Using generic collections in Silverlight user controls

Question

There is an interface:

public interface IFoo {
}

A Silverlight user control has a collection of IFoo instances:

public ObservableCollection<IFoo> Items { get; set; }

There is an abstract class that implements the interface:

abstract public class Foo : IFoo {}

And a class that further derives from that:

public class DerivedFoo : Foo {}

With all of that said, I'm trying to add instances of DerivedFoo into the control's collection via XAML, but I receive an error that DerivedFoo is not of type IFoo and cannot be used in the generic collection.

I did find a post in a forum that said this was a bug in Silverlight 3 but would be fixed (I am using Silverlight 4). Is this still a bug or am I going about this incorrectly?

Update: My code is at home and I'm at work so I can't post the actual XAML, but from memory it was along the lines of:

<my:Thing>
    <my:Thing.Items>
        <my:DerivedFoo ... />
    </my:Thing.Items>
</my:Thing>

Can you post your XAML where you add instances to the collection? — Jeff Yates
– Jeff Yates, Commented Jul 14, 2010 at 12:33
This should work in Silverlight 4 and its new XAML parser. Can you verify the version? — Jeff Wilcox
– Jeff Wilcox, Commented Jul 14, 2010 at 17:58
Yes, the Silverlight target version for the project is listed as "Silverlight 4". — redman
– redman, Commented Jul 14, 2010 at 21:09

redman · Accepted Answer · 2010-07-15 01:25:04Z

1

The answer is...

The CollectionChanged event handler for the generic collection made an improper cast during the Add action.

answered Jul 15, 2010 at 1:25

redman

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