unable to compile a lambda expression because the type isn't delegate

Question

I'm trying to use lambda expression in C#:

using System;

namespace ConsoleApplication1 {
   public struct Point {
      public int x;
      public int y;

      public Point( int x, int y ) {
         this.x = x;
         this.y = y;
      }
   }

   public interface Mobile {
      Point getPosition();
   }

   public class Program {
      public void mth( Mobile mobile ) {
         Point p = mobile.getPosition();
         Console.WriteLine( "{ " + p.x + ", " + p.y + " }" );
      }

      static void Main( string[] args ) {
         new Program().mth( () => { return new Point( 4, 5 ); } ); <<<<<<<< ERROR
      }
   }
}

Compiler error (in French):

Impossible de convertir expression lambda en type 'ConsoleApplication1.Mobile', car il ne s'agit pas d'un type délégué

International English Translation:

Unable to convert lambda expression to type 'ConsoleApplication1.Mobile', because isn't a delegate type

What's the right syntax?

Are you trying to implement that interface method using a lambda? — BoltClock
– BoltClock, Commented Nov 19, 2013 at 12:25
I'm not sure what you are trying to do here - it looks like you want to getPosition() from Mobile but you are trying to create one using an anonymous function that returns a Point... can you explain what you are trying to do? — Charleh
– Charleh, Commented Nov 19, 2013 at 12:27

Rawling · Accepted Answer · 2013-11-19 12:37:39Z

4

You appear to be confusing an interface and a delegate.

An interface is, roughly speaking, a collection of one or more methods. It has to be implemented by a class that provides implementations for each method; you can't build an implementation up from lambda methods.

A delegate is basically a named method signature. You can convert a lambda method to a delegate as long as the signature is correct, or you can create a delegate pointing at a method on a class, say.

I think what you're after to make your code work is to turn your Mobile into a delegate:

// This can represent any function that takes nothing and returns a point.
public delegate Point Mobile();

public class Program {

    // This takes any "Mobile" function, calls it and displays the result.      
    public void mth(Mobile getPosition) {
        Point p = getPosition();
        Console.WriteLine("{ " + p.x + ", " + p.y + " }");
    }

    // This calls mth with a lambda that matches the "Mobile" definition.
    static void Main( string[] args ) {
        new Program().mth(() => { return new Point( 4, 5 ); }); 
    }

    // You could also explicitly create a "Mobile".
    static void Main2( string[] args ) {
        Mobile myMobile = new Mobile(() => { return new Point( 4, 5 ); });
        new Program().mth(myMobile); 
    }
}

edited Nov 19, 2013 at 12:37

answered Nov 19, 2013 at 12:32

Rawling

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2 Comments

BoltClock Over a year ago

I'm pretty sure this is it.

Aerospace Over a year ago

Yes! That's THE answer! The concept of delegate is newer for me.

Dmytro Rudenko · Accepted Answer · 2013-11-19 12:38:08Z

-1

   public class Mobile {
      public Func<Point> GetPosition { get; set; }
   }

   public class Program {
      public void mth( Mobile mobile ) {
         Point p = mobile.GetPosition();
         Console.WriteLine( "{ " + p.x + ", " + p.y + " }" );
      }

      static void Main( string[] args ) {
         new Program().mth( new Mobile { GetPosition = () => { return new Point( 4, 5 ); } } ); 
      }
   }

edited Nov 19, 2013 at 12:38

answered Nov 19, 2013 at 12:31

Dmytro Rudenko

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1 Comment

Dmytro Rudenko Over a year ago

Sorry, I forgot adding getter-setter for GetPosition property

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