How Can Reflection Simplify Async/Await Task Creation from Events?

Task Completion from Events Using Reflection

The async/await pattern allows for easy task creation upon event firing. However, this pattern requires a specific FromEvent method for each event. To simplify this, a more generalized solution was desired, where one FromEvent method could handle any event on any instance.

While passing the event directly or as a string faced limitations, reflection offered a potential path. However, dynamically creating a handler to access the TaskCompletionSource in reflection posed challenges.

Custom DynamicMethod Invocation

The key solution involved creating a dynamic method that contained the desired logic. This method's first parameter would be a TaskCompletionSourceHolder instance. This allowed for accessing the TaskCompletionSource from within the generated IL.

Helper Class and Extension Method

A helper class, TaskCompletionSourceHolder, encapsulated the TaskCompletionSource and provided a method to set its result. The extension method FromEvent took the source object and event name as parameters. It:

• Created a new TaskCompletionSource and TaskCompletionSourceHolder instance.
• Retrieved the event information using reflection.
• Created a delegate type for the event using reflection.
• Verified that the delegate type returned void.
• Generated a dynamic method that stored the arguments passed to the event handler in an array.
• Invoked the TaskCompletionSourceHolder.SetResult method to set the task's result.
• Created a delegate from the dynamic method and added it as an event handler.

Supported Event Types

This solution supports any void-returning event, regardless of the parameter list. Additional improvements can be made to support any return value type if needed.

Example Usage

Below is an example of how to use the custom FromEvent method:

static async void Run() {
    object[] result = await new MyClass().FromEvent("Fired");
    // Output: 123, abcd
}

public class MyClass {
    // Event delegate type with two parameters
    public delegate void TwoThings(int x, string y);

    // Constructor to trigger event after a delay
    public MyClass() {
        new Thread(() => {
            Thread.Sleep(1000);
            Fired(123, "abcd");
        }).Start();
    }

    // Event using the custom TwoThings delegate
    public event TwoThings Fired;
}

Using our generalized FromEvent method, we can specify any event type without the need for an explicit task type, thus simplifying event handling in async/await scenarios.

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