Introduction To Delegates in F#


In this article I will explain Delegates and how to them in a F# console application.

What is Delegate?

A Delegate is a special function that encapsulate a method and represents a method signature. Delegates are similar to pointers to functions in C or C++. A delegate is a reference type variable that holds the address of functions. The reference can be changed at runtime. Delegates are used for implementing events and call-back functions. A delegate is referred to as type-safe function pointers, in other words if the method with two int parameters and a return type is an int then the delegate that you are declaring should also have the same signature. That is why a delegate is referred to as type-safe function.

The following is the syntax to declare a Delegate:

type delegate-typename=delegate of type1 ->type2

The following are the 3 ways to define Delegates:

  • Declaration
  • Instantiation
  • Invocation

The following are the 2 types of Delegates:

  • Simple Delegate
  • Multicast Delegate

Multicast Delegate

A Multicast Delegate holds references of one or more functions. Multicast delegates must contain only methods that have the return type void only, else there is a runtime exception. The Delegate object uses the "+" and  "-" operator s. The "+" operator composes the number of delegates by the delegate object and the "-" operator removes the component delegate from the composed delegate object.


The following are the features of Delegates:

  • Delegate are like function pointers but type safe.
  • Delegates allow methods to be passed as a parameter.
  • A Delegate can be used for callback methods.
  • Delegates improve the performance of the application.

Invoke( ) Function

The Invoke( ) Function executes the specified delegate with the specified list of arguments.

Let's use the following procedure to understand Delegates.

Step 1:

Open Visual Studio then click "Create New Project" --> "F# Console Application".


Step 2:

Write the following code in the F# application.


open System

 type DemoDelegates() =

  static member add(a : int, b : int) =

     a + b

  static member sub (a : int) (b : int) =

     a - b

  member x.Add(a : int, b : int) =

     a + b

  member x.Sub (a : int) (b : int) =

     a - b 

type Delegateobj1 = delegate of (int * int) -> int

type Delegateobj2 = delegate of int * int -> int

let InvokeDelegate1 (dlg : Delegateobj1) (a : int) (b: int) =

   dlg.Invoke(a, b)

let Delegateobj2 (dlg : Delegateobj2) (a : int) (b: int) =

   dlg.Invoke(a, b)

let del3 : Delegateobj1 = new Delegateobj1( DemoDelegates.add )

let del4 : Delegateobj2 = new Delegateobj2( DemoDelegates.sub )

let delobj = DemoDelegates()

let del1 : Delegateobj1 = new Delegateobj1( delobj.Add )

let del2 : Delegateobj2 = new Delegateobj2( delobj.Sub )

for (a, b) in [ (50,40 ); (0,0) ] do

printfn "\nAdd:%d + %d = %d" a b (InvokeDelegate1 del1 a b) 

printfn "\nSub:%d - %d = %d" a b (Delegateobj2 del2 a b)

printfn "\nadd:%d + %d = %d" a b (InvokeDelegate1 del3 a b)

printfn "\nsub:%d - %d = %d" a b (Delegateobj2 del4 a b)




Debug the application by pressing F5 and the result will be in the application as  shown in the following figure.



This article explained Delegates and how to use them in a F# console application and you saw how to declare and implement the delegates.

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