Visual Studio throws an exception: "Object reference not set to an instance of an object."

This line asumes that mSpriteTexture has been set. This is propably not the case.

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Let's talk about MVVM: http://social.msdn.microsoft.com/Forums/en-US/wpf/thread/b1a8bf14-4acd-4d77-9df8-bdb95b02dbe2