Extension methods in C#

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Extention methods allow us to add new functionality to existing classes without modifying existing implimentation.
  • An extension method is a static method
  • It must have this keyword associated with classname
  • The classname should be the first parameter in the parameter list

Extension Method Syntax:


Definition of an extension method:
First, let's create a new static class called StringExtensions. The class name is not important, but it's a common convention to use the name of the type being extended followed by "Extensions". Inside this class, we'll define a static method called Reverse:

public static class StringExtensions
{
    public static string Reverse(this string input)
    {
        char[] chars = input.ToCharArray();
        Array.Reverse(chars);
        return new string(chars);
    }
}

usage:

class Program
{
    static void Main(string[] args)
    {
        string example = "Hello, World!";
        string reversed = example.Reverse();
        Console.WriteLine(reversed); // !dlroW ,olleH
    }
}

Extension methods look and behave like instance methods, but there are a few important differences to keep in mind:

  1. Extension methods cannot access private members of the extended type.
  2. They also do not participate in inheritance or polymorphism.
  3. You cannot override an existing method with an extension method.

If the extended type has a method with the same signature as an extension method, the instance method will always take precedence. Extension methods are only called when there is no matching instance method.


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