How to run something in the STA thread?

Question

In my WPF application I do some async communication (with server). In the callback function I end up creating InkPresenter objects from the result from server. This requires the running thread to be STA, which apparently it currently isn't. Therefore I get the following exception:

Cannot create instance of 'InkPresenter' defined in assembly [..] The calling thread must be STA, because many UI components require this.

Currently my async function call is like this:

public void SearchForFooAsync(string searchString)
{
    var caller = new Func<string, Foo>(_patientProxy.SearchForFoo);
    caller.BeginInvoke(searchString, new AsyncCallback(SearchForFooCallbackMethod), null);
}

How can I make the callback - which will do the InkPresenter creation - be STA? Or invoke the XamlReader parsing in a new STA thread.

public void SearchForFooCallbackMethod(IAsyncResult ar)
{
    var foo = GetFooFromAsyncResult(ar); 
    var inkPresenter = XamlReader.Parse(foo.Xaml) as InkPresenter; // <!-- Requires STA
    [..]
}

Answer 1

You can use the Dipatcher class to execute the method call on the UI-Thread. The Dispatcher provides the static property CurrentDispatcher to get the dispatcher of a thread.

If your object of the class, that creates the InkPresenter, is created on the UI-Thread, then the CurrentDispatcher method returns the Dispatcher of the UI-Thread.

On the Dispatcher you can call the BeginInvoke-method to call the specified delegate asynchronously on the thread.

Answer 2

It should be good enough to call it on the UI thread. Therefore, use a BackgroundWorker and on the RunWorkerAsyncCompleted, you can then do the creation of the inkPresenter.

Answer 3

You can start STA Threads like so:

    Thread thread = new Thread(MethodWhichRequiresSTA);
    thread.SetApartmentState(ApartmentState.STA); //Set the thread to STA
    thread.Start(); 
    thread.Join(); //Wait for the thread to end

The only problem is that your result object must be passed along somehow.. You can use a private field for that, or dive into passing along parameters into threads. Here I set the foo data in a private field and start up the STA Thread to mutate the inkpresenter!

private var foo;
public void SearchForFooCallbackMethod(IAsyncResult ar)
{
    foo = GetFooFromAsyncResult(ar); 
    Thread thread = new Thread(ProcessInkPresenter);
    thread.SetApartmentState(ApartmentState.STA);
    thread.Start();
    thread.Join(); 
}

private void ProcessInkPresenter()
{
    var inkPresenter = XamlReader.Parse(foo.Xaml) as InkPresenter;
}

Hope this helps!

Answer 4

It's a bit of a hack, but I would use XTATestRunner So your code will look like:

    public void SearchForFooAsync(string searchString)
    {
        var caller = new Func<string, Foo>(_patientProxy.SearchForFoo);
        caller.BeginInvoke(searchString, new AsyncCallback(SearchForFooCallbackMethod), null);
    }

    public void SearchForFooCallbackMethod(IAsyncResult ar)
    {

            var foo = GetFooFromAsyncResult(ar); 
InkPresenter inkPresenter;
            new XTATestRunner().RunSTA(() => {
                        inkPresenter = XamlReader.Parse(foo.Xaml) as InkPresenter;
                    });
    }

as a bonus it's possible to catch exceptions thrown in STA (or MTA) thread like this:

try{
new XTATestRunner().RunSTA(() => {
                        throw new InvalidOperationException();
                    });
}
catch(InvalidOperationException ex){
}