A delegate is a helper object used by another object. The delegator may send the delegate certain messages, and provide a default implementation when there is no delegate or the delegate does not respond to a message. This pattern is heavily used in Cocoa framework on Mac OS X. See also wp:Delegation pattern.

Objects responsibilities:

Delegator:

 Keep an optional delegate instance.
 Implement "operation" method, returning the delegate "thing" if the delegate respond to "thing", or the string "default implementation".
Delegate:

 Implement "thing" and return the string "delegate implementation"
Show how objects are created and used. First, without a delegate, then with a delegate that does not implement "thing", and last with a delegate that implements "thing".

using System;
 
interface IOperable
{
    string Operate();
}
 
class Inoperable
{
}
 
class Operable : IOperable
{
    public string Operate()
    {
        return "Delegate implementation.";
    }
}
 
class Delegator : IOperable
{
    object Delegate;
 
    public string Operate()
    {
        var operable = Delegate as IOperable;
        return operable != null ? operable.Operate() : "Default implementation.";
    }
 
    static void Main()
    {
        var delegator = new Delegator();
        foreach (var @delegate in new object[] { null, new Inoperable(), new Operable() })
        {
            delegator.Delegate = @delegate;
            Console.WriteLine(delegator.Operate());
        }
    }
}