Extension Methods in C#

Extension methods allow you to extend an existing type with new functionality, without having to sub-class or recompile the old type.

Key Points:

         1: Extension Class must be static

         2: Extension Method must be static

         3: 1st Param in extension method must contains type which are geting extended    

Example:

    class Program

    {

        static void Main(string[] args)

        {

            //Default String

            String str = "My Name is XYZ";

            //LINQ Query to get count of vowels (use of Extension method)

            var queryChar = from ch in str.ToUpper()

                        where ch.IsVowel()

                        select ch;

            //Display result

            Console.WriteLine("Calling Char Extension Method:");

            Console.WriteLine("String \"{0}\" contains {1} Vowels", str, 
            queryChar.Count());

            Console.WriteLine();

            //Count Words in string (use of Extension method)

            int countWords = str.CountWords();

            Console.WriteLine("Calling String Extension Method:");

            Console.WriteLine("String \"{0}\" contains {1} Words", str, 
            countWords);

            Console.WriteLine();

            //Get Factorial from given integer  (use of Extension method)

            Console.WriteLine("Enter an integer to get factorial");

            int i =  int.Parse(Console.ReadLine());

            Console.WriteLine("Calling Int Extension Method:");

            Console.WriteLine("Factorial of {0} is: {1}", i, i.GetFactorial());

            Console.ReadLine();           

        }

    }

    /*Creating Extension Method*/

    static class Exten

    {

        public static bool IsVowel(this char ch)

        {

            return "AEIOU".Contains(ch);

        }

        public static int CountWords(this string str)

        {

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

                             StringSplitOptions.RemoveEmptyEntries).Length;

        }

        public static Int64 GetFactorial(this int x)

        {

            if (x <= 1) return 1;

            if (x == 2) return 2;

            else

                return x * GetFactorial(x - 1);

        }

    }

Output:
Calling Char Extension Method:

String "My Name is XYZ" contains 3 Vowels

Calling String Extension Method:

String "My Name is XYZ" contains 4 Words

Enter an integer to get factorial

8

Calling Int Extension Method:

Factorial of 8 is: 40320