Question

I have a busy indicator that I want to show when my application is doing some computation.

var uiThread = new Thread(() =>
    {
        autoResetEvent.Set();
        bussyWindowVM.Dispatcher = Dispatcher.CurrentDispatcher;
        Dispatcher.CurrentDispatcher.BeginInvoke((Action)delegate
            {
                var busyWindow = new BusyWindow
                {
                    DataContext = bussyWindowVM, 
                    Owner = Application.Current.MainWindow,
                    WindowStartupLocation = WindowStartupLocation.CenterOwner
                };
                busyWindow.Show();
            });

        Dispatcher.Run();
    });
// set single threaded apartment
uiThread.SetApartmentState(ApartmentState.STA);

// mark UI thread as background thread
uiThread.IsBackground = false;

// start the UI thread
uiThread.Start();

// wait until thread exits
autoResetEvent.WaitOne();

But when I run the application it throws

System.InvalidOperationException “{The calling thread cannot access
this object because a different thread owns it.}”

I am not able to figure out how to resolve this problem.

But if I remove Owner = Application.Current.MainWindow then everything works fine but the owner is not set correctly so wen I re-size my application the window doesnot come center of application also It stays on top when I minimize application.

Stack Trace:

   at System.Windows.Threading.DispatcherObject.VerifyAccess()
   at System.Windows.Application.get_MainWindow()
   at MyProject.MyViewModel.<>c__DisplayClass3.<CreateBusyWindow>b__2() in D:\MyStuff\Dev\Repo\MyProject\ViewModels\MyViewModel.cs:line 2482
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.Dispatcher.WrappedInvoke(Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.DispatcherOperation.InvokeImpl()
   at System.Threading.ExecutionContext.runTryCode(Object userData)
   at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Windows.Threading.DispatcherOperation.Invoke()
   at System.Windows.Threading.Dispatcher.ProcessQueue()
   at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.Dispatcher.WrappedInvoke(Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
   at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
   at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
   at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
   at CivilGeo.GeoHECRAS.ViewModels.GeoHECRASViewModel.<>c__DisplayClass3.<CreateBusyWindow>b__1() in D:\MyStuff\Dev\CivilGeoRepo\GeoHECRAS\CivilGeo.GeoHECRAS\ViewModels\GeoHECRASViewModel.cs:line 2492
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()

Answer 1

Dispatcher.CurrentDispatcher in your background thread creates a brand new Dispatcher for that thread.
This dispatcher cannot access objects from the original UI thread, such as Application.Current.MainWindow.

If you want to run a separate UI thread, you won’t be able to set Owner. Also, you shouldn’t call BeginInvoke().