What Happens to the Thread After an `await` Keyword in C# Async Programming?

Understanding Thread Behavior After the await Keyword

In asynchronous programming, the await keyword plays a crucial role in managing threads and ensuring efficient execution. However, it can be confusing to understand how the code proceeds after encountering await. This article aims to clarify this with a detailed exploration.

Consider the following code snippet as an example:

private async Task MyAsyncMethod()
{
    // Code before await
    await MyOtherAsyncMethod();
    // Code after await
}

private void MyMethod()
{
    Task task = MyAsyncMethod();
    task.Wait();
}

When the await keyword is encountered in MyAsyncMethod, control is returned to MyMethod. This is because MyAsyncMethod is marked as async. However, since task.Wait() is then called, the thread executing MyMethod is blocked, seemingly preventing the execution of the code after await.

Does a New Thread Execute the Code After await?

The answer is: maybe. The behavior depends on the implementation of the synchronization context that is "current" at the time the await expression is evaluated.

In the example provided, if the code is running on a UI thread, the continuation (the code after await) will be executed on the same UI thread. On the other hand, if the code is running on a thread-pool thread, the continuation may be executed on any available thread-pool thread.

Avoiding Thread Blockage

If the goal is to execute the code after await immediately, it is crucial to avoid blocking the thread with task.Wait() or task.Result. Instead, consider other options such as registering a callback or using await task itself.

Controlling Thread Affinity

For scenarios where specific thread affinity is required for the continuation, the ConfigureAwait method can be used. By passing false to ConfigureAwait, the continuation can be explicitly instructed to run on a different thread context.

Additional Resources

To deepen your understanding of thread behavior with await, refer to the following resources:

• MSDN documentation: https://docs.microsoft.com/en-us/dotnet/csharp/async/
• C# in Depth, 3rd edition: https://www.manning.com/books/csharp-in-depth-third-edition
• Tekpub screencast series: https://www.tekpub.com/category/257-asynchronous-programming-in-c-sharp

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