Home > Posts > Gotcha: MediaElement Source=null releases stream, SetSource(null) fails

Gotcha: MediaElement Source=null releases stream, SetSource(null) fails

This is my contribution to:


If you use MediaElement, make sure you don’t get bitten by this one: http://msdn.microsoft.com/en-us/library/cc626563(v=vs.95).aspx

ArgumentNullException – The mediaStreamSource is null.

After calling this method, MediaElement.Source returns null. If this is called and MediaElement.Source is set, the last operation wins.

If a MediaElement is removed from the UI tree while is has an opened MediaStreamSource, subsequent calls to SetSource may be ignored. To ensure featureSetSource calls will work, set the Source property to null before detaching the MediaElement from the UI tree.

naturally one would expect, if they only use SetSource(somestream) to use SetSource(null) to release the resources. Nope, they thought "better", you have to use Source=null instead to release resources and SetSource(null) throws ArgumentNullException

that is what I call a design bug (breaks the rule of "least expected" behavior and causes bugs that bite you at runtime only [unless somebody has made a static analysis rule to catch such a thing – would need metadata of course that some argument can’t be null, like in Code Contracts])

I managed to introduce this bug while refactoring some code in ClipFlair Studio‘s AudioRecorder control the other day 😦

Note that you can’t use at MediaElement something like Source = stream to open a Stream, since that is a Uri property (not an Object property to also accept Stream) and you have to use SetSource(stream) instead, so you’d also expect to be able to use SetSource(null) to release the resources.

Update: Fixed this in AudioRecorderView class (uses MVVM pattern) of AudioRecorderControl, at Audio property’s "set" accessor it needed the following null-guarding pattern:

if (mediaStreamSource != null) 

      //must set the source once, not every time we play the same audio, 
      //else with Mp3MediaSource it will throw DRM error


   player.Source = null;

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