How to Safely Await Asynchronous Events in C#?

Handling asynchronous events in C#: Safely await asynchronous operations

Problem overview:

In C#, events are usually delegates without return values ​​that are used to notify subscribers of specific events in the application. However, when trying to use an asynchronous event handler, a problem can arise: the event is fired, but the application is closed before the asynchronous handler completes execution.

Subscribe to events using an asynchronous handler:

In order to handle events asynchronously, it is not recommended to modify the event signature itself to return a Task. Instead, you can use a registration method that accepts an awaitable callback function. This provides greater flexibility and easier integration with third-party components.

Implementation using registration method:

private List<Func<Task>> ShutdownCallbacks = new List<Func<Task>>();

public void RegisterShutdownCallback(Func<Task> callback)
{
    this.ShutdownCallbacks.Add(callback);
}

public async Task Shutdown()
{
    var callbackTasks = new List<Task>();
    foreach (var callback in this.ShutdownCallbacks)
    {
        callbackTasks.Add(callback());
    }

    await Task.WhenAll(callbackTasks);
}

In this example, the ShutdownCallbacks list stores callback functions that can be awaited. The Shutdown() method iterates through these callback functions and waits for them simultaneously. This ensures that all asynchronous handlers have completed before the shutdown process continues.

Call the event using a call list:

Alternatively, if you wish to preserve the existing event paradigm, you can use the GetInvocationList() method to call handlers sequentially and wait for the returned Tasks.

class A
{
    public event Func<object, EventArgs, Task> Shutdown;

    public async Task OnShutdown()
    {
        Func<object, EventArgs, Task> handler = Shutdown;

        if (handler == null)
        {
            return;
        }

        Delegate[] invocationList = handler.GetInvocationList();
        Task[] handlerTasks = new Task[invocationList.Length];

        for (int i = 0; i < invocationList.Length; i++)
        {
            handlerTasks[i] = ((Func<object, EventArgs, Task>)invocationList[i])(this, EventArgs.Empty);
        }

        await Task.WhenAll(handlerTasks);
    }
}

Here, OnShutdown() gets the event delegate, calls handlers and waits for their completion.

Conclusion:

While asynchronous event handlers may seem tempting, this approach is generally not recommended due to the potential for deadlocks and other problems. As mentioned above, using registered methods or call lists provides a more robust and easier to manage solution for asynchronously awaiting events in C#.

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