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Delegates (F#)

A delegate represents a function call as an object. In F#, you ordinarily should use function values to represent functions as first-class values; however, delegates are used in the .NET Framework and so are needed when you interoperate with APIs that expect them. They may also be used when authoring libraries designed for use from other .NET Framework languages.

Syntax

type delegate-typename = delegate of type1 -> type2

Remarks

In the previous syntax, type1 represents the argument type or types and type2 represents the return type. The argument types that are represented by type1 are automatically curried. This suggests that for this type you use a tuple form if the arguments of the target function are curried, and a parenthesized tuple for arguments that are already in the tuple form. The automatic currying removes a set of parentheses, leaving a tuple argument that matches the target method. Refer to the code example for the syntax you should use in each case.

Delegates can be attached to F# function values, and static or instance methods. F# function values can be passed directly as arguments to delegate constructors. For a static method, you construct the delegate by using the name of the class and the method. For an instance method, you provide the object instance and method in one argument. In both cases, the member access operator (.) is used.

The Invoke method on the delegate type calls the encapsulated function. Also, delegates can be passed as function values by referencing the Invoke method name without the parentheses.

The following code shows the syntax for creating delegates that represent various methods in a class. Depending on whether the method is a static method or an instance method, and whether it has arguments in the tuple form or the curried form, the syntax for declaring and assigning the delegate is a little different.

type Test1() =
  static member add(a : int, b : int) =
     a + b
  static member add2 (a : int) (b : int) =
     a + b

  member x.Add(a : int, b : int) =
     a + b
  member x.Add2 (a : int) (b : int) =
     a + b


// Delegate1 works with tuple arguments.
type Delegate1 = delegate of (int * int) -> int
// Delegate2 works with curried arguments.
type Delegate2 = delegate of int * int -> int

let InvokeDelegate1 (dlg: Delegate1) (a: int) (b: int) =
   dlg.Invoke(a, b)
let InvokeDelegate2 (dlg: Delegate2) (a: int) (b: int) =
   dlg.Invoke(a, b)

// For static methods, use the class name, the dot operator, and the
// name of the static method.
let del1 = Delegate1(Test1.add)
let del2 = Delegate2(Test1.add2)

let testObject = Test1()

// For instance methods, use the instance value name, the dot operator, and the instance method name.
let del3 = Delegate1(testObject.Add)
let del4 = Delegate2(testObject.Add2)

for (a, b) in [ (100, 200); (10, 20) ] do
  printfn "%d + %d = %d" a b (InvokeDelegate1 del1 a b)
  printfn "%d + %d = %d" a b (InvokeDelegate2 del2 a b)
  printfn "%d + %d = %d" a b (InvokeDelegate1 del3 a b)
  printfn "%d + %d = %d" a b (InvokeDelegate2 del4 a b)

The following code shows some of the different ways you can work with delegates.

type Delegate1 = delegate of int * char -> string

let replicate n c = String.replicate n (string c)

// An F# function value constructed from an unapplied let-bound function
let function1 = replicate

// A delegate object constructed from an F# function value
let delObject = Delegate1(function1)

// An F# function value constructed from an unapplied .NET member
let functionValue = delObject.Invoke

List.map (fun c -> functionValue(5,c)) ['a'; 'b'; 'c']
|> List.iter (printfn "%s")

// Or if you want to get back the same curried signature
let replicate' n c =  delObject.Invoke(n,c)

// You can pass a lambda expression as an argument to a function expecting a compatible delegate type
// System.Array.ConvertAll takes an array and a converter delegate that transforms an element from
// one type to another according to a specified function.
let stringArray = System.Array.ConvertAll([|'a';'b'|], fun c -> replicate' 3 c)
printfn "%A" stringArray

The output of the previous code example is as follows.

aaaaa
bbbbb
ccccc
[|"aaa"; "bbb"|]

Names can be added to delegate parameters like so:

// http://www.pinvoke.net/default.aspx/user32/WinEventDelegate.html
type WinEventDelegate = delegate of hWinEventHook:nativeint * eventType:uint32 * hWnd:nativeint * idObject:int * idChild:int * dwEventThread:uint32 * dwmsEventTime:uint32 -> unit

Delegate parameter names are optional and will be shown in the Invoke method. They are not required to match the parameter names in the implementation. They are only allowed for the curried form but not the tupled form.

type D1 = delegate of item1: int * item2: string -> unit
let a = D1(fun a b -> printf "%s" b)
a.Invoke(item2 = "a", item1 = 1) // Calling with named arguments

type D2 = delegate of int * item2: string -> unit // Omitting one name
let b = D2(fun a b -> printf "%s" b)
b.Invoke(1, item2 = "a")

The output of the previous code example is as follows.

aa

Direct construction and framework metadata

To preserve parameter names and attributes for reflection-based frameworks, F# 11 can target a compatible named function or method directly instead of a generated wrapper.

For example, ASP.NET Core Minimal API parameter binding uses route names and binding attributes to populate handler arguments:

open System
open Microsoft.AspNetCore.Builder
open Microsoft.AspNetCore.Mvc
open Microsoft.Extensions.DependencyInjection

type ShipmentStore() =
    member _.Status trackingId =
        if trackingId = "SHP-1042" then "Dispatched" else "Unknown"

let status
    (trackingId: string)
    ([<FromServices>] store: ShipmentStore)
    ([<FromHeader(Name = "X-Customer")>] customer: string) =
    $"{customer}: {trackingId} is {store.Status trackingId}"

let handler = Func<string, ShipmentStore, string, string>(status)

Check the original metadata through Method:

let parameters = handler.Method.GetParameters()
let header =
    parameters[2].GetCustomAttributes(typeof<FromHeaderAttribute>, false)[0]
    :?> FromHeaderAttribute
assert (handler.Method.Name = "status")
assert (parameters[0].Name = "trackingId")
assert (parameters[1].IsDefined(typeof<FromServicesAttribute>, false))
assert (header.Name = "X-Customer")

For the complete runnable application, see the sample project.

Which targets are direct?

Prefer Func<...>(namedFunction) for direct construction in both optimized and unoptimized builds. An explicit forwarding lambda retains its wrapper in unoptimized builds. Captures, argument adaptation, and mandatory inline expansion can also require wrappers.

Direct delegates with the same method and receiver compare equal. For reliable event unsubscription, retain and reuse the subscribed delegate.

Important

Validate instance receivers before direct delegate construction. A null receiver can throw ArgumentException for a non-virtual method or NullReferenceException for a virtual method before invocation.

let receiver: System.Text.StringBuilder = null
let callback = System.Func<char, System.Text.StringBuilder>(receiver.Append)

Wrappers can use parameter names from the delegate's Invoke signature when the function supplies none; they don't preserve the original method's metadata.

See also

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