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".

type Delegator() =
  let defaultOperation() = "default implementation"
  let mutable del = null
 
  // write-only property "Delegate"
  member x.Delegate with set(d:obj) = del <- d
 
  member x.operation() =
    if del = null then
      defaultOperation()
    else
      match del.GetType().GetMethod("thing", [||]) with
      | null -> defaultOperation()
      | thing -> thing.Invoke(del, [||]) :?> string 
 
type Delegate() =
  member x.thing() = "delegate implementation"
 
let d = new Delegator()
assert (d.operation() = "default implementation")
 
d.Delegate <- "A delegate may be any object"
assert (d.operation() = "default implementation")
 
d.Delegate <- new Delegate()
assert (d.operation() = "delegate implementation")