What are extension methods in C#

What are extension methods?

Extension methods are a new feature in C# 3.0. An extension method enables us to add methods to existing types without creating a new derived type, recompiling, or modify the original types. We can say that it extends the functionality of an existing type in .NET. An extension method is a static method to the existing static class. We call an extension method in the same general way; there is no difference in calling.

Extension methods enable you to add methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type.

An extension method is a special kind of static method, but they are called as if they were instance methods on the extended type.

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.

Extension methods are only in scope when you explicitly import the namespace into your source code with a using directive.


The following list contains basic features and properties of extension methods:

1. It is a static method.
2. It must be located in a static class.
3. It uses the "this" keyword as the first parameter with a type in .NET and this method will be called by a given type instance on the client side.
4. It also shown by VS intellisense. When we press the dot (.) after a type instance, then it comes in VS intellisense.
5. An extension method should be in the same namespace as it is used or you need to import the namespace of the class by a using statement.
6. You can give any name for the class that has an extension method but the class should be static.
7. If you want to add new methods to a type and you don't have the source code for it, then the solution is to use and implement extension methods of that type.
8. If you create extension methods that have the same signature methods as the type you are extending, then the extension methods will never be called.

Benefits of extension methods

• Extension methods allow existing classes to be extended without relying on inheritance or having to change the class's source code.
• If the class is sealed than there in no concept of extending its functionality. For this a new concept is introduced, in other words extension methods.
• This feature is important for all developers, especially if you would like to use the dynamism of the C# enhancements in your class's design.

More code snippets of extension expansion methods
using System;
using System.Text;
namespace ExtensionMethod2
{
    public static class ExtMetClass
    {
        public static int IntegerExtension(this string str)
        {
            return Int32.Parse(str);
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            string str = "123456";
            int num = str.IntegerExtension();
            Console.WriteLine("The output using extension method: {0}", num);
            Console.ReadLine();
        }
    }
}

In the preceding program we have used an extension method IntegerExtension() to convert a string to a numeric type.

Important points for the use of extension methods:

• An extension method must be defined in a top-level static class.
• An extension method with the same name and signature as an instance method will not be called.
• Extension methods cannot be used to override existing methods.
• The concept of extension methods cannot be applied to fields, properties or events.
• Overuse of extension methods is not a good style of programming.