How to pass a function as a parameter in C#?

Question

Is it possible to pass a function as a parameter in C#? I can do it using the Func or Action classes, but this forces me to declare the entire function signature at once. When I try to use Delegate, I get a compile error saying it can't convert a method group to a Delegate.

I'm working on Axial and I'm trying to allow users to call web services. What I'm going for is the ability to create the Visual Studio proxy class and then pass in the generated function. The function signature doesn't matter because the generated code only uses the function name. However, I'd like to pass in the function instead of the name for two reasons: the ability to use the proxy's Url property and a compiler error if the web service doesn't exist or is updated in Visual Studio.

public void AlertIt(object o) {
    Axial.DOM.Window.Alert(o.ToString());
}
public void CallAddService() {
    object[] param = new object[] { int.Parse(txtA.Text), int.Parse(txtB.Text) };
    Axial.ServerScript.CallWebService(new WSProxy.WS().Add, param, AlertIt, AlertIt);
}

class Axial.ServerScript {
    public void CallWebService(Delegate method, object[] param, Action<object> successCallback, Action<object> failureCallback) {
        // translate to javascript (already working)
    }
}

Trismus

Answer 1

I think what you want is:

static object InvokeMethod(Delegate method, params object[] args){
    return method.DynamicInvoke(args);
}

static int Add(int a, int b){
    return a + b;
}

static void Test(){
    Console.WriteLine(InvokeMethod(new Func<int, int, int>(Add), 5, 4));
}

Prints "9".

Roethke

Is there a way to not require the "new Func<int, int, int>" part of the code? The usage pattern is pretty important. – Trismus

The only way that comes to mind would be to have InvokeMethod take Func<int, int, int> instead of Delegate. But then you'd need a different overload of InvokeMethod for each combination of type parameters to Func<> that you'll ever see. – Roethke

As a note, there is a significant cost associated with using DynamicInvoke - where possible, use a typed delegate and Invoke (much faster) – Gwynethgwynne

Thanks for this. It really helped me explain this to another dev. here. – Quinary

Answer 2

Converting a method group, anonymous method or lambda expression to a delegate requires the compiler to know the exact delegate type. However, you could potentially use lambda expressions and captured variables to make this simpler:

public void InvokeMethod(Action action)
{
    action();
}

public int Add(int a, int b)
{
    return a + b;
}

public void Test()
{    
    InvokeMethod(() => Add(2, 3));
}

That basically delays invocation in the normal way, but by wrapping the actual call to Add in a plain Action delegate.

If that doesn't fulfil your requirements, perhaps you can tell us a bit more about what you're really trying to achieve.

EDIT: If this is generated code, you can cast to a Func<...> with the right type arguments - assuming there aren't too many. Other than that, there's no real way of just passing in a method group. There's been occasional calls for an "infoof(...)" operator (like typeof but for members) which would give you a MemberInfo, but that doesn't actually exist.

Chilli

That's very close, but makes a difficult API to use. I've updated the question to say exactly what I'm doing. – Trismus

BTW, this code requires compilation with C# 3.0 tools. (Just set the project to .Net framework 3.5) – Slyke

But here the Test() and InvokeMethod() are returning void, so the result of Add() can't be returned. – Accelerando

@AbhishiktN.Jain: Nothing's trying to return the value of Add. We're just calling InvokeMethod with a delegate, which is the result of the conversion of the lambda expression. Have you tried this code? – Chilli

Answer 3

You should have a delegate first

delegate int Operation(int a, int b)

then it becomes:

public void InvokeMethod(Operation method, object target, object param)
{
    method((int) target, (int) param);
}

No need for any call to Invoke.

As with dbone I'm unsure why you would need a params[] array. Would you clarify the expanded usage for the params?

Also, I'll have to correct something in your question though, because it will cause a compilation error :p

Fantastic

Answer 4

please have a look at using delegates here is a great example

Delegate Example

why are you using reflection? will there ever be a different number of params? or do you know the method signture will remain constant (also remember C# supports the params[] keyword)

params c#

HTH

Bones

Swig

Answer 5

Look at Functional Programming Series by Justin Etheredge. You should find solution to your problem there.

Lorrimor

That series isn't necessarily related but I read it because it's interesting. It gave me the idea of a templated function, which would work if the C# type inference were better. – Trismus

Answer 6

This is much simple example, to programmer who already familiar with (C/C++/VB.NET/Python)-style pass function by pointer/ref (with C# delegate):

        delegate void CALLBACK(String s);
        static void Main(string[] args)
        {

            Get("some string", testfunc);

            Util.pause();
        }

        static void Get(String s, CALLBACK x)
        {
            x(s);
        }


        static void testfunc(String s)
        {
            Console.WriteLine(s);
        }

Businessman

Answer 7

Say If you need to pass the method as parameter as well as you need to catch the return value for further processing . Then the above examples will work fine . But say if you need to pass a method with void return type then you need to create one more version of the InvokeMethod function. Check the example below.

private static T retry<T>(Delegate method, params object[] args)
{
    for (int i = 0; i <= 3; i++)
    {
        try
        {
            return (T)method.DynamicInvoke(args);
        }
        catch (Exception ex)
        {
            if (i == 3)
            {
                logMessage(ex.Message);
            }
            Console.WriteLine("Retry count " + i);
            Thread.Sleep(10);
        }
    }
    return default(T);
}

private static void retry2(Delegate method, params object[] args)
{
    for (int i = 0; i <= 3; i++)
    {
        try
        {
            method.DynamicInvoke(args);
            break;
        }
        catch (Exception ex)
        {
            if (i == 3)
            {
                logMessage(ex.Message);
                //return default(T);
            }
            Console.WriteLine("Retry count " + i);
            Thread.Sleep(10);
        }
    }
}
static bool isSuccess = true;
static void logMessage(string msg)
{
    isSuccess = false;
    Console.WriteLine(msg);
}

static int Add(int a, int b)
{
    return a + b;
}

static void Add2(int a, int b)
{
    int c = a + b;
    Console.WriteLine(c);
}

static void Main(string[] args)
{
    int d = retry<int>(new Func<int, int, int>(Add), 6, 7.7);
    Console.Write("  " + d + "\n"+isSuccess);

    retry2(new Action<int, int>(Add2), 45, 60);

    Console.ReadKey();
}

Vanmeter

Answer 8

Something like this ought to work for you:

delegate int MyDelegate(int a, int b);
public int Add(int a, int b) {
    return a + b;
}
public void InvokeMethod(Delegate method, object[] param) {
    Console.WriteLine(method.DynamicInvoke(param));
}
public Form1() {       
    InitializeComponent();
    InvokeMethod(new MyDelegate(Add), new object[] { 1, 2 });
}

Good luck!

Daffie