What is an extension method in C# and how to create it?

An extension method is a method that allows you to add new methods to an existing type without creating a new derived type or modifying the original type. This can be especially useful when you want to add functionality to a type that you don't have the source code for, such as a third-party library.

To create an extension method, you first need to create a static class and then define the extension method as a static method within that class.

The first parameter of the extension method should be the type you want to extend, and it should be preceded by the "this" keyword. 

For example, here's how you might create an extension method that adds a "MultiplyByTwo" method to the int type:

public static class IntExtensions

{

    public static int MultiplyByTwo(this int value)

    {

        return value * 2;

    }

}

To use this extension method, you can simply call it like a regular instance method on any int value. For example:

int x = 5;

int y = x.MultiplyByTwo(); // y is now 10

You can also use extension methods to extend interfaces, in which case the extension method will be available to all types that implement the interface. For example:

public interface IEmployee

{

    void DoSomething();

}

public static class ExampleExtensions

{

    public static void DoSomethingElse(this IEmployee example)

    {

        // Implementation goes here

    }

}

Now, any type that implements IEmployee will have the "DoSomethingElse" method available to it.

One important thing to note about extension methods is that they are only available to the code that explicitly references the containing static class.

For example, if you define the IntExtensions class in a separate assembly, you will need to include a "using" statement in your code to access the extension methods. This can make it easier to manage the extension methods you are using, as you can include only the ones you need and avoid cluttering your code with unnecessary methods.

Overall, extension methods are a useful way to add functionality to existing types without modifying the original code. They can be especially useful when working with third-party libraries or when you want to add functionality to a type that you don't have the source code for.