How can I find out whether method executes on UI thread or not in a decoupled way?

Question

Here is the problem I have: I need to make sure an object is instantiated on the UI thread. If it is not, it should throw an exception. But how do I check inside a method whether it is running on the UI thread? Note: I do not want to pass any information into the object's constructor.

The perfect candidate would be the DispatcherSynchronizationContext (WPF implementation of SynchronizationContext) which internally holds a reference to Dispatcher which references the thread it's associated with, but unfortunately that field is private so there is no way for me to access it.

Answer 1

Small clarification, although there is typically only 1 UI thread there can be many UI threads. This is true for both WPF and WinForms.

The best way I've found to achieve this though is with a SynchronizationContext. Both WPF and WinForms will establish a SynchronizationContext on any thread they are running UI on. This is the function I use if I am not tied to any particular UI model.

public bool IsPossiblyUIThread() {
  return SynchronizationContext.Current != null;
}

Note, it is not in any way foolproof. It's possible for non-UI components to establish a SynchronizationContext and this would return true for a simple worker thread. Hence the non-authoritative name.

A slightly more reliable way to do this is as follows. But it requires you to reference at least a portion of WPF to implement.

public bool IsLikelyWpfUIThread() {
  var context = SynchronizationContext.Current;
  return context != null && context is DispatcherSynchronizationContext;
}

Answer 2

Dispatcher.CheckAccess() returns true if your code runs on the same Thread as the Dispatcher. It should work if there is only one Dispatcher/UIThread.