Download file in chunks (Windows Phone)

Question

In my application I can download some media files from web. Normally I used WebClient.OpenReadCompleted method to download, decrypt and save the file to IsolatedSto

Answer 1

To download a file in chunks you'll need to make multiple requests. One for each chunk.
Unfortunately it's not possible to say "get me this file and return it in chunks of size X";

Assuming that the server supports it, you can use the HTTP Range header to specify which bytes of a file the server should return in response to a request.
You then make multiple requests to get the file in pieces and then put it all back together on the device. You'll probably find it simplest to make sequential calls and start the next one once you've got and verified the previous chunk.

This approach makes it simple to resume a download when the user returns to the app. You just look at how much was downloaded previously and then get the next chunk.

I've written an app which downloads movies (up to 2.6GB) in 64K chunks and then played them back from IsolatedStorage with the MediaPlayerLauncher. Playing via the MediaElement should work too but I haven't verified. You can test this by loading a large file directly into IsolatedStorage (via Isolated Storage Explorer, or similar) and check the memory implications of playing that way.

Answer 2

Confirmed: You can use BackgroundTransferRequest to download multi-GB files but you must set TransferPreferences to None to force the download to happen while connected to an external power supply and while connected to wi-fi, else the BackgroundTransferRequest will fail.

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I wonder if it's possible to use a BackgroundTransferRequest to download large files easily and let the phone worry about the implementation details? The documentation seems to suggest that file downloads over 100 MB are possible, and the "Range" verb is reserved for it's own use, so it probably uses this automatically if it can behind the scenes.

From the documentation regarding files over 100 MB:

For files larger than 100 MB, you must set the TransferPreferences property of the transfer to None or the transfer will fail. If you do not know the size of a transfer and it is possible that it could exceed this limit, you should set the value to None, meaning that the transfer will only proceed when the phone is connected to external power and has a Wi-Fi connection.

From the documentation regarding use of the "Range" verb:

The Headers property of the BackgroundTransferRequest object is used to set the HTTP headers for a transfer request. The following headers are reserved for use by the system and cannot be used by calling applications. Adding one of the following headers to the Headers collection will cause a NotSupportedException to be thrown when the Add(BackgroundTransferRequest) method is used to queue the transfer request:

• If-Modified-Since
• If-None-Match
• If-Range
• Range
• Unless-Modified-Since

Here's the documentation: http://msdn.microsoft.com/en-us/library/windowsphone/develop/hh202955(v=vs.105).aspx