ConfigureAwait(false) doesn't make the continuation code run in another thread

Question

I have a code which runs on thread1. I call a function in a synchronic way (using async method but it shouldn't disturb me - async method doesn't turn code to be asynchronic).

I have an await with ConfigureAwait set to false, so I understood the code after it is a task continuation which suppose to run in a different thread than the code before the await (because ConfigureAwait was set to false).

By my test - All code run in the same thread. How is it? Why doesn't the code, below the await, run on a different thread? This is the code:

public async void F1()
{
    Console.WriteLine($"Thread.CurrentThread.ManagedThreadId={Thread.CurrentThread.ManagedThreadId}");
    int x = await F2().ConfigureAwait(false);
    Console.WriteLine($"Thread.CurrentThread.ManagedThreadId={Thread.CurrentThread.ManagedThreadId}");
}

private async Task<int> F2()
{
    Console.WriteLine("Begins F2");
    Console.WriteLine($"Thread.CurrentThread.ManagedThreadId={Thread.CurrentThread.ManagedThreadId}");
    Console.WriteLine("Finishes F2");
    return 7;
}

This is the output:

Thread.CurrentThread.ManagedThreadId=1
Begins F2
Thread.CurrentThread.ManagedThreadId=1
Finishes F2
Thread.CurrentThread.ManagedThreadId=1

Your code is not actually async. Try at least add await Task.Delay(...) or Task.Yield to F2 — Guru Stron
Aside to the question, async void is an anti-pattern and should not be used in most cases: markheath.net/post/async-antipatterns — Martin Costello

Hans Keﬆing Hans Keﬆing · Accepted Answer · 2020-05-22T12:53:03

It is not "supposed to run in a different ~~thread~~ context", it is allowed to. In the opposite case (ConfigureAwait(true)) it must continue on the same ~~thread~~ context.

Furthermore when there is nothing to await (inside the method), the "async" method runs synchronously so doesn't need to return to some ~~thread~~ context, it is still on it.

ConfigureAwait(false) doesn't make the continuation code run in another thread

2 Answers