Question

I’ve got two classes, MyClassA and MyClassB. MyClassB inherits from MyClassA. I’ve written a method with the following signature

public void DoSomething(MyGeneric<MyClassA> obj);

I’ve also got the following event handler.

public void MyEventHandler(Object source, EventArgs e)
{
   //source is of type MyGeneric<MyClassB>
   DoSomething((MyGeneric<MyClassA>)obj);
}

I understand that MyGeneric<MyClassA> is not of the same type MyGeneric<MyClassB> but since MyClassB is a subclass of MyClassA is there still a way to make this work?

For reference, the exact error message:

Unable to cast object of type
‘MSUA.GraphViewer.GraphControls.TreeNode1[MSUA.GraphViewer.GraphControls.MaterialConfigControl]'
to type
'MSUA.GraphViewer.GraphControls.TreeNode1[MSUA.GraphViewer.PopulatableControl]’.

Answer 1

This is type contravariance in generics.

Even though B is a subtype of A,
Generic<B> is not a subtype of Generic<A>,
so you can’t cast Generic<B> to Generic<A>.

Check: http://msdn.microsoft.com/en-us/library/dd799517.aspx for more details.

You can overload DoSomething() to DoSomething(Generic<B>), this method can then convert Generic<B> to Generic<A> and call DoSomething(Generic<A>).