Extension Methods

An extension method is a new language feature of C# starting with the 3.0 specification, as well as Visual Basic.NET starting with 9.0 and Oxygene with 2.0. 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# and Visual Basic, there is no apparent difference between calling an extension method and the methods that are actually defined in a type.

What are extension methods?

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.

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.

Extension Methods Usage

This section is optional reading section for understanding the core idea of extension methods:

1. This keyword has to be the first parameter in the extension method parameter list.
2. Extension methods are used extensively in C# 3.0 and further version specially for LINQ extensions in C#, for example:
    Collapse | Copy Code

   int[] ints = { 10, 45, 15, 39, 21, 26 };
   
   var result = ints.OrderBy(g => g);

   Here, order by is a sample for extension method from an instance of IEnumerable<T> interface, the expression inside the parenthesis is a lambda expression.
3. It is important to note that extension methods can't access the private methods in the extended type.
4. If you want to add new methods to a type, you don't have the source code for it, the ideal solution is to use and implement extension methods of that type.
5. If you create extension methods that have the same signature methods inside the type you are extending, then the extension methods will never be called.

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.

Program to show how to use extension methods
Create a project Class Library as:

using System;
using System.Text;namespace ClassLibExtMethod
{
    public class Class1    {
        public string Display()
        {
            return ("I m in Display");
        }

        public string Print()
        {
            return ("I m in Print");
        }
    }
}

Now create a new project using "File" -> "New" -> "Project...".



Add the reference of the previously created class library to this project.

Use the following code and use the ClassLibExtMEthod.dll in your namespace:using System;
using System.Text;
using ClassLibExtMethod;
namespace ExtensionMethod1
{
    public static class XX
    {
         public static void NewMethod(this Class1 ob)
        {
            Console.WriteLine("Hello I m extended method");
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Class1 ob = new Class1();
            ob.Display();
            ob.Print();
            ob.NewMethod();
            Console.ReadKey();
        }
    }
}

You can see the extension method with an arrow (different from the normal method sign), since they were instance methods on the extended type. See the figure below:

Output of the preceding program


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();
        }
    }
}