I was wondering if there was a way to programmatically retrieve the first frame of an AVI and get a bitmap image to show the user a preview. The MediaControl in Silverlight shows a preview for Silverlight supported video files but not AVI. Because I'm in a Silverlight environment I cannot use unmanaged code or libraries to do so. I only have access to the filestream.
R – Getting Bitmap for First Frame of AVI in Silverlight
The Silverlight runtime doesn't have support for AVI files natively so you won't be able to use those with the MediaElement.
What you would need to do is actually parse the AVI file by hand and pull out frames from that file. Once you get to the point where you are parsing frames, it potentially gets a little trickier.
If you plan on having this work on Silverlight 2, your AVI file would need to contain WMV frames or frames in one of Silverlight's supported image formats(JPG or PNG). If you were working with WMV, I would set up a MediaStreamSource and pass in the desired video frame to the MediaStreamSource as my first sample. If you are working with one of the image formats you should use Image and set its source to a stream. You can see an example where Joe Stegman has used this to help Silverlight 2 support non-native image formats.
If you were doing this in Silverlight 3 (it's in Beta right now) the techniques are generally the same but the media format support grows a bit to include (in addition to WMV): H264, Raw YV12, or Raw ARGB frames. Similarly on the imaging front, you now have the WriteableBitmap which you could use to draw your frame.
If your video is something like Theora, Divx, Xvid, VP6, etc. You would need to find a way to decode that frame so you could display it in Silverlight.