WP7 Silverlight Gotcha: Setting the Source on a MediaElement will stop background music playback

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Applies To:  Silverlight (the restriction applies to XNA, but the MediaElement is only Silverlight)

Quick Bits

Simply setting the Source of a MediaElement to a valid source will stop any current background playback, causing you to fail Marketplace certification, according to section 6.5.1 from the certification guide.

The Fine Print

From the certification guide:

6.5.1 Initial Launch Functionality 

When the user is already playing music on the phone when the application is launched, the application must not pause, resume, or stop the active music in the phone MediaQueue by calling the Microsoft.Xna.Framework.Media.MediaPlayer class. If the application plays its own background music or adjusts background music volume, it must ask the user for consent to stop playing/adjust the background music (e.g. message dialog or settings menu). 

Note: This requirement does not apply to applications that play sound effects through the Microsoft.Xna.Framework.Audio.SoundEffect class, as sound effects will be mixed with the MediaPlayer. The SoundEffect class should not be used to play background music. 

Note: This requirement does not apply to Music + Videos Hub applications that are described in Section 6.4 

The Setup

You have a MediaElement which only used to play back one file, so you set the Source in your XAML.

The Sting

The user starts playing some music in Zune, then launches your XAP. As soon as your XAML loads and the MediaElement's Source is set, the background music stops playing, even if the MediaElement is not playing. That's right, even if you set AutoPlay = "False", the background music will still be stopped.

Since you must ask the user for consent, you will have failed certification.

The Solution

  1. Never set a MediaElement's Source in XAML, unless that XAML is on a page that you navigate to after asking the user for consent.
  2. Check to see if background music is playing and then set the source (in code).
    Note: If you set the source and then immediately call Play(), the Play() will have no affect since the MediaElement will still be in the "Opening" state, instead set "AutoPlay = true" (works from code)