How to Handle Asynchronous Events in C#?

Detailed explanation of C# asynchronous event processing

Introduction

C# event mechanism is a powerful tool for handling asynchronous operations. However, traditional events are void, which makes it impossible to wait for their execution to complete. This article explores several alternative ways to implement asynchronous event handling.

Challenge

Consider the following code snippet, where a shutdown event needs to be handled with an asynchronous task:

public event EventHandler<EventArgs> GameShuttingDown;

public async Task ShutdownGame()
{
    // ...
    await this.NotifyGameShuttingDown();
    await this.SaveWorlds();
    this.NotifyGameShutDown();
    // ...
}

private async Task SaveWorlds()
{
    // ...
}

The problem is that the NotifyGameShuttingDown event handler is returnless and calling it within the ShutdownGame method will prevent the asynchronous save task from completing before the event fires.

Solution 1: Custom asynchronous event delegation

One way is to define a custom event delegate that returns a Task, allowing the execution of the wait handler:

public event Func<object, EventArgs, Task> Shutdown;

In the ShutdownGame method, the handler can be called like this:

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);

Solution 2: Registration-based method

Another option is to use a registration-based approach, where callbacks can be registered and executed asynchronously:

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);
}

Notes

• Design Impact: Using asynchronous event handlers may change the design of your application, so consider the potential impact carefully.
• Complexity: Custom delegate methods add additional complexity, but it ensures compatibility with existing event patterns.
• Performance: A registration-based approach may be more efficient because it avoids the overhead of reflection when calling handlers.

Recommended

The recommended approach depends on the specific needs of the application. For existing code bases and heavily event-based designs, custom delegate methods are preferred. For new applications or those that do not rely on events, a registration-based approach can provide better performance and flexibility.

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