
100000	32
1000000	64

and so on.

When you use bitmask in SQL server, the bitwise AND (&) is a very useful feature.

Suppose I have a value 9 saved in the database. If I want to see if Mango was one of the selected checkboxes, I could use Bitwise and (&)

IF( ( 9&8 ) = 8 )

BEGIN

PRINT 'mango selected!'

END

ELSE

BEGIN

PRINT ' no mango selected.'

END

The statement 9&8 does this

1>Convert both to binary

9 decimal = 1001 binary

8 decimal = 1000 binary

2> See which position matches between the two. In this case it is 1 in the thousands position

1000

3>Return the decimal of the common value

binary 1000= 8 decimal.

Therefore ( 9&8 ) returns 8.

sql stored procedure output parameters

2012-02-08T14:47:00.001-05:00

Stored Procedure with output parameter

CREATE PROCEDURE [dbo].[usp_OutPara]

@x int output,

@s varchar(max)

BEGIN

set @x=1

select @s

END

Execute the stored proc and read the output variable

DECLARE @x INT

DECLARE @x2 INT

DECLARE @s VARCHAR(MAX)

set @s='hello world'

EXEC Usp_outpara

@x=@x2 OUTPUT,

@s=@s

SELECT @x2 --Displays 1

To read the output variables in asp.net check out

http://dotnetanalysis.blogspot.com/2011/09/adonet-with-examples.html

Unable to start debugging on the web server. An unexpected DCOM error occurred while trying to automatically attach to the remote web server. Try manually attaching to the remote web server using the 'Attach To Process' dialog.

2012-02-01T15:46:00.000-05:00

This is what I did

1>Click on website in IIS 7.5. On the right side of the screen under Actions, click on "Basic settings". That opens up a pop up as shown below.

2>Click Connect as in the pop up window shown above.

3>Select Application user

Unable to start debugging on the web server. The IIS worker process for the launched URL is not currently running. (IIS 7.5)

2012-02-01T13:23:00.002-05:00

Enabling windows authentication on the website worked for me.

1>In IIS manager click on the website to see all the features

2>Double click on authentication

3>Enable Windows Aunthentication

database normalization examples (SQL Server)(normalization in database)

2012-01-29T22:41:00.003-05:00

Normalizing a database means laying out the tables in an efficient way with the main focus being on preserving the integrity of data. It is easier to explain with an example, what normalization is all about.

I once worked on this project where the developers had laid out this database for a website which had these two tables.

The Bands table held the userids of the users who belonged to the band. Any band member could log in to the website and add a new member to his band. Once a new member was added to a band, all the members of that band were to receive an email. The developers, to make their job easier, added an email column to the band. So every time they copy over a userID to the bands table, they would also copy over the email address of that user. This website ran okay for a short time, but after a few weeks users started complaining of not receiving emails when members were added to their bands. A little investigation revealed that these users who had complained of not getting emails had changed their email addresses.
Can you guess what might have happened? When the users updated their email addresses thru the website, their address got updated only in the users table, the reason being that when this page was built, the Bands table did not exist. The bands was a feature that was added later on into the system.

One of the main goals of normalization is to avoid repetition of data. If data is repeated, it can lead to inconsistent data as this clearly evident in the above example (email address is inconsistent, two tables give two separate email addresses for the same individual).

There are 7 forms of Normalization. The seven forms of Normalization are

First Normal Form
Second Normal Form
Third Normal Form
Boyce-Codd Normal Form
Fourth Normal Form
Fifth Normal Form
Domain/Key Normal Form

Many database designers normalize OLTP databases only up to the 3rd Normal form (and reporting OLAP databases up to 2nd Normal Form). The 3rd Normal form reduces redundancy to a reasonable level without complicating the design too much. In this article we will start with a Denormalized database, then follow the rules and normalize it to the third normal form.

(Before we proceed , if you are unfamiliar with SQL keys, please check out this article
http://dotnetanalysis.blogspot.com/2012/01/sql-key-super-key-candidate-key-primary.html)

Consider the table below. This database stores data for products purchased by people online. This database also stores their employer information. Lets see how to normalize this database to the third normal form.

SSN	UserName	Product1	Product2	MoreProducts	EmployerName	EmployerAddress
332345432	Amy	M			Google	1 California drive
666666666	Kevin	A	B	C,D	Facebook	22nd Street Sanfrancisco
919919919	Raj	D			Google	1 California drive

First Normal Form:

You can only have one value in a column
You should not create multiple columns for a one to many relationship.

There are more rules to the first normal form (such as two rows cannot be identical etc) which are automatically enforced by the SQL server. Hence I won't be listing them here.

SSN	UserName	EmployerName	EmployerAddress	Product
332345432	Amy	Google	1 California drive	M
666666666	Kevin	Facebook	22nd Street Sanfrancisco	A
666666666	Kevin	Facebook	22nd Street Sanfrancisco	B
666666666	Kevin	Facebook	22nd Street Sanfrancisco	C
666666666	Kevin	Facebook	22nd Street Sanfrancisco	D
919919919	Raj	Google	1 California drive	D

Second Normal Form:

The table is in First Normal Form
All the non primary key columns in the table should depend on the entire primary key

SSN	UserName	EmployerName	EmployerAddress
332345432	Amy	Google	1 California drive
666666666	Kevin	Facebook	22nd Street Sanfrancisco
919919919	Raj	Google	1 California drive

SSN	ProductName
332345432	M
666666666	A
666666666	B
666666666	C
666666666	D
919919919	D

For non transactional (reporting) applications, 2nd Normal form is good enough.

Third Normal Form

Database is in Second Normal form
There are no transitive dependencies (That is every non primary key is dependent only on the primary key.

SSN	UserName
332345432	Amy
666666666	Kevin
919919919	Raj

EmployerName	EmployerAddress
Google	1 California drive
Facebook	22nd Street Sanfrancisco

SSN	EmployerName
332345432	Google
666666666	Facebook
919919919	Google

When a database is in the third normal form, there is no redundancy (no repetition of data).

robocopy working example (copy files command prompt)

2012-01-26T11:45:00.003-05:00

ROBOCOPY "C:\source\config\dev" "C:\desti\config\Beta" connectionStrings.config

The console command above copies the file connectionStrings.config
from C:\source\config\dev
to C:\desti\config\Beta

SQL key (Super key, Candidate key, Primary key, unique key, Composite key, foreign key)

2012-01-21T18:04:00.000-05:00

SocialSecurityNumber	Name	Address	BirthDay	TaxIDNumber
818764259	John	22 washington st	1/3/1972	837650987
517881756	Raj	1 boston road	6/9/1964	767354174
788876566	Janet	23 avenue NYC	4/15/1985	939635252
748209944	Janet	23 avenue NYC	4/15/1980	643228844

Uniquely identifying a row means being able to pin point to one row. For example suppose I need you to delete the fourth row, if I tell you, delete the row where the name is Janet. There are two such rows, so that means I cannot uniquely identify a row (or pin point a row) with the name alone.

But if I tell you delete the row where Name is Janet , address is 23 avenue NYC and birth day is 4/15/1980. Then you will delete the correct row. That means a combination of Name,Address and Birthday, uniquely identifies a row.
(You can also uniquely identify a row with SocialSecurityNumber or TaxIDNumber )

Super key: A combination of columns that can uniquely identify a row in a table. A super key can have more columns than it needs to uniquely identify a row.

For example in the above table [SocialSecurityNumber+ Name+Birthday] is a super key. Note here that SocialSecurityNumber by itself can uniquely identify a row. Name and Birthday are extra columns that are not required to uniquely identify a row.

Candidate key: A minimal super key. (A sub set of super key, where if you remove even one column, you cannot identify a row uniquely any more)

Assume that a particular address you cannot have two individuals with the same name AND date of birth.

In the example above SocialSecurityNumber, TaxIDNumber and the combination [Name+Addresss+ Birthday] are all Candidate keys. But if you remove Birthday from [Name+Addresss+ Birthday] then it is not a candidate key anymore.

Primary key: The main candidate key in a table. (The main column or combination of columns that uniquely identifies a row). In the above example, SocialSecurityNumber is the primary key.

Note that when you set a column as a primary key in SQL server, it also automatically sets that column (or the combination of columns) as a “clustered index”. But if you want, you can remove the clustered index on the primary key. It is not mandatory to have the primary key as the clustered index.

Unique Key: Unique is practically (but not theoretically) the same as superkey. Unique key is a constraint that you put on a table where you want to ensure that the value is not repeated between two rows.

For example, in my SQL database table I want to make sure that no two rows have the same TaxIDNumber. To do that in SQL Server, I will right click on the table-->Design

In the new window that opens up, I will right click the table-->Indexes/Keys. Click Add. Then make the selections as shown in the image below.

Note here that you can set a unique key on a combination of columns. You can also set multiple unique keys in a table.

Difference between primary and unique key:

Primary key constraint	Unique key constraint
A table can have only one primary key.	A table can have multiple unique keys.
Doesn’t allow null in any of the columns that are a part of the primary key.	Allows nulls in all the columns that are a part of the unique key constraint.
By default a clustered index is created on the column(s) that you set the primary key on. (This index can be dropped later, it is not mandatory)	By default a non clustered index is created on the column(s) that you set the Unique key on. (This index can be dropped later, it is not mandatory)

Composite key (Compound key): A key that includes more than one column.

Foreign key constraint: This is more easily explained with an example. Consider the tables shown below.

The table Students hold the roll numbers for each student. The grade table holds a grade for each roll number. To make sure a grade for a roll number should not exist in the Grade table, if that roll number is not present in the students table, we will set a foreign key as shown below. With a foreign key set, sql server will not allow the entry of a roll number in the Grades table, unless that roll number exists in the Students table.

Note here that rollNumber has to be defined as a primary or uniqe key in the students table for this foreign key to be possible.

Further Reading:
Database Normalization :http://dotnetanalysis.blogspot.com/2012/01/database-normalization-sql-server.html

how to read uncommitted data in sql server

2012-01-18T15:39:00.002-05:00

Example

set transaction isolation level read uncommitted

select * from tblStudents

(The text highlighted in yellow prompts SQL server to return uncommitted records)

Give user read permission to all databases in SQL server ( SQL server read only permission)

2012-01-17T13:38:00.000-05:00

To give a user "read only" permissions to a database, where the user can view everything and modify nothing, he/she needs two permissions, db_datareader and VIEW DEFINITION.

In the script below replace myUserName with the username of the user that you want to give permissions to all the databases in SQL server. On running the script below, this user will get permission to view all tables, stored procedures, functions etc. But would be able to modify none.

declare @username nvarchar(max)

DECLARE @queryForServer nvarchar(max)

DECLARE @queryForEachDB nvarchar(max)

/********************************/

/*ONLY VARIABLE TO BE SET*/

/********************************/

set @username='[myUserName]'

/********************************/

set @queryForServer=

IF NOT EXISTS (SELECT * FROM sys.server_principals WHERE name = N'''+@username+''')

BEGIN

CREATE LOGIN '+@username+' FROM WINDOWS WITH DEFAULT_DATABASE=[Master], DEFAULT_LANGUAGE=[us_english]

END

set @queryForEachDB =

'use ?

IF EXISTS (SELECT * FROM sys.database_principals WHERE name = N'''+@username+''')

BEGIN

DROP USER '+@username+'

END

CREATE USER '+@username+' FOR LOGIN '+@username+'

GRANT VIEW DEFINITION to '+@username+'

EXEC sp_addrolemember N''db_datareader'', N'''+@username+'''

--If not exists add user at Server Level

EXECUTE sp_executesql @queryForServer

--Add user to each database with datareader and view definition permission

EXECUTE sp_msforeachdb @queryForEachDB

How to give an active directory group access to SQL server? (AD group SQL server)

2012-01-13T15:31:00.003-05:00

Verify the members in the active directory group

First make sure that the AD group doesn't have any member that you don't want to give access to your SQL server by running this command on the command prompt.

NET GROUP myGroupName /domain

This above command will give you all the members in the active directory group myGroupName .(only replace myGroupName , rest of the command remains as is)

Give the AD Group login permission to SQL server

CREATE LOGIN [myGroupName] FROM WINDOWS WITH DEFAULT_DATABASE=[master], DEFAULT_LANGUAGE=[us_english]

asp.net get paths/urls

2012-01-05T17:51:00.002-05:00

Get absolute path of a file :

string absoluteFilePath = Server.MapPath("~/Products.xml");

Get Complete url of the current page:

string completeUrl = HttpContext.Current.Request.Url.AbsoluteUri;

// "http://localhost:35876/Default.aspx"

Get the domain name (host header):

string domainName = HttpContext.Current.Request.Url.Host;
OR

string domainName =HttpContext.Current.Request.Headers["Host"]

// "localhost"

Get Relative path of the current page (relative to the root) :

string relativePath = HttpContext.Current.Request.Url.AbsolutePath;

// "/Default.aspx"

Further reading:
http://www.west-wind.com/weblog/posts/2009/Dec/21/Making-Sense-of-ASPNET-Paths

C# Dispose Method

2012-01-03T14:39:00.001-05:00

If a class implements the IDisposable interface, it will have a Dispose() Method. A simplified explanation for Dispose() would be, a place where you free up resources that garbage collector may not know to free. Consider the example below

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

namespace ConsoleApplication1

{

public class GetDataFromFile:IDisposable

{

public void Dispose()

{

//perform clean up here

//eg close the file handle

}

public string ReadFile()

{

string fileData = "";

//some code that reads the file will go in here

return fileData;

}

class Program

{

static void Main(string[] args)

{

GetDataFromFile GF=new GetDataFromFile();

string data;

try

{

data = GF.ReadFile();

}

finally

{

GF.Dispose();

}

Every class that implements IDisposable, would have customized use for its Dispose method. For example, in .net class library, one of the classes that implements IDisposable is SqlConnection Class. In the SqlConnection Class, the Dispose() method returns the connection object back to the connection pool.

A few points to note

If you are freeing up resources in the Dispose, it is advisable that these resources should be freed in the destructor too. The reason being, if your class is being used by someone who forgets to call the Dispose(), then the freeing up of resources would still be performed when the garbage collection kicks in.
If an object's Dispose method is called more than once, the object must ignore all calls after the first one. It should not throw an error.
As a general rule, if you do not need to free up any unmanaged resources, you do not need to implement the IDisposable interface.
try

{



    data= GF.ReadFile();

}

finally

{

    GF.Dispose();

}

can also be written as

using (GetDataFromFile GF = new GetDataFromFile())

{

    data = GF.ReadFile();

}

Predicate C#

2011-12-18T19:03:00.001-05:00

What is a Predicate?

The predicate is just a generic delegate that is defined in the .net framework class library. In the framework library, it is defined as

public delegate bool Predicate<T>(T obj);

This delegate is used for searching purposes.

Predicate Example

The FindAll() method in List<T> expects an instance of the predicate as a parameter. In the example below I show you how to do that.

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Linq.Expressions;

namespace PlayLambda

{

public class Person

{

public string name;

public int Age;

}

class Program

{

static void Main(string[] args)

{

Person P1 = new Person();

P1.name = "Mike";

P1.Age = 37;

Person P2 = new Person();

P2.name = "Tommy";

P2.Age = 13;

Person P3 = new Person();

P3.name = "Kim";

P3.Age = 27;

List<Person> People = new List<Person>();

People.Add(P1);

People.Add(P2);

People.Add(P3);

Predicate<Person> myPredicate = IsDrinkingAge;

//FindAll() expects an instance of the Predicate as a parameter

foreach (Person p in People.FindAll(myPredicate))

{

Console.WriteLine("This person can legally drink: " + p.name);

}

static bool IsDrinkingAge(Person p)

{

//The signature of this method should match the signature of the predicate

if (p.Age > 21)

{

return true;

}

else

{

return false;

}

Shown below is the output of this program

What is LINQ?

2011-12-14T20:21:00.003-05:00

Traditionally C# (like C++, java etc) has been an imperative programming language. That means if you want to fetch data from a source or dataset, we have to give the compiler a detailed set of precise instructions. For instance, suppose I have a datatable that has information on all my employees. If I want to find the salary of an employee named John, I would have to write code to loop thru every row in the datatable.

Now suppose the same table is there in a SQL database. If I need to find the salary of John, all I would do is write something like

Select salary from tblEmployees where firstName='John'

As you can notice, in SQL I don't have to write code to loop through all the rows in the table. DBMS does the job of looping through for me. This way of coding where you just declare what you want and the job gets done behind the scene by some engine for you, is called declarative programming.

So what we have learned above can be summed up as

Imperative programming: : Give detailed steps to get required data. eg C#,C++ etc
Declarative programming (functional programming) : Tell the computer what you need and it will be fetched for you. eg SQL

Clearly, compared to imperative programming, declarative programming requires a lot lesser code to retrieve data. LINQ is the way to write declarative code in C#.

Type Inference

2011-12-14T19:08:00.001-05:00

In C# you can write

var x = "hello";

Since "hello" is a string, x becomes a string at compile time. This is called type inference. (As in, the type of variable x got inferred.)

If you try something like

var x = "hello";

int b = a;

The compiler will throw an error: Cannot implicitly convert type 'int' to 'string'.

foreach C# ( IEnumerable implementation )

2011-11-26T17:23:00.001-05:00

foreach is used in C# to iterate over a collection of items.

The example below shows a simple example where foreach is used in a console application.

int[] myArray = new int[] {1,2,3,4,5 };

foreach (int i in myArray)

{

System.Console.WriteLine(i);

}

The output of this code is shown below

What we have done above is we have used foreach on an instance of the system.array class. To be able to run foreach on an instance of a class, the compiler's rule is, the class must implement IEnumerable interface.

So if you have a class, where you want to be able to run a foreach loop on an instance of the class, you could do that by implementing the IEnumerable interface in that class. In fact the compiler does not even care if you have actually inherited from the IEnumerable interface. What it looks for is a few method names with particular signatures. The example below shows the bare essentials for being able to run a foreach on an instance of a class.

using System;

using System.Text;

namespace ConsoleApplication4

{

public class Person

{

public string firstName;

public string lastName;

}

public class People//This class is practically implementing IEnumerable Interface

{

To be able to run a foreach on People class

You should have a method named GetEnumerator.

public PeopleEnum GetEnumerator()

{

/* This method should return an instance of a class that has 3 methods of the following signatures

1> public bool MoveNext()

2> public void Reset()

3> public xxx Current() --replace xxx with whatever you want the class People to be comprised of

return new PeopleEnum();

}

public class PeopleEnum//This class is practically implementing IEnumerator Interface

{

public bool MoveNext()

{

return false;

}

public void Reset()

{

}

public Person Current//This is where the compiler finds out that, the class People is comprised of Persons

{

get

{

return new Person();

}

class Program

{

static void Main(string[] args)

{

People developers = new People();

foreach (Person pp in developers)

{

}

What is shown above is what is required to satisfy the compiler's needs. For the foreach to be actually useful , you would implement IEnumerable as shown in
http://msdn.microsoft.com/en-us/library/system.collections.ienumerable(v=VS.110).aspx

Its very rare that you would actually implement the IEnumerable interface for looping thru a user defined collection. Under most circumstances you would attain this by using a class in System.Collections Or the System.Collections.Generic Namespace.

C# Dependency Inversion (or the Inversion of Control ) using constructor injection

2011-11-08T18:32:00.001-05:00

Instead of dwelling on why the term "Inversion of control" was chosen, I will instead tell you what problem it tries to solve and how it solves it. You can come up with your own explanation of what the original intention of the term was. (Please share it with us in the comments below if you don't mind )

The code shown below is an oversimplified version whose purpose is to only explain the concept. What I will try to show you here is how in a bad code, changing one part of the code requires changes in other parts of the code. And how after applying dependency inversion, I could make different parts of code, independent of each other.

Imagine I have a console application that has a class GetMessageFromDatabase that gets data from the database. I have another class DisplayMessage that has a method that displays this message on the screen.

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

namespace ConsoleApplication1

{

class Program

{

static void Main(string[] args)

{

DisplayMessage dm = new DisplayMessage();

dm.ShowMessage();

}

public class DisplayMessage

{

GetMessageFromDatabase Gmd;

public DisplayMessage()

{

Gmd = new GetMessageFromDatabase();

}

public void ShowMessage()

{

Console.WriteLine(Gmd.GetMessage());

Console.ReadLine();

}

public class GetMessageFromDatabase

{

public string GetMessage()

{

//Pretend this comes from the database

return "Hi from database";

}

Everything works fine. Suppose 3 months down the line based on the argument passed into the Main method, I have to pull data either from a database or an xml file. To make that happen I add a class GetMessageFromXML to my code. Also I make a slight modification to my Main method. The code now looks like below.

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

namespace ConsoleApplication1

{

class Program

{

static void Main(string[] args)

{

DisplayMessage dm = new DisplayMessage(args[0].ToString());

dm.ShowMessage();

}

public class DisplayMessage

{

string source;

public DisplayMessage(string s)

{

source = s;

}

public void ShowMessage()

{

if (source.ToUpper() == "DATABASE")

{

GetMessageFromDatabase Gmd = new GetMessageFromDatabase();

Console.WriteLine(Gmd.GetMessage());

Console.ReadLine();

}

else if (source.ToUpper() == "XML")

{

GetMessageFromXML Gmx = new GetMessageFromXML();

Console.WriteLine(Gmx.GetMessage());

Console.ReadLine();

}

public class GetMessageFromDatabase

{

public string GetMessage()

{

//Pretend this comes from the database

return "Hi from database";

}

public class GetMessageFromXML

{

public string GetMessage()

{

//Pretend this comes from an XML file

return "Hi from XML";

}

Now another 2 months later suppose I also need to read data from a text file based on the input parameter to the main method. What would I do? I would add another class GetMessageFromTextFile . Also I would have to modify the class DisplayMessage.

Do you see how dependent the class DisplayMessage is on the implementation (that is depending on where the data is being fetched from) ? You can see where I am going with this.

What I have shown you above so far, is bad code.

Now my question to you is this:

How do you write the code such that even if I add a new location that the data can be fetched from, the code in the class DisplayMessage does not have to change?

If you solve the above problem, you have invented your own inversion of control !

Inversion of Control attempts to make the class DisplayMessage and class GetMessageFromTextFile totally independent of each other. Change in one class shouldn't require a change in another class.

There are three popular ways of inversion of control namely

Constructor Injection
Method Injection
Property Injection

In the code below, I have demonstrated inversion of control using constructor injection

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

namespace ConsoleApplication1

{

public interface IGetData

{

string GetMessage();

}

class Program

{

static void Main(string[] args)

{

IGetData IG;

string source = args[0].ToString();

if (source.ToUpper() == "DATABASE")

{

IG = new GetMessageFromDatabase();

}

else if (source.ToUpper() == "XML")

{

IG = new GetMessageFromXML();

}

else if (source.ToUpper() == "TEXT")

{

IG = new GetMessageFromTextFile();

}

else

{

IG = new GetMessageFromDatabase();//default set to database

}

DisplayMessage dm = new DisplayMessage(IG);

dm.ShowMessage();

}

public class DisplayMessage

{

IGetData IGLocal;

public DisplayMessage(IGetData IG)

{

IGLocal = IG;

}

public void ShowMessage()

{

Console.WriteLine(IGLocal.GetMessage());

}

public class GetMessageFromDatabase : IGetData

{

public string GetMessage()

{

//Pretend this comes from the database

return "Hi from database";

}

public class GetMessageFromXML : IGetData

{

public string GetMessage()

{

//Pretend this comes from an XML file

return "Hi from XML";

}

public class GetMessageFromTextFile : IGetData

{

public string GetMessage()

{

//Pretend this comes from an Text file

return "Hi from Text file";

}

Now like you see above, injecting the dependencies through the constructor, I am decoupling DisplayMessage class from the class GetMessageFromDatabase or any other class that the DisplayMessage class might end up utilizing. I can add as many new classes as I want, to fetch data, without modifying the DisplayMessage class, as long as they inherit from the IGetData interface.
So what happened here is that, everything depends on the interface. Which is ideal.

Now you might argue that I actually ended up shifting the dependency to the Main method. You are absolutely correct. But like I said earlier, I am trying to strip this example to bare essentials. So that the root concept is easier to grasp. If you are curious to know, the dependency can be removed from the Main method, in multiple ways. One way being, to move the dependency to another class by applying the factory pattern, whose responsibility would be to pass the right IGetData object, based on the source.

Further reading
http://dotnetanalysis.blogspot.com/2011/10/principles-of-design-patterns.html

Principles of software design

2011-10-31T18:47:00.002-04:00

Every software application, whether you apply design patterns or not, you should adhere to the design principles. Applying design principles makes your application robust and easy to maintain. Also design principles must be learned before you can embark on a journey to learn design patterns. Allow me to introduce you to design principles with the so called SOLID design principles.

S: Single Responsibility Principle

Like the name suggests, what this means is that every object should be built for one reason only. You should avoid writing multi purpose classes. For example in the .net class library, the string functions are in its own single purpose (sting functions) System.String class and the math functions are in System.Math class (for math functions).

O: Open-Closed Principle
A class should be open for extension and closed for modification. This means once a class has had a stable release, the existing functionality should never be modified. It it however okay to add new functionality.

L: Liskov Substitution Principle

This principle states that you should be able to use a derived class in place on the parent class and the derived class should behave the same as its parent class.

I: Interface Segregation Principle

This principle is tells you to separate interfaces based on their responsibility. Do not have one huge interface for everything. For example imagine the IEnumerable interface also made you implement a lot of methods that you don't need for a foreach loop. Thats obviously an unwanted inconvenience, if all you want is to apply a foreach loop. This is avoided by having only the methods required for a foreach loop as a part of the IEnumerable interface.

D: Dependency Inversion Principle (or the Inversion of Control Principle )
The Dependency Inversion Principle states when one class uses another class, both the code in both classes can be totally independent of each other and should only depend on interfaces (or abstract classes).
There are three ways to achieve this, namely

Constructor Injection
Method Injection
Property Injection

The Dependency Inversion Principle is a concept that will take a little more reading to understand. I would recommend checking out this article, which explains this concept in a lot more detail.

http://dotnetanalysis.blogspot.com/2011/11/dependency-inversion-or-inversion-of.html

Introduction to Generics C#

2011-10-30T21:36:00.000-04:00

Generics is a simple concept that sounds sophisticated, due to the use of the fancy word "Generics". Generics basically is anything that uses the keyword <T>. <T> can be thought of as "any type"

Consider the function below

public static void Display(string s)

{

Console.WriteLine(s);

}

Now suppose you want to be able to pass variables of any type(int, string etc) into this method. This is what you could do.

public static void Display<T>(T anyType)

{

Console.WriteLine(anyType.ToString());

}

Given below is the whole code.

using System;

using System.Text;

namespace ConsoleApplication1

{

class Program

{

static void Main(string[] args)

{

int a = 1;

string s = "hey";

NonGenericDisplay(s);//Displays "hey"

GenericDisplay<int>(a);//Displays 1

GenericDisplay<string>(s);//Displays "hey"

}

public static void NonGenericDisplay(string s)

{

//This method can only accept strings

Console.WriteLine(s);

}

public static void GenericDisplay<T>(T anyType)

{
//This method can accept int,string,bool etc..

Console.WriteLine(anyType.ToString());

}

The article gave you an oversimplified idea of what generics is all about.

In a more practical sense, generics is usually used where you want to declare a type at time of instantiating a class.

Further Reading:
http://dotnetanalysis.blogspot.com/2011/04/list.html