WPF ProgressBar - TargetParameterCountException

Question

I am making my first WPF application, where I use the Youtube .NET API to upload a video to Youtube using the ResumableUploader.

This ResumableUploader works asynchronously and provides an event AsyncOperationProgress to periodically report its progress percentage.

I want a ProgressBar that will display this progress percentage. Here is some of the code I have for that:

void BtnUpload_Click(object sender, RoutedEventArgs e) {
    // generate video

    uploader = new ResumableUploader();
    uploader.AsyncOperationCompleted += OnDone;
    uploader.AsyncOperationProgress += OnProgress;
    uploader.InsertAsync(authenticator, newVideo.YouTubeEntry, new UserState());
}

void OnProgress(object sender, AsyncOperationProgressEventArgs e) {
    Dispatcher.BeginInvoke((SendOrPostCallback)delegate {
        PgbUpload.Value = e.ProgressPercentage;
    }, DispatcherPriority.Background, null);
}

Where PgbUpload is my progress bar and the other identifiers are not important for the purpose of this question.

When I run this, OnProgress will be hit a few times, and then I will get a TargetParameterCountException. I have tried several different syntax for invoking the method asynchronously, none of which worked. I am sure the problem is the delegate because if I comment it out, the code works fine (but the ProgressBar isn't updated of course).

Here is the Exception Detail (partially in French):

System.Reflection.TargetParameterCountException was unhandled Message=Nombre de paramètres incorrects.
Source=mscorlib StackTrace: à System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks) à System.Delegate.DynamicInvokeImpl(Object[] args) à System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Boolean isSingleParameter) à System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler) à System.Windows.Threading.Dispatcher.WrappedInvoke(Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler) à System.Windows.Threading.DispatcherOperation.InvokeImpl() à System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object state) à System.Threading.ExecutionContext.runTryCode(Object userData) à System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData) à System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state) à System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) à System.Windows.Threading.DispatcherOperation.Invoke() à System.Windows.Threading.Dispatcher.ProcessQueue() à System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled) à MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled) à MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o) à System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Boolean isSingleParameter) à System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler) à System.Windows.Threading.Dispatcher.WrappedInvoke(Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler) à System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Boolean isSingleParameter) à System.Windows.Threading.Dispatcher.Invoke(DispatcherPriority priority, Delegate method, Object arg) à MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam) à MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg) à System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame) à System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame) à System.Windows.Threading.Dispatcher.Run() à System.Windows.Application.RunDispatcher(Object ignore) à System.Windows.Application.RunInternal(Window window) à System.Windows.Application.Run(Window window) à System.Windows.Application.Run() à WpfApplication3.App.Main() dans h:\razor\documents\visual studio 2010\Projects\WpfApplication3\WpfApplication3\obj\x86\Debug\App.g.cs:ligne 0 à System.AppDomain._nExecuteAssembly(Assembly assembly, String[] args) à System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args) à Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly() à System.Threading.ThreadHelper.ThreadStart_Context(Object state) à System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) à System.Threading.ThreadHelper.ThreadStart() InnerException:

Thanks for any help.

Edit: I just found out that if I don't use the Dispatcher and just call set the value directly, it works fine! Is OnProgress called on main UI thread? How can that be?

Can you provide the StackTrace of the TargetParameterCountException? — Jehof

Chad Chad · Accepted Answer · 2010-06-19T01:14:45

The uploader (or any async component) can sync with the thread that created it. There are probably a variety of ways to do this, but the one I've seen before goes like this:

public class ResumableUploader {
  private SynchronizationContext _syncContext;
  public event EventHandler<ProgressChangedEventArgs> OnProgressChanged;

  public ResumableUploader() {
    _syncContext = SynchronizationContext.Current; //Think of this as the current thread
  }

  private ReportProgressChanged(int progress) {
     if(OnProgressChanged != null) {
         _syncContext.Send(s => { OnProgressChanged(this, new ProgressChangedEventArgs(progress)); }, null);  //s is any data you want to pass in, here it is unused
     }
  }
}

Or, slightly more flexible but also more complex for the user/client would be if the user/client supplied the SynchronizationContext at instance creation:

public ResumableUploader(SynchronizationContext syncContext)

WPF ProgressBar - TargetParameterCountException

Edit: I just found out that if I don't use the Dispatcher and just call set the value directly, it works fine! Is OnProgress called on main UI thread? How can that be?

1 Answers