How to detect if we're on a UI thread?

Question

For the sake of argument, consider a UI thread as a thread that has had a call to Application.Run() or one of it's overloads called on it and has an active message loop running.

Is there a way of detecting if we're currently executing on such a thread?

The reason I want this is because I have a class with a private function that is long-running. The class itself is already multithreaded, and the usage of this class is such that it might be used from either the UI or from background threads doing processing. This function also falls into this net. But I don't want it to block up the UI thread. So I want to detect if I am running on a UI thread and if so, fork the function call into a background thread (probably ThreadPool, but that's a non-issue for this discussion). This is entirely well-behaved, but the background threads are probably relying on the output of the function, so blocking for them is better, whereas the UI thread is accessing it in a more "set-and-forget" manner.

Answer 1

If you have access to a Form or a Control, you can check the InvokeRequired property; this will return false if you are on the UI thread and true if you are not.. If it happens in a context where you cannot check against a Control, you could easily set up a static property in your program that you could check against. Store a reference to Thread.CurrentThread at startup, and compare Thread.CurrentThread to that reference when you need to know:

static class Program
{
    private static Thread _startupThread = null;

    [STAThread]
    static void Main()
    {
        _startupThread = Thread.CurrentThread;

        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);
        Application.Run(new Form1());
    }

    public static bool IsRunningOnStartupThread()
    {
        return Thread.CurrentThread == _startupThread;
    }
}

By calling Program.IsRunningOnStartupThread you will get a bool saying if you are or not.