WPF: Cannot perform this operation while dispatcher processing is suspended when checking if window is modal

Question

I am getting the following exception when checking if a window is modal:

System.InvalidOperationException: Cannot perform this operation while dispatcher processing is suspended.
at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
at System.Windows.Threading.DispatcherOperation.Wait(TimeSpan timeout)
at System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherOperation operation, CancellationToken cancellationToken, TimeSpan timeout)
at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)

I am checking if the window is modal in the following manner:

(bool)AssociatedObject.Dispatcher.Invoke(new Func<bool>(() => ComponentDispatcher.IsThreadModal));

I am using .Net Framework 4.0

What might be causing this error and how to fix it?

Answer 1

I was able to fix the problem by changing the way I check if a given window is modal. I changed the code to:

return (bool)typeof(Window).GetField("_showingAsDialog", BindingFlags.Instance | BindingFlags.NonPublic).GetValue(AssociatedObject);

When examining the Window class one can notice that this field is being set to true when the window is being shown as dialog.

https://referencesource.microsoft.com/#PresentationFramework/src/Framework/System/Windows/Window.cs,61d39b218b53dbbe,references

Above link points to the code for .Net Framework 4.8. However, the same applies for .Net Framework 4.0.