Call F# code from C#


I am playing around with F# and C#, and would like to call F# code from C#.

I managed to get it to work the other way around in Visual Studio by having two projects in the same solution, and adding a reference of the C# code to the F# project. After doing this, I could call C# code and even step through it while debugging.

What I am trying to do is F# code FROM C# instead of C# code from F#. I added a reference to the F# project to the C# project, but it isn't working the way it did before. I would like to know if this is possible without doing it manually.

1/25/2009 11:40:28 PM

Accepted Answer

Below is a working example of calling F# from C#.

As you encountered, I was not able to add a reference by selecting from the "Add Reference ... Projects" tab. Instead I did have to do it manually, by browsing to the F# assembly in the "Add Reference ... Browse" tab.

------ F# MODULE -----

// First implement a foldl function, with the signature (a->b->a) -> a -> [b] -> a
// Now use your foldl function to implement a map function, with the signature (a->b) -> [a] -> [b]
// Finally use your map function to convert an array of strings to upper case
// Test cases are in TestFoldMapUCase.cs
// Note: F# provides standard implementations of the fold and map operations, but the 
// exercise here is to build them up from primitive elements...

module FoldMapUCase.Zumbro

let AlwaysTwo =

let rec foldl fn seed vals = 
   match vals with
   | head :: tail -> foldl fn (fn seed head) tail
   | _ -> seed

let map fn vals =
   let gn lst x =
      fn( x ) :: lst
   List.rev (foldl gn [] vals)

let ucase vals =
   map String.uppercase vals


// Test cases for FoldMapUCase.fs
// For this example, I have written my NUnit test cases in C#.  This requires constructing some F#
// types in order to invoke the F# functions under test.

using System;
using Microsoft.FSharp.Core;
using Microsoft.FSharp.Collections;
using NUnit.Framework;

namespace FoldMapUCase
    public class TestFoldMapUCase
        public TestFoldMapUCase()

        public void CheckAlwaysTwo()
            // simple example to show how to access F# function from C#
            int n = Zumbro.AlwaysTwo;
            Assert.AreEqual(2, n);

        class Helper<T>
            public static List<T> mkList(params T[] ar)
                List<T> foo = List<T>.Nil;
                for (int n = ar.Length - 1; n >= 0; n--)
                    foo = List<T>.Cons(ar[n], foo);
                return foo;

        public void foldl1()
            int seed = 64;
            List<int> values = Helper<int>.mkList( 4, 2, 4 );
            FastFunc<int, FastFunc<int,int>> fn =
                FuncConvert.ToFastFunc( (Converter<int,int,int>) delegate( int a, int b ) { return a/b; } );

            int result = Zumbro.foldl<int, int>( fn, seed, values);
            Assert.AreEqual(2, result);

        public void foldl0()
            string seed = "hi mom";
            List<string> values = Helper<string>.mkList();
            FastFunc<string, FastFunc<string, string>> fn =
                FuncConvert.ToFastFunc((Converter<string, string, string>)delegate(string a, string b) { throw new Exception("should never be invoked"); });

            string result = Zumbro.foldl<string, string>(fn, seed, values);
            Assert.AreEqual(seed, result);

        public void map()
            FastFunc<int, int> fn =
                FuncConvert.ToFastFunc((Converter<int, int>)delegate(int a) { return a*a; });

            List<int> vals = Helper<int>.mkList(1, 2, 3);
            List<int> res =<int, int>(fn, vals);

            Assert.AreEqual(res.Length, 3);
            Assert.AreEqual(1, res.Head);
            Assert.AreEqual(4, res.Tail.Head);
            Assert.AreEqual(9, res.Tail.Tail.Head);

        public void ucase()
            List<string> vals = Helper<string>.mkList("arnold", "BOB", "crAIg");
            List<string> exp = Helper<string>.mkList( "ARNOLD", "BOB", "CRAIG" );
            List<string> res = Zumbro.ucase(vals);
            Assert.AreEqual(exp.Length, res.Length);
            Assert.AreEqual(exp.Head, res.Head);
            Assert.AreEqual(exp.Tail.Head, res.Tail.Head);
            Assert.AreEqual(exp.Tail.Tail.Head, res.Tail.Tail.Head);

7/1/2019 8:28:25 AM

It should 'just work', though you might have to build the F# project before a project-to-project reference from C# works (I forget).

A common source of issues is namespaces/modules. If your F# code does not start with a namespace declaration, it gets put in a module with the same name as the filename, so that e.g. from C# your type might appear as "Program.Foo" rather than just "Foo" (if Foo is an F# type defined in Program.fs).

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