Code Arsenal - Programming Tutorials and Code Samples

Memo - persistent, local-first memory for AI coding agents

2026-03-15T16:32:00.003+02:00

The problem with AI coding assistants: every session starts from zero.

You spend an hour with Claude figuring out how your auth flow works, why that API returns 429s under load, or how to properly configure your test database. Real progress.

Next week, different session — you're explaining it all over again.

I built memo — a local memory layer for AI coding agents (Claude Code, Cursor, Codex, or anything MCP-compatible). It gives your AI assistant persistent, searchable memory across all sessions and projects.

Written in Go, runs 100% locally. Nothing leaves your machine!

Uses SQLite + vector search and an on-device embedding model, so it finds relevant memories even when you phrase things differently.

How I use it:

When I figure out why something works a certain way — an undocumented API quirk, a tricky config, a non-obvious design decision — I tell Claude to save it. Next time that context is needed, it's already there.

When onboarding onto a new codebase, I save architecture notes and gotchas as I learn them. Weeks later, Claude already has the context instead of me re-reading the same code.

Over time it becomes a personal knowledge base that your AI assistant actually uses — and it starts surfacing relevant context without you asking.

brew install ybonda/tap/memo
claude mcp add --scope user memo -- memo serve

That's it. Persistent memory across all your projects — everything saved locally in ~/.memo/memories.db

GitHub repo

TTSensei — The Ultimate Table Tennis Equipment Database

2026-02-04T23:39:00.001+02:00

Choosing the right table tennis blade sounds straightforward until you actually start looking. Hundreds of brands, thousands of models, and every one of them described with vague marketing language like "excellent feel" or "offensive power." For players who want real data before spending their money, the options have always been limited — scattered forum threads, a handful of outdated review sites, and word of mouth.

TTSensei takes a completely different approach.

What Makes TTSensei Different?

The platform hosts one of the most detailed table tennis equipment databases you'll find anywhere. Over 2,400 blades from 100+ brands are catalogued with more than 35 specifications each — not just the basics like weight and speed class, but granular details like ply composition, layer materials, blade thickness, and construction method.

What really stands out is how the ratings work. Each blade gets scored on speed, control, spin, and other dimensions, but every score comes with a written breakdown explaining what it means in practice. You won't just see a number — you'll understand whether a blade's speed comes from stiffness or flex, and what that implies for your technique. The rubber section works the same way, with in-depth characteristics and performance analysis that help you understand how a rubber pairs with different blades.

More Than a Database

TTSensei has grown into a full table tennis hub. The platform tracks professional players with detailed profiles covering their equipment choices, career history, and playing styles. It provides match coverage with scores and tactical breakdowns from professional events, giving you more than just results.

There's also a news section that keeps up with tournament outcomes, new product releases, and what's happening in the sport globally. Players can join the community forum to swap equipment opinions, discuss technique, or get advice from other enthusiasts. And for anyone looking to save money or offload gear they no longer use, TTSensei runs a community marketplace where registered members can buy and sell used blades, rubbers, and accessories directly.

Independent and Unbiased

TTSensei doesn't accept paid reviews or sponsored placements. Ratings are driven by equipment performance and data, not by who's paying for advertising. The team uses AI technologies in their research workflows to ensure consistency across thousands of products, but the priority is always accuracy over promotion.

Worth a Bookmark

Whether you're an experienced competitive player fine-tuning your setup, a coach guiding students through equipment decisions, or a recreational player who just wants to know what the pros are using — TTSensei has something useful for you. The marketplace alone makes it worth checking out if you're on a budget.

Take a look at ttsensei.com

AIgregator - All AI Tools in One Place

2025-04-21T19:56:00.002+03:00

We are happy to announce our new project: AIgregator.com!

In today's rapidly evolving AI landscape, finding the right tools can feel overwhelming. That's where AIgregator comes in—a simple yet powerful platform designed to simplify your search for AI solutions.

AIgregator (aigregator.com) serves as a centralized hub for discovering a wide range of AI tools, applications, and resources. Whether you're a professional looking for industry-specific solutions, a developer seeking new tools, or simply curious about what AI can do for you, this website offers a structured approach to navigating the AI ecosystem.

What makes AIgregator particularly useful is its organized categorization system. Users can browse through various sections including AI image generators, text tools, audio applications, and more. This classification helps visitors quickly find tools relevant to their specific needs without wading through endless search results.

The team behind the platform verifies and regularly updates all tools information, ensuring that users always have access to the most current details about features, pricing, and availability.

Registered users can bookmark their favorite AI tools, creating a personalized collection that's easily accessible with each visit.

Best of all, AIgregator is completely free to use. There are no subscription fees, hidden costs, or premium tiers—the entire platform is accessible to everyone at no charge.

Visit aigregator.com today and start exploring the future of AI at your fingertips!

C# 7 New Features

2018-03-28T09:23:00.001+03:00

C# 7 adds a number of new features and focuses on data consumption, code simplification and performance. The biggest features have already been considered - tuples, local functions, matching with the pattern and throwing expressions. But there are other new opportunities, both large and small. They all combine to make the code more efficient and understandable, so that everyone is happy and productive.

Let's look at the rest of the features of the new version of C #. So! Let's start!

Out

Currently, in C #, using out parameters is not as flexible as we would like. Before calling a method with out parameters, you first need to declare the variables to pass to the method. Because these variables are usually not initialized (since they will be overwritten by the method), you can not use var for their declaration, so you must always specify the full type:

public void PrintCoordinates(Point p)
{
  int x, y; // have to "predeclare"
  p.GetCoordinates(out x, out y);
  WriteLine($"({x}, {y})");
}

Literals Improvements

var d = 123_456;
var x = 0xAB_CD_EF;

You can put this _ symbol anywhere between the numbers the right number of times to improve readability. It does not affect the value. This separator can be used with the types byte, int, long, decimal, float and double.

Ref returns and locals

You can now pass method parameters by reference (with the ref modifier), you can also return them by reference, and also save them by reference in local variables.

public ref int Find(int number, int[] numbers)
{
  for (int i = 0; i < numbers.Length; i++)
  {
    if (numbers[i] == number)
    {
      return ref numbers[i]; // return the storage location, not the value
    }
  }
  throw new IndexOutOfRangeException($"{nameof(number)} not found");
}

int[] array = { 1, 15, -39, 0, 7, 14, -12 };
ref int place = ref Find(7, array); // aliases 7's place in the array
place = 9; // replaces 7 with 9 in the array
WriteLine(array[4]); // prints 9

This is useful for returning specific fields from large data structures. For example, a game can contain its data in a large pre-distributed array of structures (to avoid pauses in garbage collection). Now methods can return a reference directly to such a structure through which the calling method can read and modify it.

There are some security limitations:

You can return only those links that are "safe to return": only those that were sent to you, and those that point to fields in objects
Local ref variables are initialized in a specific storage location and can not be changed to point to another

Returned generic types in async methods

Until now, async methods in C# have to either return void, Task, or Task <T>. C# 7 allows you to define other types in such a way that they can be returned from the asynchronous method. The return type must still match the asynchronous pattern, which means that the GetAwaiter method must be available. A concrete example: in the .NET Framework, a new ValueTask type is added, allowing you to use this new language feature

Since Task and Task <T> are reference types, allocating memory in performance-affecting segments (especially when allocating memory in limited cycles) can seriously reduce performance. Support for generic return value types allows you to return a small meaningful type instead of a reference type, so you can prevent excessive allocation of memory.

class Program
{
  static Random rnd;

  static void Main()
  {
    Console.WriteLine($"You rolled {GetDiceRoll().Result}");
  }

  private static async ValueTask GetDiceRoll()
  {
    Console.WriteLine("...Shaking the dice...");
    int roll1 = await Roll();
    int roll2 = await Roll();
    return roll1 + roll2;
  }

  private static async ValueTask Roll()
  {
    if (rnd == null)
    rnd = new Random();

    await Task.Delay(500);
    int diceRoll = rnd.Next(1, 7);
    return diceRoll;
  }
}

Become a Published Author on CodeFlex.co

2018-02-22T19:03:00.000+02:00

Codeflex.co is a software development & DevOps community web portal that allows to every developer become a recognized expert in the field by publishing their articles directly on CodeFlex website.

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You’ll build your reputation as an expert in the software development and DevOps.
Recruiters from the best companies will be impressed by your expertise - you'll get your job easily.
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You’ll contribute to the community, sharing is caring.

Submission Guidelines

The CodeFlex articles may be of any topic but related to the technology field. We would love to publish articles in various categories including, but not limited to, programming languages, iOS/Android development, software architecture, open source, DevOps techniques etc.

The newest technologies like Big Data, AI, Machine Learning, Microservices, Cloud Technologies, DevOps are more than welcomed.

Please follow the guidelines below when submitting an article:

The article you provide has to be exclusively written for CodeFlex.co.
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Java File to String in one Line

2018-02-22T14:51:00.002+02:00

This is the shortest version of how to read a file and convert it to String in Java:

String myFile = "";
try{
    myFile = new String(Files.readAllBytes(Paths.get(myFile.txt)) );
}
catch (IOException e)
{
    e.printStackTrace();
}

C# 6.0 First Look

2016-10-09T17:35:00.000+03:00

Check out new features in C# 6.0.

Enjoy!

Redis Cluster Setup

2016-09-26T08:12:00.001+03:00

Redis is NoSQL key-value in-memory distributed data store. It can be used as super-fast database, cache or message broker.

Please check out this very useful article about Redis Cluster Setup on Linux: Configuring and Running Redis Cluster on Linux

Java 8 Finding Specific Element in List with Lambda

2016-02-21T11:01:00.001+02:00

As you probably know in Java 8 we can use Lambda expression for different manipulations with collections.

If you have a List with some custom objects inside and you want to find specific element in it you can use stream().filter procedure of Lambda instead of ForEach as you probably used to. Below is a simple example of this case.

List<RepositoryFile> fileList = response.getRepositoryFileList();
RepositoryFile file1 = fileList.stream().filter(f -> f.getName().contains("my-file.txt")).findFirst().orElse(null);

So here we have filtered entire collection by specific condition. Notice that due to orElse(null) at the end of the expression if no file was found then Lambda will return null.

Java JSON Parser Example with Regex

2016-02-18T11:36:00.000+02:00

Recently I came across with problem where I needed to recognize and parse custom exception message inside JSON on Java server side. For that particular problem I used Regex.

The JSON response format is like this:

And this is the code:

protected void checkException(String response) throws ApiException {
        
    String errorCode = null;
    String message = null;
    Map<String, String> properties = null;

    Pattern errorCodePattern = Pattern.compile("\"code\"\\s*:\\s*\"([^,]*)\",");
    Pattern messagePattern = Pattern.compile("\"message\"\\s*:\\s*\"([^,]*)\",");
    Pattern statusPattern = Pattern.compile("\"status\"\\s*:\\s*\"(FAILURE)\"");        

    Matcher errorCodeMatcher = errorCodePattern.matcher(response);
    Matcher messageMatcher = messagePattern.matcher(response);
    Matcher statusMatcher = statusPattern.matcher(response);        

    boolean errorCodeFlag = errorCodeMatcher.find();
    boolean messageFlag = messageMatcher.find();
    boolean statusFlag = statusMatcher.find();                

    if (errorCodeFlag && messageFlag && statusFlag) {
        
        Pattern propertiesPattern = Pattern.compile("\"properties\"\\s*:\\s*\\[(\\{[^\\]]*)");
        Pattern propertiesValuePattern = Pattern.compile("\"value\"\\s*:\\s*\"([^\"]*)\"");
        Pattern propertiesKeyPattern = Pattern.compile("\"key\"\\s*:\\s*\"([^\"]*)\"");

        Matcher propertiesMatcher = propertiesPattern.matcher(response);
        
        boolean propertiesFlag = propertiesMatcher.find();
        
        boolean keyFlag = false;
        boolean valueFlag = false;
        
        errorCode = errorCodeMatcher.group(1);
        message = messageMatcher.group(1);

        System.out.println("\nJSON response contains exception:\n");
        System.out.println("code:        " + errorCode);
        System.out.println("message:     " + message);
        System.out.println("status:      " + statusMatcher.group(1));

        if (propertiesFlag) {

            properties = new HashMap<String, String>();
            String propertiesData = propertiesMatcher.group(1);

            Matcher keyMatcher = propertiesKeyPattern.matcher(propertiesData);
            Matcher valueMatcher = propertiesValuePattern.matcher(propertiesData);

            keyFlag = keyMatcher.find();
            valueFlag = valueMatcher.find();

            System.out.println("properties:    ");
            if (keyFlag && valueFlag && keyMatcher.groupCount() == valueMatcher.groupCount()) {
                for (int i = 1; i <= keyMatcher.groupCount(); i++) {
                    System.out.println("key: " + keyMatcher.group(i) + "  value: " + valueMatcher.group(i));
                    properties.put(keyMatcher.group(i), valueMatcher.group(i));
                }
            }
            System.out.println("");
        }
        throw new ApiException(errorCode, message, properties);
    }
}

The output:

AngularJS Binding Syntax and Examples

2016-02-17T19:05:00.001+02:00

AngularJS bindings cheat sheet:

One-way(data source -> view target):

{{expression}}
[target] = "expression"
bind-target = "expression"

One-way(view target -> data source):

(target) = "statement"
on-target = "statement"

Two-way:

[(target)] = "expression"
bindon-target = "expression"

Now let's see some real life AngularJS binding examples:

Property:

<img [src] = "carImageUrl">
<car-detail [car]="currentCar"></car-detail>
<div [ngClass] = "{selected: isSelected}"></div>

Event:

<button (click) = "onSave()">Save</button>
<car-detail (deleted)="onCarDeleted()"></car-detail>
<div myClick (myClick)="clicked=$event">click me</div>

Two-Way:

<input [(ngModel)]="carName">

Attribute:

<button [attr.aria-label]="help">help</button>

Class:

<div [class.special]="isSpecial">Special</div>

Style:

<button [style.color] = "isSpecial ? 'red' : 'blue'">

What is Interpolation in AngularJS

2016-02-15T19:25:00.002+02:00

Interpolation is such a process when AngularJS fetches a calculated string into the text between HTML element tag and within attribute assignments.

Let's see example of interpolation:

<h3>
   {{title}}
    <img src = "{{myImageUrl}}" >
</h3>

In this example {{myImageUrl}} first is been evaluated by AngularJS and only then it converts it to a string. And remember that template expression cannot refer to anything in the global namespace.

How to Become a Software Developer

2016-01-14T18:32:00.002+02:00

If you come over this post I'd like to cheer you and say that you are the very special person!

Why? Because you probably a software developer, or might want to become one.

There are a list of steps that are required to obtain a recognized professional qualification. The necessary conditions, you become a professional in the relevant field. I did not mean that it is easy to become a doctor, but everyone knows how to do it!

Now ask Google «how to become a software developer." Answers like that I used, are reduced to vague platitudes: "learn a programming language," "Participate in the creation of open source," "join the local community of programmers."

There is no clear plan for building a career or any guarantees about when or whether you will become a professional at all, not a word about how you will manage to get a paying job in the future!

Check out the results of a survey of software developers conducted by Stack Overflow in 2015:

As you can see here guys, most of software developers are self-taught. Only a third of respondents received a diploma in computer science or in some related field.

What I'm trying to say here is that a software developer is such a beast that can learn anything and able to adapt to anything!

And what do you think about all this? Guys share your thoughts in comments!

Windows Batch Script for Incremental Backup

2015-10-26T15:47:00.001+02:00

Recently I faced with the problem when I wanted to do a backup of some directory just by one click. I didn't want to rename or change something manually each time I need to do a backup. All I want is just copy the content of one folder to some destination many times by one single mouse click.

This means that each time I run a backup I want my script to check if there already exist some previous backup folder and if yes then to increment it's name and actually perform the backup. Thus if I will run the script 5 times then the destination directory will look like this:

Well I must confess it was not easy to find a solution for this relatively simple task. Therefore I decided to put a result of my research here in order to do someone's life easier.
Here is a Windows batch script that will do incremental backup for you:

@echo off

:: copy content of this folder
set source=c:\temp\test\1
:: to this destination
set dest_path=c:\temp\test\2\

:: this command works for UNC paths. 'cd' works only on local paths.
pushd %dest_path%

:: this block handles incrimentation.
:: for the first time this block will create "Backup1" directory
:: otherwise it will create directory with incremented name
setlocal enableDelayedExpansion
set "dest_folder_name=Backup"
set "n=0"
for /f "delims=" %%F in (
'2^>nul dir /b /ad "%dest_folder_name%*."^|findstr /xri "%dest_folder_name%[0-9]*"'
) do (
set "name=%%F"
set "name=!name:*%dest_folder_name%=!"
if !name! gtr !n! set "n=!name!"
)
set /a n+=1
set final_destination="%dest_path%%dest_folder_name%%n%"
md %final_destination%
:: ---
robocopy %source% %final_destination% /E /R:3 /W:10 /FFT /NP /NDL
popd

@echo on

Create a batch execution file with bat extension, copy paste this script into it, change source and destination paths and run it.
Enjoy!

Java Spring Restful Web Service with Configurable MongoDB

2015-10-15T11:48:00.000+03:00

Today I'll show you how to build Java RESTful Web Service that uses MongoDB database for storing and retrieving data. The service is written using Java Spring 4 technology.

In this project I will show how to configure MongoDB database parameters in java properties file and how to consume it using Java Spring. If you're new to MongoDB then I suggest to read my previous article on How to Install and Run MongoDB on Windows.

I will not describe in details maven pom.xml structure just because you can simply download the whole project by clicking on the download link at the bottom of this page.

Here is the structure of the project:

Ok, so what this all about?

We got here Person class which represent our domain object to be persistent to MongoDB.

@Document

public class Person {

    @Id

    private String id;

    private String name;

    private Integer age;

    private String country;

    public Person(){}

    public Person(String name, Integer age, String country){

        setName(name);

        setAge(age);

        setCountry(country);

    }

    public String getId() {

        return id;

    }

    public void setId(String id) {

        this.id = id;

    }

    public String getName() {

        return name;

    }

    public void setName(String name) {

        this.name = name;

    }

    public Integer getAge() {

        return age;

    }

    public void setAge(Integer age) {

        this.age = age;

    }

    public String getCountry() {

        return country;

    }

    public void setCountry(String country) {

        this.country = country;

    }

    public String toString() {

        return "[" + getName() 

                + ", " + getAge()

                + ", " + getCountry()

                + "]";

    }

}

PersonRepository class is responsible for actual operations via MongoDB for the Person. In our case these are: addPerson, getPersonByName, getAllPersons, deletePerson and updatePerson.

@Repository

public class PersonRepository {

    public static final String PERSON_COLLECTION_NAME = "person";

    @Autowired

    private MongoTemplate mongo;

    public void addPerson(Person person){

        if(!mongo.collectionExists(Person.class)){

            mongo.createCollection(Person.class);

        }

        mongo.insert(person, PERSON_COLLECTION_NAME);

    }

    public Person getPersonByName(String name) {

        return mongo.findOne(Query.query(Criteria.where("name").is(name)),

                        Person.class, PERSON_COLLECTION_NAME);

    }

    public List getAllPersons() {

        return mongo.findAll(Person.class, PERSON_COLLECTION_NAME);

    }

    public Person deletePerson(String name) {

        Person person = mongo.findOne(Query.query(Criteria.where("name").is(name)), 

                         Person.class, PERSON_COLLECTION_NAME);

        mongo.remove(person, PERSON_COLLECTION_NAME);

        return person;

    }

    public Person updatePerson(String name, Person person) {

        Query query = new Query();

        query.addCriteria(Criteria.where("name").is(name));

        Update update = new Update();

        update.set("name", person.getName());

        update.set("age", person.getAge());

        update.set("country", person.getCountry());

        mongo.updateFirst(query, update, Person.class);

        return person;

    }

}

As you can see from the code above I use here MongoTemplate and notice the @Autowired Spring annotation. Now let's see our AppConfig class:

@Configuration

@ComponentScan(basePackages = { "com.codearsenal.*" })

@PropertySource(value="classpath:config.properties", ignoreResourceNotFound=true)

public class AppConfig {

    @Autowired

    private Environment env;

    @Bean

    public MongoTemplate mongoTemplate() throws Exception {

        String db_host = env.getProperty("mongodb.host");        

        String db_port = env.getProperty("mongodb.port");

        String db_name = env.getProperty("mongodb.db");

        MongoClient mongo = new MongoClient(db_host, Integer.valueOf(db_port));

        MongoDbFactory mongoDbFactory = new SimpleMongoDbFactory(mongo, db_name);

        return new MongoTemplate(mongoDbFactory);

    }

}

Here in AppConfig I set up MongoTemplate configurations. @PropertySource annotation tells to Spring to load properties file config.properties. Inside config.properties I specified my MongoDB connection settings:

   mongodb.host=127.0.0.1
   mongodb.port=27017
   mongodb.db=codearsenal_db

Now let's see PersonController class. This is the entry point of all requests from client to our server.

@Controller

public class PersonController {

    private static final Logger logger = LoggerFactory

            .getLogger(PersonController.class);

    @Autowired

    private PersonRepository personRepository;

    @RequestMapping(value = "/", method = RequestMethod.GET)

    public String home(Locale locale, Model model) {

        logger.info(

                "This is Default Home REST page.\n\n The client locale is {}.",

                locale);

        Date date = new Date();

        DateFormat dateFormat = DateFormat.getDateTimeInstance(DateFormat.LONG,

                DateFormat.LONG, locale);

        String formattedDate = dateFormat.format(date);

        model.addAttribute("serverTime", formattedDate);

        return "status";

    }

    @RequestMapping(value = "/persons", method = RequestMethod.GET)

    @ResponseBody

    public MultiplePersonResponse getAllPersons() {

        logger.info("getAllPersons()...");

        List persons = personRepository.getAllPersons();

        MultiplePersonResponse resp = new MultiplePersonResponse(true, persons);

        logger.info("...getAllPersons()");

        return resp;

    }

    @RequestMapping(value = "/person/{name}", method = RequestMethod.GET)

    @ResponseBody

    public SinglePersonResponse getPersonByName(

            @PathVariable("name") String name) {

        logger.info("getPersonByName()...");

        Person myPerson = personRepository.getPersonByName(name);

        if (myPerson != null) {

            logger.info("returned: " + myPerson.toString());

        } else {

            logger.info("name: " + name + ", NOT FOUND!");

        }

        SinglePersonResponse resp = new SinglePersonResponse(true, myPerson);

        logger.info("...getPersonByName()");

        return resp;

    }

    @RequestMapping(value = "/person/delete/{name}", method = RequestMethod.DELETE)

    @ResponseBody

    public BasicResponse deletePersonByName(@PathVariable("name") String name) {

        logger.info("deletePersonByName()...");

        BasicResponse resp;

        Person person = personRepository.deletePerson(name);

        if (person != null) {

            logger.info("deleted: " + person.toString());

            resp = new BasicResponse(true,

                    "Successfully deleted Person: " + person.toString());

        } else {

            logger.info("name: " + name + ", NOT FOUND!");

            resp = new BasicResponse(false, "Failed to delete name: " + name);

        }

        logger.info("...deletePersonByName()");

        return resp;

    }

    @RequestMapping(value = "/person/update/{name}", method = RequestMethod.PUT)

    @ResponseBody

    public BasicResponse updatePersonByName(@PathVariable("name") String name,

            @ModelAttribute("person") Person person) {

        logger.info("updatePersonByName()...");

        BasicResponse resp;

        Person myPerson = personRepository.updatePerson(name, person);

        if (myPerson != null) {

            logger.info("updated: " + myPerson.toString());

            resp = new BasicResponse(true,

                    "Successfully updated Person: " + myPerson.toString());

        } else {

            logger.info("name: " + name + ", NOT FOUND!");

            resp = new BasicResponse(false, "Failed to update Person: " + name);

        }

        logger.info("...updatePersonByName()");

        return resp;

    }

    @RequestMapping(value = "/person/addPerson", method = RequestMethod.POST)

    @ResponseBody

    public BasicResponse addPerson(@RequestBody Person person) {

        logger.info("addPerson()...");

        BasicResponse resp;

        if (person.getName() != null && person.getName().length() > 0) {

            logger.info("adding: " + person.toString());

            personRepository.addPerson(person);

            resp = new BasicResponse(true,

                    "Successfully added Person: " + person.getName());

        } else {

            logger.info("Failed to insert...");

            resp = new BasicResponse(false, "Failed to insert...");

        }

        logger.info("..addPerson()");

        return resp;

    }

}

Ok, now we're ready to launch our Tomcat server to see how all this thing works, but first we should start MongoDB database server. Read my previous post if you do not know how to do this - How to Install and Run MongoDB on Windows.

Let's test our application, First we'll add a new person to our system, I will use Postman to simulate JSON requests. Just make sure that Tomcat and MongoDB servers are running.

addPerson:

Request method: POST
URL: http://localhost:8080/CodeArsenalApplication/person/addPerson
JSON:

               {

                  "name":"Yuri",

                  "age":35,

                  "country":"USA"

               }

JSON response from the server:

                 {

                    "success": true,

                    "message": "Successfully added Person: Yuri"

                 }

Now we can run Robomongo to ensure that the person was really added to the database:

getPersonByName:

In the same manner you can test other APIs.

If you have any questions you're welcome to ask in the comments section.

Download spring-rest-webservice-mongodb project (Eclipse/Maven).

How to Install and Run MongoDB on Windows

2015-10-12T18:36:00.001+03:00

This particular article is for those who are going to install and run MongoDB database for the first time.

First, download MongoDB from official MongoDB website

Once you done Unzip and put its content to your location, for instance C:\MongoDB\

This is how MongoDB server directory tree looks like after extraction

Then go to C:\MongoDB\Server\config\ and create MongoDB configuration file - mongo.config

Fill in corresponding data, for example this is content of my mongo.config:

## database directory

dbpath = c:\MongoDB\Server\data

## database port number

port = 27017

## logs will be created here

logpath = c:\MongoDB\Server\logs\mongo.log

Now you're ready to run your MongoDB server! Last thing to do is to create a shortcut to mongod.exe (C:\MongoDB\Server\3.0\bin\mongod.exe), with following parameter:

--config C:\mongodb\server\config\mongodb.conf

This is it! Now, by clicking on this shortcut you're starting MongoDB server with parameters you set previously in mongo.config file.

I also suggest to use Robomongo management tool for observing data in your MongoDB. It is free.

Enjoy codding and if you got any questions please do not hesitate to ask in the comments!

Good Luck!

C# Dependency Injection Simple Example

2015-03-23T13:10:00.000+02:00

People are often confused about what Dependency Injection is and when they might need or want to use it. Some time ago I wrote an article Managing Dependency Injection in C# with Autofac which explains how to manage DI in C#, but today I want to show by simple code sample what actually Dependency Injection is.

Imaging situation where you have a class, let's say Employee, and two or more different loggers for that class. Each logger prints messages in his own particular way and you want to have control of which logger to use for Employee during its instantiation.

Just take a look at this code and I'm sure you will get the idea of Dependency Injection:

public class Employee
{
    public Employee(ILogger logger)
    {
        logger.WriteToLog("New employee created");
    }
}

public interface ILogger
{
    void WriteToLog(string text);
}

public class LoggerOne : ILogger
{
    public void WriteToLog(string text)
    {
        Console.WriteLine(text);
    }
}

public class LoggerTwo : ILogger
{
    public void WriteToLog(string text)
    {
        Console.WriteLine("***********\n {0}\n***********", text);
    }
}

Now let's instantiate an Employee with two different loggers:

Employee employee1 = new Employee(new LoggerOne());
Employee employee2 = new Employee(new LoggerTwo());

And the output:

New employee created

*******************
New employee created
*******************

How to Avoid Any Global Variables in JavaScript

2014-11-27T11:43:00.000+02:00

In JavaScript, objects and functions, are also variables. And any variable declared outside a function has global scope. You probably heard that because JavaScript is a dynamically type language global variables are bad and that they easily become source of confusion and bugs. So how can we avoid of creating any global variable, but still have our code executed?

There is such technic, it's called Immediately Invoked Function Expression (IIFE).

Suppose we have this JavaScript code:

var func = function(){
  
  console.log("entering func");
  
  var counter = 0;
  
  var innerFunc1 = function(){
    counter += 1;
    console.log("innerFunc1: " + counter);
  }
  
  var innerFunc2 = function(){
    counter += 1;
    console.log("innerFunc2: " + counter);
  }
  
  return {
    inner1: innerFunc1,
    inner2: innerFunc2,
  };
}

var f = func();
f.inner1();
f.inner2();

So here we have two global variables - func and f.

Now I'll show how to run this code without creating ANY global variable by applying IIFE.

(function() {
  var func = function() {

    console.log("entering func");

    var counter = 0;

    var innerFunc1 = function() {
      counter += 1;
      console.log("innerFunc1: " + counter);
    }

    var innerFunc2 = function() {
      counter += 1;
      console.log("innerFunc2: " + counter);
    }

    return {
      inner1: innerFunc1,
      inner2: innerFunc2,
    };
  }

  var f = func();
  f.inner1();
  f.inner2();

}());

So actually we wrapped our code inside anonymous function that immediately invokes itself or in other words Immediately Invoked Function Expression (IIFE).

The code still executes exactly as it did before but with creating no global variables at all.
Happy codding! :)

WPF Customized ComboBox with Non Existing Value

2014-11-03T15:20:00.001+02:00

How do you show a non existing value inside a WPF ComboBox? Good question! And the sad truth is that you just cannot do it! WPF ComboBox shows only the values that are in its ItemSource.

But don't worry, today I'll show you a workaround to this problem. In this particular example inside a ComboBox will be shown a phantom value and when user clicks on it and choose another value from the list he will be prompted to confirm in order to give him a chance not to loose this phantom value in case if he clicked on the ComboBox by mistake.

The trick is that I create additional custom ComboBox with TextBlock, binding it to the original one and overlaying it.

Check out the main window XAML:

<ComboBox x:Name="OriginalComboBox"
          Grid.Column="1" Grid.Row="3"
          VerticalAlignment="Top"
          ItemsSource="{Binding Employees}"
          SelectedValue="{Binding Selected, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>

<phantom:PhantomValueComboBoxUserControl Grid.Column="1" Grid.Row="3"
                                      SelectedOptionText="{Binding Selected, Mode=OneWay}"
                                      RelatedComboBox="{Binding ElementName=OriginalComboBox}"/>

Now let's see the XAML of the Phantom ComboBox:

<UserControl x:Class="PhantomComboBoxExample.PhantomValueComboBoxUserControl.PhantomValueComboBoxUserControl"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
      xmlns:phantom="clr-namespace:PhantomComboBoxExample.PhantomValueComboBoxUserControl"
      mc:Ignorable="d" >
    
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition  Height="Auto" />
            <RowDefinition  Height="Auto" />
        </Grid.RowDefinitions>
        <ComboBox x:Name="MainComboBox"
               HorizontalAlignment="Stretch"
               ToolTipService.ToolTip="{Binding RelativeSource={RelativeSource Mode=FindAncestor, 
                     AncestorType=phantom:PhantomValueComboBoxUserControl}, 
                     Path=PhantomToolTip}" />

        <TextBlock IsHitTestVisible="False"
               x:Name="Watermark"
               Visibility="{Binding ElementName=MainCombo, Path=SelectedIndex}"
               Text="{Binding RelativeSource={RelativeSource Mode=FindAncestor,
                    AncestorType=phantom:PhantomValueComboBoxUserControl}, 
                    Path=SelectedOptionText}"
               FontStyle="Italic"
               Foreground="DarkSlateGray"
               Padding="5,4,25,2"
               HorizontalAlignment="Left"
               Margin="{Binding ElementName=MainCombo, Path=Margin}" />
    </Grid>
</UserControl>

And all the magic is in code behind:

public static readonly DependencyProperty RelatedComboBoxProperty =
            DependencyProperty.Register("RelatedComboBox", typeof(ComboBox), 
            typeof(PhantomValueComboBoxUserControl), new PropertyMetadata(null, RelatedComboBoxChangedCallback));

private static void RelatedComboBoxChangedCallback(DependencyObject dependencyObject, 
                    DependencyPropertyChangedEventArgs dependencyPropertyChangedEventArgs)
{
    var thisControl = dependencyObject as PhantomValueComboBoxUserControl;
    var relatedCombo = dependencyPropertyChangedEventArgs.NewValue as ComboBox;
    if (relatedCombo != null && thisControl != null)
    {
        thisControl.FontFamily = relatedCombo.FontFamily;
        thisControl.FontSize = relatedCombo.FontSize;
        thisControl.MainComboBox.Style = relatedCombo.Style;
        thisControl.Margin = new Thickness(0);
        thisControl.HorizontalAlignment = relatedCombo.HorizontalAlignment;
        thisControl.MainComboBox.HorizontalAlignment = relatedCombo.HorizontalAlignment;
        thisControl.MainComboBox.VerticalAlignment = relatedCombo.VerticalAlignment;
        thisControl.VerticalAlignment = relatedCombo.VerticalAlignment;
        relatedCombo.SelectionChanged += (sender, args) =>
        {
            thisControl._internalIndexChange = true;
            if (relatedCombo.SelectedIndex < thisControl.MainComboBox.Items.Count)
            {
                thisControl.MainComboBox.SelectedIndex = relatedCombo.SelectedIndex;
            }
            thisControl._internalIndexChange = false;
        };

        var mb = new MultiBinding
        {
            Converter = new MathToVisibilityMultiConverter(),                 
        };

        var mb1 = new Binding
        {
            Source = relatedCombo,
            Path = new PropertyPath("SelectedIndex")
        };

        var mb2 = new Binding
        {
            Source = thisControl,
            Path = new PropertyPath("SelectedOptionText")
        };

        mb.Bindings.Add(mb1);
        mb.Bindings.Add(mb2);
        thisControl.SetBinding(VisibilityProperty, mb);
        
        var b1 = new Binding
        {
            Source = relatedCombo,
            Path = new PropertyPath("IsEnabled")
        };
        thisControl.SetBinding(IsEnabledProperty, b1);

        var b2 = new Binding
        {
            Source = relatedCombo,
            Path = new PropertyPath("ItemsSource")
        };
        thisControl.SetBinding(ItemsSourceProperty, b2);
    }
}

Enjoy!
Download the source code (Visual Studio 2013 project).

Java 8 Lambda Expressions Example

2014-03-20T16:20:00.000+02:00

Finally Oracle officially presented Java 8. A key feature of Java 8 - Support for lambda expressions that allow developers to effectively apply the simultaneous computation and callback functions in programming, especially in the popular cloud applications. Function can be treated as method arguments, and code as data, which makes the final results more compact.

Java Lambda syntax is pretty similar to what we know in C#.

Let's see.

Starting a new Thread:

new Thread(() -> { System.out.println("Java 8 Lambda"); }).start();

Working with Collections:

List<Employee> employees = new ArrayList<>();

     employees.add(new Employee("John"));

     employees.add(new Employee("Robert"));

     employees.add(new Employee("Anna"));

Iteration:

employees.forEach(e -> e.setAge(35));

Filter:

List<Employee> filtered = employees.stream().parallel().filter

                (f -> f.getName().equals("Anna") || f.getName().equals("John"))

                .collect(Collectors.toList());

Optional<Employee> findFirst = employees.stream()

                .filter(e -> e.getName().equals("Anna")).findFirst();

Parallelism:

List<Employee> list = employees.stream()

                .parallel()

                .filter(p -> p.getAge() > 18).collect(Collectors.toList());

Happy codding friends! :)

Best Logging for Java - Logback with SLF4J

2014-03-16T15:49:00.000+02:00

If you're looking for good logging mechanism for Java I suggest you to use SLF4J Facade with Logback. For my opinion it's the best combination for logging in Java. Today I will show how to configure your Java project in order to use SLF4J Logback logging framework to print logs to console and file output.

First, download SLF4J files and Logback files.

Add these files to your CLASSPATH:

slf4j-api-X.X.X.jar
logback-core-X.X.X.jar
logback-classic-X.X.X.jar

Now look how simple is to use Logback! In the code all you need is this:

public class LogbackFun {

  final static Logger logback = LoggerFactory.getLogger(LogbackFun.class);

  public static void main(String[] args) {

    logback.info("Test info");

    logback.debug("Test debug");

    logback.trace("Test trace");

    logback.warn("Test warn");

    logback.error("Test error");

  }

}

Ok, how to configure the log settings? Very simple! When the program starts this what's going on:

Logback tries to find a file called logback.groovy in the CLASSPATH
If no such file is found, Logback looking for file called logback-test.xml in the CLASSPATH
If no such file is found, it checks for the file logback.xml in the CLASSPATH
If neither file is found, Logback configures itself automatically using the BasicConfigurator which will print logs to the console

So, we create logback.xml file and put it in CLASSPATH directory (src folder). Now let's take a look on the file content:

<configuration>
  <appender name="FILE" class="ch.qos.logback.core.rolling.RollingFileAppender">
    <file>logs/mylog.log</file>
      <rollingPolicy class="ch.qos.logback.core.rolling.FixedWindowRollingPolicy">
          <fileNamePattern>logs/mylog.%i.log.zip</fileNamePattern>
          <minIndex>1</minIndex>
          <maxIndex>3</maxIndex>
      </rollingPolicy>

      <triggeringPolicy class="ch.qos.logback.core.rolling.SizeBasedTriggeringPolicy">
          <maxFileSize>5MB</maxFileSize>
      </triggeringPolicy>
    <encoder>
      <pattern>%date{YYYY-MM-dd HH:mm:ss} %level [%thread] %logger{10} [%file:%line] %msg%n</pattern>
    </encoder>
  </appender>

  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%msg%n</pattern>
    </encoder>
  </appender>

  <root level="trace">
    <appender-ref ref="FILE" />
    <appender-ref ref="STDOUT" />
  </root>
</configuration>

Simply brilliant stuff! Appender tells where to print the log and contains its configurations. Here we have two appenders. One is for printing logs to a file and the second one is for printing into console. We can also set the pattern of the message, the log level set to trace here. RollingPolicy allows to control the file behavior - maximum file size, naming convention etc.

This is it! As simple as that!

If you have any questions please don't hesitate to ask in the comments section. Good luck!

SQLite4Java Select, Update Example

2014-03-16T11:20:00.000+02:00

Recently I started to work on some small Java project where I need to read and update data on SQLite data base. I searched for some good SQLite wrapper for Java and finally decided to use SQLiteJava wrapper. I found it more intuitive and faster than JDBC driver. Today I'll show how to use it in your Java application.

First of all you'll need to download SQLite4Java wrapper files from here.

Once you're done do this:

sqlite4java.jar put it on your CLASSPATH

sqlite4java-win32-x64.dll put in the same directory with the jar file

sqlite4java-win32-x86.dll put in the same directory with the jar file

I created SQLite4Java.java class which handles all my activities via SQLite file:

import java.io.File;

import java.util.ArrayList;

import com.almworks.sqlite4java.SQLiteConnection;

import com.almworks.sqlite4java.SQLiteConstants;

import com.almworks.sqlite4java.SQLiteException;

import com.almworks.sqlite4java.SQLiteStatement;

public class SQLite4Java {

    private SQLiteConnection _db;

    private String _delimiter = "[,]";

    public SQLite4Java(String inputFile) throws SQLiteException {

        _db = new SQLiteConnection(new File(inputFile));

        _db.open(true);

    }

    public String selectSingle(String query, String args) throws SQLiteException {

        SQLiteStatement st = _db.prepare(query);

        String[] parameters = args.split(_delimiter);

        String value = null;

        try {

            for (int i = 0; i < parameters.length; i++) {

                st.bind(i + 1, parameters[i]);

            }

            int columnsCount = st.columnCount();

            if (columnsCount > 1) {

                System.err.println("Error: More than a single column!");

                return null;

            }

            if (st.step()) {

                int type = st.columnType(0);

                switch (type) {

                    case SQLiteConstants.SQLITE_INTEGER:

                        value = String.valueOf(st.columnInt(0));

                    case SQLiteConstants.SQLITE_TEXT:

                        value = st.columnString(0);

                    default:

                        break;

                }

            }

        }

        catch (Exception e) {

            System.err.println(e.getMessage());

        }

        finally {

            st.dispose();

        }

        return value;

    }

    public ArrayList selectMany(String query, String args) throws SQLiteException {

        SQLiteStatement st = _db.prepare(query);

        String[] parameters;

        if (!args.isEmpty() && args != null) {

            parameters = args.split(_delimiter);

            for (int i = 0; i < parameters.length; i++) {

                st.bind(i + 1, parameters[i]);

            }

        }

        ArrayList list = new ArrayList();

        try {

            int columnsCount = st.columnCount();

            if (columnsCount > 1) {

                System.err.println("Error: More than a single column!");

                return null;

            }

            while (st.step()) {

                int type = st.columnType(0);

                switch (type) {

                    case SQLiteConstants.SQLITE_INTEGER:

                        list.add(String.valueOf(st.columnInt(0)));

                        break;

                    case SQLiteConstants.SQLITE_TEXT:

                        list.add(st.columnString(0));

                        break;

                    default:

                        break;

                }

            }

        }

        catch (Exception e) {

            System.err.println(e.getMessage());

        }

        finally {

            st.dispose();

        }

        return list;

    }

    public boolean updateSingleField(String query, String args) 

            throws SQLiteException {

        _db.exec("BEGIN TRANSACTION; ");

        SQLiteStatement st = _db.prepare(query);

        String[] parameters = args.split(_delimiter);

        try {

            for (int i = 0; i < parameters.length; i++) {

                st.bind(i + 1, parameters[i]);

            }

            st.step();

            _db.exec("COMMIT;");

        }

        catch (SQLiteException e) {

            System.err.println(e.getMessage());

        }

        finally {

            st.dispose();

        }

        return success;

    }

}

So what we got here?

Constructor receives a path to the SQLite data base file and opens connection.
selectSingle method receives query and arguments separated by comma delimiter and returns single value (In my project I need to work only with single fields, but you can edit the method in order to be able to handle data with multiple columns)
selectMany method is similar to previous with only difference that it returns a list of strings. I also check the value type and always cast it to String
updateSingleField updates single field in data base.

Here how you use it:

SQLite4Java sqlite = new SQLite4Java("c:\\db\\db");

String value = sqlite.selectSingle("SELECT _title FROM _issue WHERE id = ?", 4);

sqlite.updateSingleField("update _issue " +

                          "set _indicator = strftime('%Y%m%d%H%M%f', 'now') " +

                          "where _issue.id = ?", 3);

You can customize this class for your purposes and use it in your application. Good luck!

C# Multithreading Loop with Parallel.For or Parallel.ForEach

2013-12-18T11:01:00.000+02:00

Loop is such a trivial and so frequently used thing. And sometimes loops just killing performance of the whole application. I want the loop perform faster, but what can I do?

Starting from .NET Framework 4 we can run loops in parallel mode using Parallel.For method instead of regular for or Parallel.ForEach instead of regular foreach. And the most beautiful thing is that .NET Framework automatically manages the threads that service the loop!

But don't expect to receive performance boost each time you replace your for loop with Parallel.For, especially if your loop is relatively simple and short. And keep in mind that results will not come in proper order because of parallelism.

Now let's see the code. Here is our regular loop:

for (int i = 1; i < 1000; i++)
{
    var result = HeavyFoo(i);
    Console.WriteLine("Line {0}, Result : {1} ", i, result);
}

And here is how we write it with Parallel.For:

Parallel.For(1, 1000, (i) =>
{
    var result = HeavyFoo(i);
    Console.WriteLine("Line {0}, Result : {1} ", i, result);
});

Same story with Parallel.ForEach:

foreach(string s in stringList)
{
    var result = HeavyFoo(s);
    Console.WriteLine("Initial string: {0}, Result : {1} ", s, result);
}

Parallel.ForEach(stringList, s =>
{
    var result = HeavyFoo(s);
    Console.WriteLine("Initial string {0}, Result : {1} ", s, result);
});

Use with caution! :)

WPF MultiBinding Example (MultiValueConverter)

2013-12-01T09:40:00.001+02:00

As you probably know the main key of WPF is binding. We bind WPF control to a property and data flows from business logic to UI and vice versa. But what if we need to bind several properties to a single control and depending on the state of these properties we will decide what to display? This technique called MultiBinding.
MultiBinding in WPF is always accompanied with MultiValueConverter. Inside it we define the logic that will decide what value to pass to our WPF control.

Today I'll show a simple example of MultiBinding in WPF. In my sample application I have three TextBox controls and a single TextBlock. I want to achieve this scenario: I want to show the text in the TextBlock, but only if each of three TextBoxes contain some text.

First, take a look at our XAML:

<Window x:Class="WPF_MultiBinding.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="CodeArsenal.net"
        Height="158" Width="245"
        xmlns:local="clr-namespace:WPF_MultiBinding">
    <Window.Resources>
        <local:MultiValueConverter x:Key="MultiValueConverter" />
    </Window.Resources>
    
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="10" />
            <ColumnDefinition Width="65" />
            <ColumnDefinition Width="Auto" />
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="10" />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>

        <TextBox Name="textBoxOne" Grid.Row="0" Height="20" MaxLength="8" Grid.Column="1"
                 Text="{Binding TextOne, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
        
        <TextBox Name="textBoxTwo" Grid.Row="1" Height="20" MaxLength="8" Grid.Column="1" 
                 Text="{Binding TextTwo, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
        
        <TextBox Name="textBoxThree" Grid.Row="2" Height="20" MaxLength="8" Grid.Column="1"
                 Text="{Binding TextThree, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>

        <TextBlock Name="textBlockOutput" Grid.Row="4" Grid.Column="2">
            <TextBlock.Text>
                <MultiBinding Converter="{StaticResource MultiValueConverter}">
                    <Binding Path="TextOne" />
                    <Binding Path="TextTwo" />
                    <Binding Path="TextThree" />
                </MultiBinding>
            </TextBlock.Text>
        </TextBlock>
    </Grid>
</Window>

Did you notice that we bind three properties and define Converter for the TextBlock? By the way here is our ViewModel:

public class ViewModel : INotifyPropertyChanged
{
    public ViewModel() { }

    public event PropertyChangedEventHandler PropertyChanged;
    private string _textOne;
    public string TextOne 
    {
        get { return _textOne; } 
        set
        {
            _textOne = value;
            NotifyPropertyChanged("TextOne");
        } 
    }

    private string _textTwo;
    public string TextTwo
    {
        get { return _textTwo; }
        set
        {
            _textTwo = value;
            NotifyPropertyChanged("TextTwo");
        } 
    }

    private string _textThree;
    public string TextThree
    {
        get { return _textThree; }
        set
        {
            _textThree = value;
            NotifyPropertyChanged("TextThree");
        } 
    }

    public void NotifyPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

As you can see nothing special here. Just three properties for our TextBoxes. The real key feature of MultiBinding is Converter. We must implement IMultiValueConverter interface that allows us to apply our custom logic. The logic is pretty simple - I combine all the text that comes from TextBoxes, but only when all three of them contain text.

public class MultiValueConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        string one = values[0] as string;
        string two = values[1] as string;
        string three = values[2] as string;
        if(!string.IsNullOrEmpty(one) && !string.IsNullOrEmpty(two) && !string.IsNullOrEmpty(three))
        {
            return one + two + three;
        }
        return null;
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

Enjoy! Download the source code (Visual Studio 2013 project).

MEF Simple Application in C#

2013-11-27T15:05:00.000+02:00

I believe the best way to learn something new is to learn by simple and easy to understand example. Once you got the principles you can always proceed to more complex stuff. So today I'll show a simple example of MEF that will give you the basics.

Managed Extensibility Framework (MEF) is a component of .NET 4 and it allows to developer to build extensible applications. Imagine situation when you have some application and you know that in the future you will want to add more features to it, but you don't know exactly what and when they will be added, or may be you want another developer to develop new feature and integrate it seamlessly into the main application. So it looks like we need some module structured application here, right? And this is exactly what MEF provides.
Now to our sample application. When user starts it he asked to enter a string and a command (name of operation) he want to apply on that string. Once it's done the program will print the result string. So every operation we want to be a separate module. Enough bla bla! Let's see the code! :)

First, the Core class which is a part of the main application. Core class concentrates and launches all available extensions:

public interface ICore
{
    String PerformOperations(string data, string command);
}

[Export(typeof(ICore))]
public class Core : ICore
{
    [ImportMany]
    private IEnumerable<Lazy<IOperation, IOperationCommand>> _operations;

    public string PerformOperations(string data, string command)
    {
        foreach (Lazy<IOperation, IOperationCommand> i in _operations)
        {
            if (i.Metadata.Command.Equals(command))
                return i.Value.Operate(data);
        }
        return "Unrecognized operation";
    }
}

IOperation interface describes the operation itself and IOperationCommand describes the command name of operation:

public interface IOperation
{
    string Operate(string data);
}

public interface IOperationCommand
{
    String Command { get; }
}

Now look how simple is to write new operations. First one turn the input string into upper case, second one turns all the characters to lower case and the third one reverses the string:

[Export(typeof(MEF_Example.IOperation))]
[ExportMetadata("Command", "upper")]
public class UpperCase : MEF_Example.IOperation
{
    public string Operate(string data)
    {
        return data.ToUpper();
    }
}

[Export(typeof(MEF_Example.IOperation))]
[ExportMetadata("Command", "lower")]
public class LowerCase : MEF_Example.IOperation
{
    public string Operate(string data)
    {
        return data.ToLower();
    }
}

[Export(typeof(MEF_Example.IOperation))]
[ExportMetadata("Command", "reverse")]
public class Reverse : MEF_Example.IOperation
{
    public string Operate(string data)
    {
        return new string(data.ToCharArray().Reverse().ToArray());
    }
}

Now all we need is to initialize our application, here how we do it:

internal class Program
{
    private CompositionContainer _compositionContainer;

    [Import(typeof(ICore))]
    public ICore core;

    private Program()
    {
        var agregateCatalog = new AggregateCatalog();
        agregateCatalog.Catalogs.Add(new AssemblyCatalog(typeof(Program).Assembly));
        agregateCatalog.Catalogs.Add(new DirectoryCatalog("Extensions"));

        _compositionContainer = new CompositionContainer(agregateCatalog);

        try
        {
            this._compositionContainer.ComposeParts(this);
        }
        catch (CompositionException compositionException)
        {
            Console.WriteLine(compositionException.ToString());
        }
    }

    private static void Main(string[] args)
    {
        Program program = new Program();
        string data, command;
        while (true)
        {
            Console.Clear();
            Console.Write("Please enter data:    ");
            data = Console.ReadLine();
            Console.Write("Please enter command: ");
            command = Console.ReadLine();
            Console.Write("Result: " + program.core.PerformOperations(data, command));
            Console.ReadLine();
        }
    }
}

This is it! As you see the code is pretty simple. This is how looks the application output:

Download the source code (Visual Studio 2013 project).