Call F# code from C#

From this link they seem to have a number of possible solutions, but the one that seemed the simplest was a comment:

F# Code:

type FCallback = delegate of int*int -> int;;
type FCallback =
delegate of int * int -> int


let f3 (f:FCallback) a b = f.Invoke(a,b);;
val f3 : FCallback -> int -> int -> int

C# Code:

int a = Module1.f3(Module1.f2, 10, 20); // method gets converted to the delegate automatically in C#

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
#light




let AlwaysTwo =
2


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

----- C# UNIT TESTS FOR THE MODULE -----

// 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
{
[TestFixture]
public class TestFoldMapUCase
{
public TestFoldMapUCase()
{
}


[Test]
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;
}
}




[Test]
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);
}


[Test]
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);
}


[Test]
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 = Zumbro.map<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);
}


[Test]
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);
}


}
}

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).

// Test.fs :

module meGlobal


type meList() =
member this.quicksort = function
| [] -> []  //  if list is empty return list
| first::rest ->
let smaller,larger = List.partition((>=) first) rest
List.concat[this.quicksort smaller; [first]; this.quicksort larger]

// Test.cs :

List<int> A = new List<int> { 13, 23, 7, 2 };
meGlobal.meList S = new meGlobal.meList();


var cquicksort = Microsoft.FSharp.Core.FSharpFunc<FSharpList<IComparable>,     FSharpList<IComparable>>.ToConverter(S.quicksort);


FSharpList<IComparable> FI = ListModule.OfSeq(A.Cast<IComparable>());
var R = cquicksort(FI);