Extension Methods - C#

Extension methods enable you to "add" methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. Extension methods are a special kind of static method, but they are called as if they were instance methods on the extended type. For client code written in C#, F# and Visual Basic, there is no apparent difference between calling an extension method and the methods that are actually defined in a type.

In other word, Extension methods, as the name suggests, are additional methods. Extension methods allow you to inject additional methods without modifying, deriving or recompiling the original class, struct or interface. Extension methods can be added to your own custom class, .NET framework classes, or third party classes or interfaces.

In the following example, IsGreaterThan() is an extension method for int type, which returns true if the value of the int variable is greater than the supplied integer parameter.

Example: Extension Method
int i = 10;

bool result = i.IsGreaterThan(100); //returns false 

The IsGreaterThan() method is not a method of int data type (Int32 struct). It is an extension method written by the programmer for the int data type. The IsGreaterThan() extension method will be available throughout the application by including the namespace in which it has been defined.

Now let's see how to write an extension method.

Example: Define an Extension Method
namespace ExtensionMethods
{
    public static class IntExtensions
     {
        public static bool IsGreaterThan(this int i, int value)
        {
            return i > value;
        }
    }
}


Now, you can include the ExtensionMethods namespace wherever you want to use this extension method.

Example: Extension method
using ExtensionMethods;

class Program
{
    static void Main(string[] args)
    {
        int i = 10;

        bool result = i.IsGreaterThan(100);

        Console.WriteLine(result);
    }
}

How to use extension methods?

An extension method is a static method of a static class, where the "this" modifier is applied to the first parameter. The type of the first parameter will be the type that is extended.

Note:
The only difference between a regular static method and an extension method is that the first parameter of the extension method specifies the type that it is going to operator on, preceded by the this keyword.

Points to Remember :

• Extension methods are additional custom methods which were originally not included with the class.
• Extension methods can be added to custom, .NET Framework or third party classes, structs or interfaces.
• The first parameter of the extension method must be of the type for which the extension method is applibable, preceded by the this keyword.
• Extension methods can be used anywhere in the application by including the namespace of the extension method.