Extension methods allow an existing type to be extended with new methods, without altering the definition of the original type. An extension method is a static method of a static class, where the this modifier is applied to the first parameter. The type of the first parameter will be the type that is extended. For example:

public static class StringHelper
{
  public static bool IsCapitalized (this string s)
  {
    if (string.IsNullOrEmpty (s)) return false;
    return char.IsUpper (s[0]);
  }
}

The IsCapitalized extension method can be called as though it were an instance method on a string, as follows:

Console.Write ("Perth".IsCapitalized());

An extension method call, when compiled, is translated back into an ordinary static method call:

Console.Write (StringHelper.IsCapitalized ("Perth"));

Interfaces can be extended, too:

public static T First<T> (this IEnumerable<T> sequence)
{
  foreach (T element in sequence)
    return element;
  throw new InvalidOperationException ("No elements!");
}
...
Console.WriteLine ("Seattle".First());   // S

public static class StringHelper
{
  public static string Pluralize (this string s) {...}
  public static string Capitalize (this string s) {...}
}

string x = "sausage".Pluralize().Capitalize();

string y = StringHelper.Capitalize
           (StringHelper.Pluralize ("sausage"));