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. 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.

An Extension Method has simplified calling syntax. It represents static methods as instance methods. An extension method uses the this-keyword in its parameter list. It must be located in a static class.

  public static class ExtensionMethods

    {

        public static string UppercaseFirstLetter(this string value)

        {

 //public static string UppercaseFirstLetter( string value,this string value1)///Invalid -'this' which is not on the first parameter

        //public static string UppercaseFirstLetter(this string value,string value1)///valid

        public static string UppercaseFirstLetter(this string value)

        {

            // Uppercase the first letter in the string this extension is called on.

            if (value.Length > 0)

            {

                char[] array = value.ToCharArray();

                array[0] = char.ToUpper(array[0]);

                return new string(array);

            }

            return value;

        }

        public static int WordCount(this String str)

        {

            return str.Split(new char[] { ' ', '.', '?' },

                             StringSplitOptions.RemoveEmptyEntries).Length;

        }

    }

static void Main(string[] args)

 {

   // Use the string extension method on this value.

   string value = "hello extension methods";

   value = value.UppercaseFirstLetter(); /// Called like an instance method.

   int i = value.WordCount();/// Called like an instance method.

   Console.WriteLine(value);

   Console.WriteLine(i);

 }

 


Output

Hello extension methods
3

Refer
http://msdn.microsoft.com/en-IN/library/bb383977.aspx
http://www.dotnetperls.com/extension