Thread invoke the main window?

Question

So basically I have a MainWindow, a class SetupViewModel defined in a separate cs file, and another class ImageViewModel in a separate cs file; I am wondering is there anyway I can invoke the operation in MainWindow from ImageViewModel. More details listed below:

• in ImageViewModel, invoke the change in MainWindow as following (following code in MainWindow):

  this.Dispatcher.BeginInvoke(System.Windows.Threading.DispatcherPriority.Normal,
                  new Action(
                      delegate()
                      {
                              ((SetupViewModel)this.DataContext).ViewType = Convert.ToInt32(ViewTypes.ViewType2D);
                      }
                  )
              );
  

• The ViewType variable is defined in SetupViewModel as follows

  private int _viewType;
      public int ViewType
      {
          get
          {
              return _viewType;
          }
          set
          {
              _viewType = value;
              OnPropertyChanged("ViewType");
          }
      }
  

• Here is how I did, but was not successful; The invoke code in ImageViewModel:

  try { Action SwitchTo2DView = delegate() { ((CaptureSetupViewModel)System.Windows.Application.Current.MainWindow.DataContext).ViewType = Convert.ToInt32(ViewTypes.ViewType2D); };

              System.Windows.Application.Current.MainWindow.Dispatcher.Invoke(System.Windows.Threading.DispatcherPriority.Normal, SwitchTo2DView);
          }
          catch (Exception ex)
          {
              Console.WriteLine("Caught exception: " + ex.ToString());
          }
  

and the exception:

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.NullReferenceException: Object reference not set to an instance of an object.
   at CaptureSetupDll.ViewModel.LiveImageViewModel.<StartZStackPreview>b__2c()
   --- End of inner exception stack trace ---
   at System.RuntimeMethodHandle._InvokeMethodFast(Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner)
   at System.RuntimeMethodHandle.InvokeMethodFast(Object target, Object[] arguments, Signature sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks)
   at System.Delegate.DynamicInvokeImpl(Object[] args)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Boolean isSingleParameter)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler)
   at System.Windows.Threading.Dispatcher.WrappedInvoke(Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler)
   at System.Windows.Threading.DispatcherOperation.InvokeImpl()
   at System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object state)
   at System.Threading.ExecutionContext.runTryCode(Object userData)
   at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
   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, Boolean isSingleParameter)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler)
   at System.Windows.Threading.Dispatcher.WrappedInvoke(Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler)
   at System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Boolean isSingleParameter)
   at System.Windows.Threading.Dispatcher.Invoke(DispatcherPriority priority, Delegate method, Object arg)
   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 System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
   at System.Windows.Threading.DispatcherOperation.Wait(TimeSpan timeout)
   at System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Boolean isSingleParameter)
   at System.Windows.Threading.Dispatcher.Invoke(DispatcherPriority priority, Delegate method)
   at CaptureSetupDll.ViewModel.LiveImageViewModel.StartZStackPreview()

I am a layman to C#; Any idea how I can make this work? Thanks a lot.

Answer 1

I'll bet you five bucks your DataContext is null at the time this is being called. Which means its probably being called too early. Which probably means you should push whatever you're doing off on the Dispatcher.

While you may assume your application is completely loaded at the time this method is being called, it isn't necessarily true. To test this, set a breakpoint during debugging and check the DataContext to see if it's null.

If it is null, you have two choices. First, just return. Depending on how the method is being called, it may be called multiple times during load. A later call may reveal a non-null context.

The other option is to use the Dispatcher to re-call the method later, once the application is loaded.

Here's a little pseudocode which looks kind of like C# and may even compile:

public void CheckTheDataContext()
{
    // is it null?
    if(this.DataContext == null)
    {
        // then drop an Action re-invoking this method later
        // when the application idles out a bit
        Dispatcher.BeginInvoke((Action)(() =>
        {
            CheckTheDataContext();
        }), System.Windows.Threading.DispatcherPriority.ApplicationIdle);
        return;
    }
    DoSomethingElseWithTheContext(DataContext);
}