Calling F# sharp from C# in .net4.0

1) Add a reference FSharp.Core dll in your project
2) Reference Microsoft.FSharp.Core namespace in your code

using FS = Microsoft.FSharp.Core;

3) Implement your function, for example:

public static IEnumerable<U> map<T, U>(FS.FSharpFunc<T, U> mapper, 
 IEnumerable<T> input)
{
    foreach (T el in input)
    {
        yield return mapper.Invoke(el);
    }
}

Note: there are differences depending on the version of the .Net runtime
(4.0 vs. 3.5)
Bonus: return F# functions from C# code.

You can also implement F# functions in C# code and pass them to F#

//this is unit->string in F# speak
public class ReturnString: FSharpFunc<Unit, string>
{
    public FSharpFunc<Unit,string> asFSharpFunc()
    {
        return this;
    }

    public override string Invoke(Unit unitVar0)
    {
        return "a string";
    }
}

In F# do:

let funcFromCSharp = (ReturnString()).asFSharpFunc() //get it
funcFromCSharp () //call it

or this

let inline toFunc<'a,'b,'c when 'c :> FSharpFunc<'a,'b>> (x: 'c ): ('a -> 'b) = (# "" x : 'a -> 'b #)

let funcFromCSharp = ReturnString()|> toFunc
funcFromCSharp () //call it