function onSourceDownloadProgressChanged(sender, eventArgs) {
    var myHost = document.getElementById("silverlightControl");
    var rectBar = myHost.content.findName("rectBar");  
    var rectBorder = myHost.content.findName("rectBorder");
    
    if (eventArgs.progress) rectBar.Width = eventArgs.progress * rectBorder.Width;
    else rectBar.Width = eventArgs.get_progress() * rectBorder.Width;
}