C# Async void of event handler got warning/error

Question

I understand the async should not use void as return type unless it is an event handler. But I have code snippet above, when I turn on warning as error in my project setting I got the error above when I compile code.

RECS0165 Asynchronous method '' should not return void

If I remove async then I get another compile error

The 'await' operator can only be used within an async lambda expression. Consider marking this lambda expression with the 'async' modifier.

The suggested fix is to add async to the anonymous function. It is a dead lock.

Do I do something wrong here?

The steps to reproduce the issue:

• Create a blank UWP project in VS2017, it will create app,xaml and MainPage.Xaml.

• Add following code in in MainPage.Xaml.cs

  namespace App4 { using System; using System.Threading.Tasks; using Windows.ApplicationModel.Core; using Windows.UI.Core; using Windows.UI.Xaml; using Windows.UI.Xaml.Controls;

  public sealed partial class MainPage : Page
  {
      private DispatcherTimer refreshTimer;
  
      public MainPage()
      {
          this.InitializeComponent();
  
          this.refreshTimer = new DispatcherTimer()
          {
              Interval = new TimeSpan(0, 0, 30)
          };
  
          refreshTimer.Tick += async (sender, e) => { await DisplayMostRecentLocationData(string.Empty); };
      }
  
      private async Task DisplayMostRecentLocationData(string s)
      {
          await CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
          {
  
          });
      }
  }
  

  }

Answer 1

The issue actually comes from Refactoring Essentials.

https://social.msdn.microsoft.com/Forums/vstudio/en-US/a9b67355-99c8-429f-91d0-65e578b2a7f1/c-async-void-return-type-of-event-handler-got-warningerror?forum=csharpgeneral&prof=required

https://github.com/icsharpcode/RefactoringEssentials/issues/280

Answer 2

The Tick event handler delegate is incorrect.

Use

async (sender, e) => ...

or use an EventArg derived class for e

async (object sender, EventArgs e) => ...    

What you currently have is an anonymous object that you are trying to assign as an event handler. The compiler does not allow that hence the error.