CODEWITHSUNDEEP
c#eventsasynchronous
vpetrovic
vpetrovic

Reputation: 527

Async event handler callback through event arguments

I have a problem when I need to collect event argument data that has been changed through event hadnler that contains async calls. As you can see in code below, MessageReceiver raises event and collects data from property "Change", and continues processing. Problem is when event handler is async because it calles asny method with "await". In method OnMyEvent code continues immediately after calling the handler, it does not wait for handler to finish, so "Change" property is not set... How to make things more "in sync"?

public class MyEventArgs:EventArgs
{
    public bool Change { get; set; }
}

public delegate void MyEventHandler(object sender, MyEventArgs e);

public class MessageReceiver
{
    public event MyEventHandler MyEvent;

    public void RaiseEvent()
    {
        OnMyEvent();
    }

    protected void OnMyEvent()
    {
        MyEventArgs e = new MyEventArgs();
        if (MyEvent != null)
            MyEvent(this, e);
        if (e.Change)
            Console.WriteLine("Change occured");
    }
}
public class Form
{
    MessageReceiver mr;
    public Form(MessageReceiver MR)
    {
        mr = MR;
        mr.MyEvent += Mr_MyEvent;
    }

    private async void Mr_MyEvent(object sender, MyEventArgs e)
    {
        string s = await GetString();
        e.Change = true;
    }

    public async Task<string> GetString()
    {
        return await Task.Factory.StartNew(() =>
        {
            return Guid.NewGuid().ToString();
        }
            );
    }
}
public class Program
{
    static void Main(string[] args)
    {
        MessageReceiver mr = new MessageReceiver();
        Form frm = new Form(mr);
        mr.RaiseEvent();
    }
}

Upvotes: 3

Views: 1104

Answers (1)

Stephen Cleary
Stephen Cleary

Reputation: 456497

I discuss a variety of approaches for asynchronous events on my blog. I recommend the deferral approach, which you can implement using the DeferralManager and IDeferralSource types from my Nito.AsyncEx.Coordination library:

public class MyEventArgs: EventArgs, IDeferralSource
{
  private readonly DeferralManager _deferralManager;
  public MyEventArgs(DeferralManager deferralManager)
  {
    _deferralManager = deferralManager;
  }
  public bool Change { get; set; }
  public IDisposable GetDeferral() { return _deferralManager.GetDeferral(); }
}

public class MessageReceiver
{
  protected async Task OnMyEventAsync()
  {
    if (MyEvent != null)
    {
      DeferralManager deferralManager = new DeferralManager();
      MyEventArgs e = new MyEventArgs(deferralManager);
      MyEvent(this, e);
      await deferralManager.WaitForDeferralsAsync();
    }
    if (e.Change)
      Console.WriteLine("Change occured");
  }
}

public class Form
{
  private async void Mr_MyEvent(object sender, MyEventArgs e)
  {
    using (e.GetDeferral())
    {
      string s = await GetString();
      e.Change = true;
    }
  }
}

Upvotes: 2

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