Call a higher order F# function from C#

Question

Given the F# higher order function (taking a function in parameter):

let ApplyOn2 (f:int->int) = f(2)  

and the C# function

public static int Increment(int a) { return a++; } 

How do I call ApplyOn2 with Increment as parameter (from C#)? Note that ApplyOn2 is exported as Microsoft.FSharp.Core.FSharpFunc<int,int> which do not match with Increment's signature.

Answer 1

To get an FSharpFunc from the equivalent C# function use:

Func<int,int> cs_func = (i) => ++i;
var fsharp_func = Microsoft.FSharp.Core.FSharpFunc<int,int>.FromConverter(
    new Converter<int,int>(cs_func));

To get a C# function from the equivalent FSharpFunc, use

var cs_func = Microsoft.FSharp.Core.FSharpFunc<int,int>.ToConverter(fsharp_func);
int i = cs_func(2);

So, this particular case, your code might look like:

Func<int, int> cs_func = (int i) => ++i;
int result = ApplyOn22(Microsoft.FSharp.Core.FSharpFunc<int, int>.FromConverter(
            new Converter<int, int>(cs_func)));

Answer 2

If you would like to provide a more friendly interop experience, consider using the System.Func delegate type directly in F#:

let ApplyOn2 (f : System.Func<int, int>) = f.Invoke(2)

You would be able to call your F# function very easily in C# like this:

MyFSharpModule.ApplyOn2(Increment); // 3

There is an issue with the Increment function as you have written it, however. You need the prefix form of the increment operator in order for your function to return the correct result:

public static int Increment(int a) { return ++a; }