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Extension Methods in C#

Posted by Puran Mehra Articles | C# Language August 31, 2009
This article explains the new extension methods feature in C# 3.0 and how to user it.
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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.

Program to show how to use extension methods

Create a project Class Library as:

extMetLib.gif

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...".

extMetProject.gif

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

addRefLib.gif

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:

extensionMethod.gif

Output of the preceding program

extensionMethod1.gif

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.
Conclusion

I hope this article will help you to understand Extension Methods in C# 3.0 and above.

Your feedback and constructive contributions are welcome. Please feel free to contact me for feedback or comments you may have about this article.

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