Error casting delegate in non-generic class

Kalvin Work

I started with a generic class which has a delegate. The consumer of this class could set a method callback which requires invoking but they may not also. To address this I check if the callback is ISyncronizeInvoke, if it is then I will call BeginInvoke. This works great as long as the class is a generic class:

public class TestClassGeneric<T>
{
public delegate void GetMe();
private GetMe _call;

public TestClassGeneric(GetMe theCall)
{
  \_call = theCall;

  System.ComponentModel.ISynchronizeInvoke si =
    \_call as System.ComponentModel.ISynchronizeInvoke;
}

}

Since I don't need the class to be generic I want to make it just a standard class:

public class TestClassGeneric
{
public delegate void GetMe();
private GetMe _call;

public TestClassGeneric(GetMe theCall)
{
  \_call = theCall;

  System.ComponentModel.ISynchronizeInvoke si =
    \_call as System.ComponentModel.ISynchronizeInvoke;
}

}

But then I get a compile error: "Cannot convert type 'PureLibrary.TestClassGeneric.GetMe' to 'System.ComponentModel.ISynchronizeInvoke' via a reference conversion, boxing conversion, unboxing conversion, wrapping conversion, or null type conversion" Could someone please help me understand why this makes such a difference? Since the delegate signature didn't change it seems the casting would be the same. Thank you.

- Kalvin

Dimitri Witkowski

Very interesting question. I have an idea why this might happen. 1. Generic class: TestClassGeneric<T>. GetMe is unknown during the compile-time, because it's defined inside a generic class and the typeparam T may vary at run-time (actually, GetMe in this case will be TestClassGeneric<T>.GetMe). 2. Non-ceneric class: TestClassGeneric. GetMe is non-generic too, and the compiler may "guess" that the conversion will fail. Some facts:

System.Action is built-in delegate, it's defined as public delegate void Action() If you replace GetMe with Action, you will get an error in two cases.
At the run-time, the conversion will fail, and si will be null.
If you replace si = _call as System.ComponentModel.ISynchronizeInvoke; with si = (System.ComponentModel.ISynchronizeInvoke)_call;, both generic and non-generic will fail.

Thanks! I'll remember this - it's a very good task and needs understanding of generics in .NET Framework :thumbsup:

Die Energie der Welt ist konstant. Die Entropie der Welt strebt einem Maximum zu.

modified on Wednesday, June 9, 2010 2:13 PM