Many people are not aware of this. It is possible since .NET 3.5 to extend any class in c# to make it appear that extra functions have been added to a class. Here is a short example of being able to add an extra function to the bool class / type that exists in c#

public static class ExtenderBool
{
    public static string ToHuman(this bool b)
    {
        if (b)
            return "Yes";
        return "No";
    }

}

You can then use this in c# so long as the namespace is in scope from the calling function. Here is a quick example of a program to test it.

class Program
    {
        static void Main(string[] args)
        {
            bool tmp = true;

            Console.WriteLine("Value: {0}", tmp.ToHuman());
            Console.ReadLine();
        }
    }

The above works because the compiler will figure out that you want it to be able to use the function with that class as a argument to the static function though It does not actually add it to the class it self.