This is very strange because if you're able to create the stream object there shouldn't be any difference for the media player. Could you please elaborate a little bit more on your scenario:
1) What error do you get ?
2) What is your exact dll version ?
3) Are you using the same video file for the tests ?
4) Have you tried to directly use the Microsoft MediaElement ?
I've attached my test project so that you could examine it and tell us what you're doing differently.
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