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Asked: 2026-05-16T09:52:57+00:00

I am having a strange issue opening up my UserControl in Expression Blend when

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I am having a strange issue opening up my UserControl in Expression Blend when using a Silverlight Toolkit control. My UserControl uses the toolkit’s ListBoxDragDropTarget as follows:

<controlsToolkit:ListBoxDragDropTarget mswindows:DragDrop.AllowDrop="True" HorizontalContentAlignment="Stretch" VerticalContentAlignment="Stretch">
        <ListBox ItemsSource="{Binding MyItemControls}" ScrollViewer.HorizontalScrollBarVisibility="Disabled">
            <ListBox.ItemsPanel>
                <ItemsPanelTemplate>
                    <controlsToolkit:WrapPanel/>
                </ItemsPanelTemplate>
            </ListBox.ItemsPanel>
        </ListBox>
</controlsToolkit:ListBoxDragDropTarget>

Everything works as expected at runtime and looks fine in Visual Studio 2008. However, when I try to open my UserControl in Blend I get XamlParseException: [Line: 0 Position: 0] and I can not see anything in the design view. More specifically Blend complains:

The element “ListBoxDragDropTarget” could not be displayed because of a problem with System.Windows.Controls.ListBoxDragDropTarget: TargetType mismatch.

My silverlight application is referencing System.Windows.Controls.Toolkit from the Nov. 2009 toolkit release, and I’ve made sure to include these namespace declarations for the ListBoxDragDropTarget:

xmlns:controlsToolkit="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Toolkit"
xmlns:mswindows="clr-namespace:Microsoft.Windows;assembly=System.Windows.Controls.Toolkit"

If I comment out the ListBoxDragDropTarget control wrapper and just leave the ListBox I can see everything fine in the design view without errors. Furthermore, I realized this is happening with a variety of Silverlight Toolkit controls because if I comment out ListBoxDragDropTarget and replace it with

<controlsToolkit:BusyIndicator />

the same exact error occurs in Blend. What is even weirder is that if I start a brand new silverlight application in blend I can add these toolkit elements without any kind of error, so it seems like something dumb that is happening with my project references to the toolkit assemblies.

I’m pretty sure this has something to do with loading the default styles for the toolkit controls from its generic.xaml, since the error has to do with the TargetType and Blend is probably trying to load up the default styles.

Has anyone encountered this issue before or have any ideas as to what may be my problem?

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1 Answer

  1. Editorial Team

    Hi we had exactly the same issue, we solved it by checking the references in the project which was having this problem. All referenced toolkit assemblies should be in the same directory on disk.

    Our project reference to the System.Windows.Controls.Toolkit.dll always ‘jumped’ back to the orginal path which was causing our issue. We solved by editing the project file in notepad++ (or any of your favorite text-editor) and hardcode the path where it could find the assembly.

    Hope this helps.

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