How to Insert Characters to a File Using C#

What thread runs the code after the `await` keyword?

Code as posted will "Deadlock" in Winform App if called from main thread because you're blocking the main thread with the Wait().

But in console app this works. but how?

Answer is hidden in the SynchronizationContext.Current. await captures the "SynchronizationContext" and when the task is completed it will continue in the same "SynchronizationContext".

In winform app SynchronizationContext.Current will be set to WindowsFormsSynchronizationContext which will post to the call to "Message loop", but who is going to process that? out main thread is waiting in Wait().

In console app SynchronizationContext.Current will not be set by default so it will be null when no "SynchronizationContext" available for await to capture so it will schedule the continuation to ThreadPool(TaskScheduler.Default which is ThreadpoolTaskScheduler) and so the code after await works(through threadpool thread).

Aforementioned capturing behavior can be controlled using Task.ConfigureAwait(false); which will prevent winform app from deadlocking but code after await no longer runs in UI thread.

Why does the code after await sometimes run on another thread?

Because it can.

You awaited so if the Task wasn't already completed the thread you were running on went back to the pool, potentially to do something useful. Then the Task you were awaiting finished, so a thread was needed to continue. There's no reason why it should be the same thread as before.

does async/await method run in the same thread as caller?

Have I missed something?

Sort of. await doesn't know anything about threads. It causes the code after the await keyword (by default) to run in the same SynchronizationContext as the caller, if it exists.

In the case of a WPF Application, the current SynchronizationContext is setup to marshal the call back to the UI thread, which means it will end up on the UI thread. (The same is true in Windows Forms). In a Console Application, there is no SynchronizationContext, so it can't post back onto that context, which means you'll end up on a ThreadPool thread.

If you were to install a custom SynchronizationContext into SynchronizationContext.Current, the call would post there. This is not common in Console Applications, however, as it typically requires something like a "message loop" to exist to work properly.

How and when to use ‘async’ and ‘await’

When using async and await the compiler generates a state machine in the background.

Here's an example on which I hope I can explain some of the high-level details that are going on:

public async Task MyMethodAsync()
{
    Task<int> longRunningTask = LongRunningOperationAsync();
    // independent work which doesn't need the result of LongRunningOperationAsync can be done here

    //and now we call await on the task 
    int result = await longRunningTask;
    //use the result 
    Console.WriteLine(result);
}

public async Task<int> LongRunningOperationAsync() // assume we return an int from this long running operation 
{
    await Task.Delay(1000); // 1 second delay
    return 1;
}

OK, so what happens here:

1. Task<int> longRunningTask = LongRunningOperationAsync(); starts executing LongRunningOperation

2. Independent work is done on let's assume the Main Thread (Thread ID = 1) then await longRunningTask is reached.

   Now, if the longRunningTask hasn't finished and it is still running, MyMethodAsync() will return to its calling method, thus the main thread doesn't get blocked. When the longRunningTask is done then a thread from the ThreadPool (can be any thread) will return to MyMethodAsync() in its previous context and continue execution (in this case printing the result to the console).

A second case would be that the longRunningTask has already finished its execution and the result is available. When reaching the await longRunningTask we already have the result so the code will continue executing on the very same thread. (in this case printing result to console). Of course this is not the case for the above example, where there's a Task.Delay(1000) involved.

Does the use of async/await create a new thread?

In short NO

From Asynchronous Programming with Async and Await : Threads

The async and await keywords don't cause additional threads to be
created. Async methods don't require multithreading because an async
method doesn't run on its own thread. The method runs on the current
synchronization context and uses time on the thread only when the
method is active. You can use Task.Run to move CPU-bound work to a
background thread, but a background thread doesn't help with a process
that's just waiting for results to become available.

await keyword blocks main thread

I think you misunderstand what async means. It doesn't mean that the method runs in another thread!!

An async method runs synchronously until the first await, then returns a Task to the caller (unless it's async void, then it returns nothing). When the task that is being awaited completes, execution resumes after the await, usually on the same thread (if it has a SynchronizationContext).

In your case, the Thread.Sleep is before the first await, so it's executed synchronously, before control is returned to the caller. But even if it was after the await, it would still block the UI thread, unless you specifically configured the the awaiter not to capture the synchronization context (using ConfigureAwait(false)).

Thread.Sleep is a blocking method. If you want an async equivalent, use await Task.Delay(3000), as suggested in Sriram Sakthivel's answer. It will return immediately, and resume after 3 seconds, without blocking the UI thread.

It's a common misconception that async is related to multithreading. It can be, but in many cases it's not. A new thread is not implicitly spawned just because the method is async; for a new thread to spawn, it has to be done explicitly at some point. If you specifically want the method to run on a different thread, use Task.Run.

Does async run in a separate thread?

No, it does not. It MAY start another thread internally and return that task, but the general idea is that it does not run on any thread.

Let me explain. The general use of async is if you are not CPU bound and that means IO and all IO in windows has callback operated interfaces at the lowest level, so - a network request sends the request and then goes on with work - but there is no thread attached. At all. The general use case for async is that async runs on a thread and when nothing is there to do it will then use the threads for the tasks completed, allowing multiple operations on one thread - AND... the IO will not use up a thread.

Your method basically turns into a state engine relinquishing control to the task scheduler waiting for a completed task being signalled.

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