How to 'await' raising an EventHandler event

Question

Sometimes the event pattern is used to raise events in MVVM applications by or a child viewmodel to send a message to its parent viewmodel in a loosely coupled way like this.

Parent ViewModel

searchWidgetViewModel.SearchRequest += (s,e) =>  {     SearchOrders(searchWidgitViewModel.SearchCriteria); }; 

SearchWidget ViewModel

public event EventHandler SearchRequest;  SearchCommand = new RelayCommand(() => {      IsSearching = true;     if (SearchRequest != null)      {         SearchRequest(this, EventArgs.Empty);     }     IsSearching = false; }); 

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In refactoring my application for .NET4.5 I am making as much as code possible to use async and await. However the following doesn't work (well I really wasn't expecting it to)

 await SearchRequest(this, EventArgs.Empty); 

The framework definitely does this to call event handlers such as this, but I'm not sure how it does it?

private async void button1_Click(object sender, RoutedEventArgs e) {    textBlock1.Text = "Click Started";    await DoWork();    textBlock2.Text = "Click Finished"; } 

Anything I've found on the subject of raising events asynchrously is ancient but I can't find something in the framework to support this.

How can I await the calling of an event but remain on the UI thread.

Answer 1

Edit: This doesn't work well for multiple subscribers, so unless you only have one I wouldn't recommend using this.

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Feels slightly hacky - but I have never found anything better:

Declare a delegate. This is identical to EventHandler but returns a task instead of void

public delegate Task AsyncEventHandler(object sender, EventArgs e); 

You can then run the following and as long as the handler declared in the parent uses async and await properly then this will run asynchronously:

if (SearchRequest != null)  {     Debug.WriteLine("Starting...");     await SearchRequest(this, EventArgs.Empty);     Debug.WriteLine("Completed"); } 

Sample handler:

 // declare handler for search request  myViewModel.SearchRequest += async (s, e) =>  {                          await SearchOrders();  }; 

Note: I've never tested this with multiple subscribers and not sure how this will work - so if you need multiple subscribers then make sure to test it carefully.