Threadline

Which thread runs the code after await?

Read the explanation
What if

FetchAsync starts on the caller's thread. After await Task.Delay, the rest of it, and of Main, runs on a thread-pool thread.

Predict first

Main starts on thread 1 and awaits Task.Delay inside FetchAsync. Which thread runs the code after the await?

012345678thread 1: Main startsMainthread 1: FetchAsync startsFetchwaits for Main's Task to finishpool thread: FetchAsync resumesresumespool thread: return "data"returnpool thread: Main resumesMain

Tap any block to see what that thread is doing.

  • Running
  • Waiting / parked
Step 1/5

Main starts on thread 1.

1public static async Task Main()2{3    Log("Main starts");4    string data = await FetchAsync();5    Log($"Main resumes with {data}");6}7 8static async Task<string> FetchAsync()9{10    Log("FetchAsync starts"); // still the caller's thread11    await Task.Delay(100); // a network call: no thread waits for it12    Log("FetchAsync resumes"); // a thread-pool thread13    return "data";14}

Explanation

What just happened

Main started on thread 1 and called FetchAsync, which ran on that same thread up to its await. Task.Delay needs no thread while it waits: FetchAsync returned to its caller, and a timer resumed it later. The rest of FetchAsync, and then Main, ran on a thread-pool thread, not thread 1.

Try the what-if switch

Why

await splits a method in two: the part before runs now, the part after is scheduled when the awaited task finishes. Where that second part runs depends on the SynchronizationContext. A console app has none, so it runs on the pool.

Watch out

Don’t assume you’re on the same thread after await. Anything tied to the thread, like [ThreadStatic] fields or ThreadLocal<T> values, can be different on the other side.

Common questions

Does await create a new thread in C#?

No. The code before the first await runs on the caller's thread. While the awaited work is pending, the method has returned to its caller; when it completes, the rest runs on whichever thread the context picks.

Which thread runs the code after await?

In a console app, a thread-pool thread: there's no SynchronizationContext to return to. In a UI app, the context posts it back to the UI thread.

When should I use Task.Run?

For CPU-bound work you want off the current thread. Task.Run queues it on the thread pool; awaiting it frees the caller until it's done.

Verified: this timeline matches a real program, checked in CI on .NET 8 and 10. How we verify. Unclear or wrong? Tell us