WPF MVVM 异步事件调用

Question

I am lost in this one, i want my Viewmodel to use a event delegate so i can subscribe to it, open some dialog and wait for the dialog result. Later the ViewModel should do whatever it wants with the dialog result.

Here is how i implemented it (resumed code):

public class MyViewModel()
{
   public delegate TributaryDocument SearchDocumentEventHandler();
   public event SearchDocumentEventHandler SearchDocument;

   //Command for the search button
   public CommandRelay SearchDocumentCommand { get; set; }

   //Document that i found in the dialog.
   public TributaryDocument Document { get; set; }

   public MyViewModel()
   {
      SearchDocumentCommand = new CommandRelay(DoSearchDocument);
   }

   //The command execution
   public void DoSearchDocument()
   {
       //Event used here !
       Document = SearchDocument?.Invoke();
   }
}

public class MyUIControl : UserControl
{
    public MainWindow MainWindow { get; }

    public MyUIControl()
    {
       MainWindow = Application.Current.Windows[0] as MainWindow;
       DataContextChanged += MyUIControl_DataContextChanged;
    }

    private void MyUIControl_DataContextChanged(object sender, DependencyPropertyChangedEventArgs e)
    {
        var modelView = (MyViewModel)DataContext;
        modelView.SearchDocument += MyUIControl_SearchDocument;
    }

    private TributaryDocument MyUIControl_SearchDocument()
    {
       //Dont know what to do here... i am lost on this part.
       return await MainWindow.ShowDialog(new MyDocumentSearcherDialog());
    }
}

//The signature for MainWindow.ShowDialog
public async Task<object> ShowDialog(object dialog)
{
   return await DialogHost.Show(dialog, "MainDialog");
}

MyDocumentSearcherDialog is just a dialog where i search and return a TributaryDocument object.

The problem to my understanding comes from this part (since i cant compile it):

private TributaryDocument MyUIControl_SearchDocument()
{
   return await MainWindow.ShowDialog(new MyDocumentSearcherDialog());
}

I cant use await without changing the method signature to async. If i change it to async then i must return a Task<TributaryDocument> and change the event delegate:

    public delegate Task<TributaryDocument> SearchDocumentEventHandler();

    //On MyUIControl
    private Task<TributaryDocument> MyUIControl_SearchDocument()
    {
       return await MainWindow.ShowDialog(new MyDocumentSearcherDialog());
    }

   //On MyViewModel
   public async void DoSearchDocument()
   {
       //Event used here !
       Document = await Task.Run(async () => await SearchDocument?.Invoke());
   }

If i do this i get the following exception:

Additional information: The calling thread must be STA, because many UI components require this.

Answer 1

It seems like all you need to do is to remove the Task.Run (there is no need to Offload to another thread in this situation). The Task.Run will definitely give you a STA Thread Exception if you are doing UI work from within.

However, in short the Async and Await Pattern will create a continuation with the current SynchronisationContext , so there is no need to worry about it.

public async void DoSearchDocument()
{ 
   await SearchDocument?.Invoke();
}

_{Note : Since this is an event, it's about the only place it's OK to use async void .}