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VSTHRD001 Avoid legacy thread switching methods

Switching to the UI thread should be done using JoinableTaskFactory.SwitchToMainThreadAsync rather than legacy methods such as Dispatcher.Invoke or ThreadHelper.Invoke. This avoids deadlocks and can reduce threadpool starvation.

Examples of patterns that are flagged by this analyzer

ThreadHelper.Generic.Invoke(delegate {
    DoSomething();
});

or

Dispatcher.CurrentDispatcher.BeginInvoke(delegate {
    DoSomething();
});

Solution

Use await SwitchToMainThreadAsync() instead, wrapping with the JoinableTaskFactory's Run or RunAsync method if necessary:

void Foo() {
    ThreadHelper.JoinableTaskFactory.Run(async delegate {
        await ThreadHelper.JoinableTaskFactory.SwitchToMainThreadAsync();
        DoSomething();
    });
}

In the above example, we obtain a JoinableTaskFactory instance from the ThreadHelper.JoinableTaskFactory static property as it exists within Visual Studio itself. Other applications should create and expose their own JoinableTaskContext and/or JoinableTaskFactory for use in code that run in these applications. See our doc on consuming JoinableTaskFactory from a library for more information.

Replacing Dispatcher.BeginInvoke

When updating calls to Dispatcher.BeginInvoke, there are a few considerations to consider.

  1. BeginInvoke schedules the delegate for execution later.
  2. BeginInvoke always executes the delegate on the dispatcher's thread.
  3. BeginInvoke schedules the delegate at some given priority, or default priority determined by the dispatcher.

To resolve a warning for such code, it is often sufficient to replace it with this, which is roughly equivalent:

await joinableTaskFactory.RunAsync(async delegate {
    await joinableTaskFactory.SwitchToMainThreadAsync(alwaysYield: true);
    DoSomething();
})

The first line in the delegate is necessary to match the behaviors of 1 and 2 on the above list. When the caller is known to already be on the main thread, you can simplify it slightly to this:

await joinableTaskFactory.RunAsync(async delegate {
    await Task.Yield();
    DoSomething();
})

Matching behavior 3 on the list above may be important when the dispatcher priority is specified in the BeginInvoke call and was chosen for a particular reason. In such a case, you can ensure that JoinableTaskFactory matches that priority instead of using its default by creating a special JoinableTaskFactory instance with the priority setting you require using the JoinableTaskFactory.WithPriority method.

Altogether, this might look like:

await joinableTaskFactory.WithPriority(DispatcherPriority.DataBind).RunAsync(async delegate {
    await joinableTaskFactory.SwitchToMainThreadAsync(alwaysYield: true);
    DoSomething();
})

Configuration

This analyzer is configurable via the vs-threading.LegacyThreadSwitchingMembers.txt file. See our configuration topic for more information.

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