I used '3dtools.dll' and it was right wrok in 'expression Blend4'. add reference Like this: xmlns:tools="clr-namespace:_3DTools;assembly=3DTools"} . . <tools:TrackballDecorator> . . </tools:TrackballDecorator> . . <tools:TrackballDecorator> but it does not work in 'MS Blend for Visual Studio(...