LINQ params public, private return static this

Extension

An extension method has simplified calling syntax. It represents a static method as an instance method. Extension methods can lead to simpler syntax.

This keyword

An extension method uses the this-keyword in its parameter list. It must be located in a static class. We often use special "Extensions" classes.

An example

Here is a custom extension method. Usually you will want to store your extension method class in a separate source file (like ExtensionMethods.cs) in the project.

Note This file should store a static class with public static extension methods.

Then In the rest of your source code, you can invoke these extension methods in the same way as instance methods.

Detail An extension method must be static and can be public so you can use it anywhere in your source code.

Important You must specify the this-keyword before the appropriate parameter you want the method to be called upon.

using System;

public static class ExtensionMethods
{
    public static string UppercaseFirstLetter(this string value)
    {
        // Uppercase the first letter in the string.
        if (value.Length > 0)
        {
            char[] array = value.ToCharArray();
            array[0] = char.ToUpper(array[0]);
            return new string(array);
        }
        return value;
    }
}

class Program
{
    static void Main()
    {
        // Use the string extension method on this value.
        string value = "dot net perls";
        value = value.UppercaseFirstLetter();
        Console.WriteLine(value);
    }
}
Dot net perls

Second parameter

Here is an extension method that acts on ints. It receives a parameter—the multiplier for the operand. It returns another int so can be used in function call chains.

using System;

static class Extensions
{
    public static int MultiplyBy(this int value, int multiplier)
    {
        // Uses a second parameter after the instance parameter.
        return value * multiplier;
    }
}

class Program
{
    static void Main()
    {
        // Ten times 2 is 20.
        // Twenty times 2 is 40.
        int result = 10.MultiplyBy(2).MultiplyBy(2);
        Console.WriteLine(result);
    }
}
40

Notes, keyword

The only difference in the declaration between a regular static method and an extension method is the "this" keyword in the parameter list.

Important If you want the method to receive other parameters, you can include those at the end.

LINQ

There are many extension methods available. These extension methods were written by Microsoft developers and are available in all C# programs targeting recent .NET Framework versions.

Tip On most of the extension methods, you need to add the "using System.Linq" directive to the top of the code.

A discussion

Extension methods can have many arguments. We can use variable "params" arguments with extension methods. Extension methods are static methods—so there is no performance loss.

So Extension methods affect the high-level representation of the code, not the low-level implementation.

A summary

You can add extension methods to any type, even a value type. The original representation of the type does not change. Extension methods affect syntax, not execution.