如何从Silverlight中的HttpWebRequest.BeginGetRequestStream中更新UI

如何做到这一点,我已经尝试从将数据推送到流中的循环中调用Dispatch.BeginInvoke(如下所示),但它会锁定浏览器直到完成,因此它似乎处于某种工作线程/ui线程死锁状态。

这是我正在做的工作的代码片段:

class RequestState
{
    public HttpWebRequest request;  // holds the request
    public FileDialogFileInfo file; // store our file stream data

    public RequestState( HttpWebRequest request, FileDialogFileInfo file )
    {
        this.request = request;
        this.file = file;
    }
}

private void UploadFile( FileDialogFileInfo file )
{
    UriBuilder ub = new UriBuilder( app.receiverURL );
    ub.Query = string.Format( "filename={0}", file.Name );

    // Open the selected file to read.
    HttpWebRequest request = (HttpWebRequest)WebRequest.Create( ub.Uri );
    request.Method = "POST";

    RequestState state = new RequestState( request, file );
    request.BeginGetRequestStream( new AsyncCallback( OnUploadReadCallback ), state );
}

private void OnUploadReadCallback( IAsyncResult asynchronousResult )
{
    RequestState state = (RequestState)asynchronousResult.AsyncState;
    HttpWebRequest request = (HttpWebRequest)state.request;

    Stream postStream = request.EndGetRequestStream( asynchronousResult );
    PushData( state.file, postStream );
    postStream.Close();

    state.request.BeginGetResponse( new AsyncCallback( OnUploadResponseCallback ), state.request );
}

private void PushData( FileDialogFileInfo file, Stream output )
{
    byte[] buffer = new byte[ 4096 ];
    int bytesRead = 0;

    Stream input = file.OpenRead();
    while( ( bytesRead = input.Read( buffer, 0, buffer.Length ) ) != 0 )
    {
        output.Write( buffer, 0, bytesRead );
        bytesReadTotal += bytesRead;

        App app = App.Current as App;
        int totalPercentage = Convert.ToInt32( ( bytesReadTotal / app.totalBytesToUpload ) * 100 );

        // enabling the following locks up my UI and browser
        Dispatcher.BeginInvoke( () =>
        {
            this.ProgressBarWithPercentage.Percentage = totalPercentage;
        } );
    }
}