F# Define function to act as delegate to .net Action

Question

in System.Activities.WorkflowApplication there is a delegate property:

        public Action<WorkflowApplicationCompletedEventArgs> Completed { get; set; }

In my program so far, I have a variable that is an instance of this class

I want to define an F# function to set that:

let f (e: WorkflowApplicationCompletedEventArgs) = 
    // body
myInst.Completed <- f

but this produces the error:

Error 102 This expression was expected to have type Action but here has type 'a -> unit

how do I complete function "f" to satisfy the compiler?

Sidoney

let f = System.Action<WorkflowApplicationCompletedEventArgs>(fun e -> (*body to unit*)) – Catechu

Works like a charm!! – Sidoney

Answer 1

If you pass an anonymous function fun a -> ... to a method or a constructor that expects a System.Action<...> or a System.Func<...>, then it is automatically converted; in any other case, you need to convert it explicitly like @Funk indicated.

let f = System.Action<WorkflowApplicationCompletedEventArgs>(fun e ->
    // body
)
myInst.Completed <- f

// Another solution:

let f (e: WorkflowApplicationCompletedEventArgs) = 
    // body
myInst.Completed <- System.Action<_>(f)

Pokeberry

that last one is a new form to me. What does the underscore represent here? – Sidoney

It lets the type inference system guess the type. Based on the type of f, it can deduce what type the Action's parameter is. I could just as well have written Action<WorkflowApplicationCompletedEventArgs>, but this is shorter and just as clear. – Pokeberry