Monday, 5 September 2011

Extension Methods in C# 3.0


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.
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.
Create a project Class Library as ClassLibExtMethod:
using System;
using
System.Text;
namespace ClassLibExtMethod
{
    public class Class1
    {
        public string Display()
        {
            return ("I am in Display");
        }

        public string Print()
        {
            return ("I am in Print");
        }
    }
}
Now create a new project File -> New -> Project
Add the reference of the previously created class library in this project.
using System;
using
System.Text;
using
ClassLibExtMethod;
namespace
ExtensionMethod1
{
    public static class XX
    {
         public static void NewMethod(this Class1 ob)
        {
            Console.WriteLine("Hello I am 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 down arrow (different from normal method sign), as they were instance methods on the extended type.

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 i.e. extension methods.
  • This feature is important for all developers especially if you would like to use the dynamism of the C# enhancements to be taken place in your classes design.


More Code Snippet to 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 above program we have used extension method IntegerExtension() to convert string to numeric type.

Some important points while using 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 good style of programming.

No comments:

Post a Comment