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What kind of ComboBox control are you testing against? I was not able to reproduce such an issue in my local environment against our public samples. Are you able to reproduce the behavior against our Silverlight demos
The exception you are getting, the path or logic to find this control may not be correct at the moment you call to ToggleDropDown(). As a result, instead finding canvas it finds a FrameworkElement. One thing you can try is to call the RefreshVisualTrees() method of the app right before the ComboBox.ToggleDropDown() method.
If my suggestion does not help and you are not able to reproduce such behavior against our demo, please provide us with direct access to your application along with a copy of your framework test. If direct access is not possible (eg. app is local and/or private), we can instead attempt to "simulate" your app through a captured Fiddler trace which you can also attach to this ticket in a zip. If you are unfamiliar with how to create one, this link will provide you with step-by-step instructions for download and use. Alongside the previous instructions, please make sure to enable the 'Store binaries' and 'Decrypt HTTPS traffic' options prior to starting the capture.
Thank you for helping us advise you.