How Does Async-Await Improve Application Responsiveness Without Creating New Threads?

Async-Await: Enhancing Responsiveness Without Extra Threads

A common misunderstanding is that async-await creates new threads. This article clarifies how async-await improves application responsiveness without requiring additional threads.

The Async-Await Mechanism

Async-await cleverly divides a method into two parts:

• Pre-Await: Code executed before the await keyword.
• Post-Await: Code executed after the await keyword.

The Process

1. Initial Execution: The pre-await code runs on the main thread, managed by the message loop.
2. Await and Return: Upon encountering await, the method returns to its caller, signaling it's waiting for an asynchronous operation (like GetSomethingAsync).
3. Message Loop Continues: The message loop, unblocked, processes other events, keeping the UI responsive.
4. Asynchronous Completion: Once the asynchronous operation finishes, a callback is triggered, notifying the SynchronizationContext.
5. Message Queueing: The SynchronizationContext adds a message to the message loop queue, indicating the post-await code is ready.
6. Method Resumption: The message loop retrieves the message and resumes execution of the method from where it left off after await.

Preventing UI Freezes

This division of the method ensures UI tasks (redrawing, message handling) continue even during lengthy asynchronous operations.

Clearing Up the Misconception

While async-await doesn't create threads itself, underlying asynchronous operations might use threads. However, this thread usage is separate from async-await's responsiveness enhancement.

Further Reading

• Asynchronous Programming with Async and Await
• SynchronizationContext Class
• Stephen Cleary - There is no thread
• Channel 9 - Mads Torgersen: Inside C# Async

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