Using class extensions in .NET 3.0 like in .NET 3.5

A customer had the requirement to develop his solution with .NET 3.0 framework.
I was already used to the new and nice features of .NET 3.5 framework, especially the class extension feature.

A simple example for a class extension would be like this:

public class ReverseString
{
   public static void Main()
   {
      string s = "Hello World!";
      string r = s.Reverse();
   }
}

static class StringHelpers
{
   public static string Reverse(this string s)
   {
      char[] c = s.ToCharArray();
      Array.Reverse(c);
      return new string(c);
   }
}

In order to use the same feature under .NET 3.0 you can use the following trick:

First create an empty class file (e.g. AttributeExtension.cs) and add this class to your project.

Secondly put the following code in that class:

using System;

namespace System.Runtime.CompilerServices
{
    /// <summary>
    /// Indicates that a method is an extension method, 
    /// or that a class or assembly contains extension methods.
    /// </summary>
    [AttributeUsage(AttributeTargets.Method)]
    public sealed class ExtensionAttribute : Attribute
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="ExtensionAttribute"/> class.
        /// </summary>
        public ExtensionAttribute() { }
    }
}

Visual Studio 2008 will recognise the extension and there you go. From now on class extensions are available for .NET 3.0 applications.