Demonstration of extension methods. Note the static class PersonExtensions, the static method NameLenght, and the this keyword in front of the Person parameter.

ExtensionMethods.cs

class Person
{
  private string name;

  public Person(string name)
  {
    this.name = name;
  }

  public string Name { get { return this.name; } }

}

static class PersonExtensions
{
  public static int NameLength(this Person person)
  {
    return person.Name.Length;
  }
}

class Work
{
  static void Main(string[] args)
  {
    Person p = new Person("John");
    Console.WriteLine("Name length: {0}", p.NameLength());
  }
}