Thursday, January 6, 2011

Cannot convert lambda expression to type 'System.Delegate' because it is not a delegate type

Last week I was trying to pass a lambda as a parameter to a method that expected a delegate. However doing this resulted in the following compiler error

“Cannot convert lambda expression to type 'System.Delegate' because it is not a delegate type.”   

   1:  public void CreateAndExecuteLambda()
   2:  {
   3:     //Does not compile
   4:     InvokeDelegate(() => MessageBox.Show("Hello World")); 
   5:  }
   6:   
   7:  public void InvokeDelegate(Delegate @delegate) 
   8:  {
   9:     @delegate.DynamicInvoke();
  10:   }

At first I found this very strange, if I remembered it well a lambda expression is just a new syntax for anonymous methods and anonymous methods are nothing more than inline delegates. So why the compiler does not agree?

The reason is that a lambda expression can either be converted to a delegate type or an expression tree - but it has to know which delegate type. Just knowing the signature isn't enough. For instance, suppose I have:

   1:  public delegate void Action1();
   2:  public delegate void Action2();   
   3:   
   4:  Delegate x = () => Console.WriteLine("hello world"); 

What would you expect the concrete type of the object referred to by x to be?

If you want to make it work you have to help the compiler by explicitly casting the lambda expression to the correct delegate type.

   1:  public void CreateAndExecuteLambda()
   2:  {
   3:     //This works!
   4:     InvokeDelegate((Action)(() => MessageBox.Show("Hello World"))); 
   5:   }
   6:   
   7:  public void InvokeDelegate(Delegate @delegate)
   8:  {
   9:     @delegate.DynamicInvoke();
  10:  }

1 comment:

Anonymous said...

You could modify InvokeDelegate to accept an Action (and not a plain old delegate) as a parameter.

That's how Parallel.Invoke works (and it doesn't require the explicit cast).