Why LiveConnectClient.BackgroundDownloadAsync fails if trying to "await" it?

Question

I wrote a simple function to download a file from SkyDrive into IsolatedStorage.

    public static async Task<T> DownloadFileData<T>( string fileID, string filename )
        {
        var liveClient = new LiveConnectClient( Session );

        // Download the file
        await liveClient.BackgroundDownloadAsync( fileID + "/Content", new Uri( "/shared/transfers/" + filename, UriKind.Relative ) );

        // Get a reference to the Local Folder
        string root = ApplicationData.Current.LocalFolder.Path;
        var storageFolder = await StorageFolder.GetFolderFromPathAsync( root + @"\shared\transfers" );

        // Read the file
        var FileData = await StorageHelper.ReadFileAsync<T>( storageFolder, filename );
        return FileData;
        }

The function fails running the line:

// Download the file
await liveClient.BackgroundDownloadAsync( fileID + "/Content", new Uri( "/shared/transfers/" + filename, UriKind.Relative ) );

With the error: "An exception of type 'System.InvalidOperationException' occurred in mscorlib.ni.dll but was not handled in user code

The request has already been submitted"

The function succeeds if I modify the line to (removing the await):

// Download the file
liveClient.BackgroundDownloadAsync( fileID + "/Content", new Uri( "/shared/transfers/" + filename, UriKind.Relative ) );

Why is that?

Thx

Answer 1

There is a need to check if BackgroundTransferService.Request is empty and if not to remove any pending request.

I modified my code like this, and it seems to work fine:

public static async Task<T> DownloadFileData<T>( string skydriveFileId, string isolatedStorageFileName )
    {
    var liveClient = new LiveConnectClient( Session );

    // Prepare for download, make sure there are no previous requests
    var reqList = BackgroundTransferService.Requests.ToList();
    foreach ( var req in reqList )
        {
        if ( req.DownloadLocation.Equals( new Uri( @"\shared\transfers\" + isolatedStorageFileName, UriKind.Relative ) ) )
            {
            BackgroundTransferService.Remove( BackgroundTransferService.Find( req.RequestId ) );
            }
        }

    // Download the file into IsolatedStorage file named @"\shared\transfers\isolatedStorageFileName"
    try
        {
        await liveClient.BackgroundDownloadAsync( skydriveFileId + "/Content", new Uri( @"\shared\transfers\" + isolatedStorageFileName, UriKind.Relative ) );
        }
    catch ( TaskCanceledException exception )
        {
        Debug.WriteLine( "Download canceled: " + exception.Message );
        }

    // Get a reference to the Local Folder
    var storageFolder = await GetSharedTransfersFolder<T>();

    // Read the file data
    var fileData = await StorageHelper.ReadFileAsync<T>( storageFolder, isolatedStorageFileName );
    return fileData;
    }