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c#winformsmultithreadingui-thread
Matthew Scharley
Matthew Scharley

Reputation: 132264

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

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.

Upvotes: 16

Views: 20331

Answers (6)

nawfal
nawfal

Reputation: 73173

You could use any of these:

bool isUIThread1 = SynchronizationContext.Current != null;
bool isUIThread2 = Application.MessageLoop;
bool isUIThread3 = Thread.CurrentThread.GetApartmentState() == ApartmentState.STA;

For SynchronizationContext.Current to have a value, the first form should be shown. Its also possible to setSynchronizationContext.Current to something from non-UI contexts as well as to set it to null from UI context (this is often seen to by-pass UI deadlocks). So in a way it is not completely reliable.

Application.MessageLoop is a better choice but that again requires the message pump to be started (i.e. first form shown).

GetApartmentState can be used in your Main method, even before showing any form but you need [STAThread] set on Main. It would be extremely rare for serious applications to miss that. In a way this approach is the soundest.

For all practical purposes you can use any of those.

Upvotes: 4

henon
henon

Reputation: 2501

There is no answer for WPF yet, so for the record, in WPF you can use the return value of Application.Current.Dispatcher.CheckAccess() to know if you are on the GUI thread or not.

It returns true if the current thread is the UI thread in WPF.

Upvotes: 7

Fredrik Mörk
Fredrik Mörk

Reputation: 158309

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.

Upvotes: 13

Ross Anderson
Ross Anderson

Reputation: 111

bool isMessageLoopThread = System.Windows.Forms.Application.MessageLoop;

Upvotes: 11

Jon Skeet
Jon Skeet

Reputation: 1500515

I would suggest that it's the kind of decision the caller should make. You could always write wrapper methods to make it easier - but it means that you won't have problems with the caller being in an "odd" situation (e.g. a UI framework you don't know about, or something else with an event loop) and you making the wrong decision for them.

If the method ever needs to provide feedback in the right thread, I'd pass in an ISynchronizeInvoke (implemented by Control) to do that in a UI-agnostic way.

Upvotes: 7

Mitch Wheat
Mitch Wheat

Reputation: 300549

Please see Detecting running in Main Thread in C# library

Upvotes: 6

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