Detecting whether on UI thread in WPF and Winforms

Question

I've written an assertion method Ensure.CurrentlyOnUiThread(), below, that checks that the current thread is a UI thread.

• Is this going to be reliable in detecting the Winforms UI thread?
• Our app is mixed WPF and Winforms, how best to detect a valid WPF UI thread?
• Is there a better way to do this? Perhaps code contracts?

Ensure.cs

using System.Diagnostics;
using System.Windows.Forms;

public static class Ensure
{
    [Conditional("DEBUG")]
    public static void CurrentlyOnUiThread()
    {
        if (!Application.MessageLoop)
        {
            throw new ThreadStateException("Assertion failed: not on the UI thread");
        }
    }
}

Answer 1

Don't use

if(Dispatcher.CurrentDispatcher.Thread == Thread.CurrentThread)
{
   // Do something
}

Dispatcher.CurrentDispatcher will, if the current thread do not have a dispatcher, create and return a new Dispatcher associated with the current thread.

Instead do like this

Dispatcher dispatcher = Dispatcher.FromThread(Thread.CurrentThread);
if (dispatcher != null)
{
   // We know the thread have a dispatcher that we can use.
}

To be sure you have the correct dispatcher or are on the correct thread you have the following options

Dispatcher _myDispatcher;

public void UnknownThreadCalling()
{
    if (_myDispatcher.CheckAccess())
    {
        // Calling thread is associated with the Dispatcher
    }

    try
    {
        _myDispatcher.VerifyAccess();

        // Calling thread is associated with the Dispatcher
    }
    catch (InvalidOperationException)
    {
        // Thread can't use dispatcher
    }
}

CheckAccess() and VerifyAccess() do not show up in intellisense.

Also, if you have to resort to these kinds of things its likely due to bad design. You should know which threads run what code in your program.