The following example implements an extension method named WordCount in the MyExtensions.StringExtension class. The method operates on the String class, which is specified as the first method parameter. The MyExtensions namespace is imported into the application namespace, and the method is called inside the Main method.

using System.Linq;
using System.Text;
using System;

namespace CustomExtensions
{
    //Extension methods must be defined in a static class
    public static class StringExtension
    {
        // This is the extension method.
        // The first parameter takes the "this" modifier
        // and specifies the type for which the method is defined.
        public static int WordCount(this String str)
        {
            return str.Split(new char[] {' ', '.','?'}, StringSplitOptions.RemoveEmptyEntries).Length;
        }
    }
}
namespace Extension_Methods_Simple
{
    //Import the extension method namespace.
    using CustomExtensions;
    class Program
    {
        static void Main(string[] args)
        {
            string s = "The quick brown fox jumped over the lazy dog.";
            //  Call the method as if it were an 
            //  instance method on the type. Note that the first
            //  parameter is not specified by the calling code.
            int i = s.WordCount();
            System.Console.WriteLine("Word count of s is {0}", i);
        }
    }
}

To run this code, copy and paste it into a Visual C# console application project that has been created in Visual Studio. By default, this project targets version 3.5 of the .NET Framework, and it has a reference to System.Core.dll and a using directive for System.Linq. If one or more of these requirements are missing from the project, you can add them manually. For more information, see How to: Create a LINQ Project.

Extension methods present no specific security vulnerabilities. They can never be used to impersonate existing methods on a type, because all name collisions are resolved in favor of the instance or static method defined by the type itself. Extension methods cannot access any private data in the extended class.

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