How to call delegate from string in C#?

Question

is it possible to call a delegate stored in a variable by its variable name (as a string)? i guess i'd have to use reflection mechanism, but i'm not getting anywhere

example code:

class Demo {
  public delegate int DemoDelegate();

  private static int One() {
    return 1;
  }
  private static void CallDelegate(string name) {
    // somehow get the value of the variable with the name
    // stored in "name" and call the delegate using reflection
  }
  private static void CallDelegate(string name, DemoDelegate d) {
    d();
  }
  static void main(string[] args) {
    DemoDelegate one = Demo.One;
    CallDelegate(one);
    // this works, but i want to avoid writing the name of the variable/delegate twice:
    CallDelegate("one", one);
  }

}

is this even possible? if so how? if not, then i have to use the second form, bad luck

Having `DemoDelegate d` in the second overload completely defeats the purpose of `string name`, doesn't it? — BoltClock, May 15 '11 at 19:08
@boltclock: yes, that's why i want to avoid it (in my real code, i want to store the name of the variable to `Console.Out.WriteLine()` it later) — knittl, May 15 '11 at 19:14
There are a number of fundamental errors here. `One()` is an instance method for one and you're accessing `this` from within a static method. What are you trying to call? The instance method `One()` or the delegate stored in the local variable in `main()`, `one`? — Jeff Mercado, May 15 '11 at 19:24
@knittl if you only want to store the name of the variable, why not just use "this.delegateMember.Method.Name"? this makes more sense to me than doing dodgy reflection hacks? — peteisace, May 15 '11 at 19:26
@jeff: thanks for mentioning, fixed it. and i want to call the delegate stored in the local variable "one" (lowercase) — knittl, May 15 '11 at 19:28
@knittl: Maybe I'm missing the point here but you already have an instance of your delegate in `one`. You don't need its name to be able to invoke it. If you needed to access the `One()` method from a different class, that would be a different case however. — Jeff Mercado, May 15 '11 at 19:31
@jeff: this is only example code, to make it clear i want to call a delegate (and not a method directly) — knittl, May 15 '11 at 19:39
I have changed my answer to write the variable name, to the console, later, as you suggested in a previous comment. — Miguel Angelo, May 15 '11 at 19:54

score 6 · Answer 1 · answered May 15 '11 at 19:15

6

Variables barely exist. The only way to reliably call-by-string (in this scenario) would be to store the delegates in a dictionary:

Dictionary<string, DemoDelegate> calls = new Dictionary<string, DemoDelegate>
{
    {"one",one}, {"two",two}
}

Now store that dictionary somewhere (in a field, typically), and do something like:

private int CallDelegate(string name) {
    return calls[name].Invoke(); // <==== args there if needed
}

answered May 15 '11 at 19:15

Marc Gravell

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This is quite not true. It is possible to use variable outside their native contexts using closures (whatever the name is, in C# they are called delegates, lamdas, etc..). – Miguel Angelo May 15 '11 at 19:53
This fact combined with linq expressions, makes the compiler "print" info I want about a variable, using the lamda: () => varIWishToGetInfo. – Miguel Angelo May 15 '11 at 20:05
@Miguel well, you can... sometimes. Don't try it from F# though ;p In reality, I don't think I recommend it – Marc Gravell May 15 '11 at 21:52

Miguel Angelo · Accepted Answer · 2011-05-15T19:48:08.883

Yes, it is possible, as long as you use Linq Expressions, and little reflection.

Take a look at this code, it does something simillar to what I think you want:

using System;
using System.Linq.Expressions;
using System.Reflection;
using System.Collections.Generic;

namespace q6010555
{
    class Demo
    {
        static List<string> varNamesUsed = new List<string>();

        public delegate int DemoDelegate();

        private static int One()
        {
            return 1;
        }
        private static void CallDelegate(Expression<Func<DemoDelegate>> expr)
        {
            var lambda = expr as LambdaExpression;
            var body = lambda.Body;
            var field = body as MemberExpression;
            var name = field.Member.Name;
            var constant = field.Expression as ConstantExpression;
            var value = (DemoDelegate)((field.Member as FieldInfo).GetValue(constant.Value));

            // now you have the variable name... you may use it somehow!
            // You could log the variable name.
            varNamesUsed.Add(name);

            value();
        }
        static void Main(string[] args)
        {
            DemoDelegate one = Demo.One;
            CallDelegate(() => one);

            // show used variable names
            foreach (var item in varNamesUsed)
                Console.WriteLine(item);
            Console.ReadKey();
        }
    }
}

I have changed my answer, including the delegate call. But I don't understand yet, why you need the name of the variable. Is it for logging? — Miguel Angelo, May 15 '11 at 19:38

score 1 · Answer 3 · answered May 15 '11 at 19:18

1

public void Fire(string name)
        {
            FieldInfo field = this.GetType().GetField(name, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);

            if (field != null)
            {
                Delegate method = field.GetValue(this) as Delegate;

                if (method != null)
                {
                    method.Method.Invoke(method.Target, new object[0]);
                }
            }
        }

Obviously restricts you from having parameterized delegates.

answered May 15 '11 at 19:18

peteisace

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1
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this works for delegates stored in member variables, but not for local variables, if i understand correctly? – knittl May 15 '11 at 19:32
@knittl: If you're using local variables, you should **know** their name already. In that case, you could just use a `switch` or a `Dictionary` (which is what Marc suggests in [his answer](http://stackoverflow.com/questions/6010555/how-to-call-delegate-from-string-in-c/6010611#6010611). – Brian May 16 '11 at 14:39
@brian: local variables before the function call. i know then mase, but my code does not ;) – knittl May 16 '11 at 17:03

score 0 · Answer 4 · answered May 15 '11 at 19:14

You can't really access variables in another stack frame (although I think it is possible using hackery around the StackFrame class). Instead, you'll be wanting to pass an Delegate object around and use methods like DynamicInvoke, if you want to invoke a generalised delegate in a reflection-like manner.

How to call delegate from string in C#?

4 Answers4

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