How to Safely Call an Async Method in C# Without Await

How to safely call an async method in C# without await

If you want to get the exception "asynchronously", you could do:

  MyAsyncMethod().
    ContinueWith(t => Console.WriteLine(t.Exception),
        TaskContinuationOptions.OnlyOnFaulted);

This will allow you to deal with an exception on a thread other than the "main" thread. This means you don't have to "wait" for the call to MyAsyncMethod() from the thread that calls MyAsyncMethod; but, still allows you to do something with an exception--but only if an exception occurs.

Update:

technically, you could do something similar with await:

try
{
    await MyAsyncMethod().ConfigureAwait(false);
}
catch (Exception ex)
{
    Trace.WriteLine(ex);
}

...which would be useful if you needed to specifically use try/catch (or using) but I find the ContinueWith to be a little more explicit because you have to know what ConfigureAwait(false) means.

C# How to start an async method without await its complete?

If you truly just want to fire and forget. Simply don't call use await.

// It is a good idea to add CancellationTokens
var asyncProcedure = SomeHTTPAction(cancellationToken).ConfigureAwait(false);

// Or If not simply do:
var asyncProcedure = SomeHTTPAction().ConfigureAwait(false);

If you want to use the result output later its gets trickier. But if it is truly fire and forget the above should work

A Cancellation token allows interrupts and canceling procedures. If you are using Cancellation token you will need to use it everywhere from the retrieval straight through to the calling method (Turtles all the way down).

I used ConfigureAwait(false) to prevent deadlocks. Here for more information

Use async without await with a function that holds another function that returns void

It's important to distinguish asynchronous from parallel.

Asynchronous means not blocking the current thread while you're waiting for something to happen. This lets the current thread go do something else while waiting.

Parallel means doing more than one thing at the same time. This requires separate threads for each task.

You cannot call FunctionReturningVoid asynchronously because it is not an asynchronous method. In your example, Console.WriteLine() is written in a way that will block the thread until it completes. You can't change that. But I understand that's just your example for this question. If your actual method is doing some kind of I/O operation, like a network request or writing a file, you could rewrite it to use asynchronous methods. But if it's doing CPU-heavy work, or you just can't rewrite it, then you're stuck with it being synchronous - it will block the current thread while it runs.

However, you can run FunctionReturningVoid in parallel (on another thread) and wait for it asynchronously (so it doesn't block the current thread). This would be wise if this is a desktop application - you don't want to lock up your UI while it runs.

To do that, you can use Task.Run, which will start running code on another thread and return a Task that you can use to know when it completes. That means your WrapperFunction would look like this:

public Task WrapperFunction()
{
    return Task.Run(() => this.FunctionReturningVoid("aParameter"));
}

Side point: Notice I removed the async keyword. It's not necessary since you can just pass the Task to the calling method. There is more information about this here.

Microsoft has some well-written articles about Asynchronous programming with async and await that are worth the read.

call async method without await #2

If you call an async method from a single threaded execution context, such as a UI thread, and wait for the result synchronously, there is a high probability for deadlock. In your example, that probability is 100%

Think about it. What happens when you call

ValidateRequestAsync(userName, password).Result

You call the method ValidateRequestAsync. In there you call ReadAsStringAsync. The result is that a task will be returned to the UI thread, with a continuation scheduled to continue executing on the UI thread when it becomes available. But of course, it will never become available, because it is waiting (blocked) for the task to finish. But the task can't finish, because it is waiting for the UI thread to become available. Deadlock.

There are ways to prevent this deadlock, but they are all a Bad Idea. Just for completeness sake, the following might work:

Task.Run(async () => await ValidateRequestAsync(userName, password)).Result;

This is a bad idea, because you still block your UI thread waiting and doing nothing useful.

So what is the solution then? Go async all the way. The original caller on the UI thread is probably some event handler, so make sure that is async.

Are awaits in async method called without await still asynchronous?

Async methods always start running synchronously. The magic happens at await, but only when await is given a Task that has not completed.

In your example, this is what will happen when you call Run():

1. Jump to DoStuffAsync()
2. Jump to PerformCalc()
3. Jump to DoLongTaskAsync()
4. If DoLongTaskAsync() is a truly asynchronous operation and returns an incomplete Task, then await does its job and DoStuffAsync() returns an incomplete Task to Run().
5. Since the task is not awaited, Run() completes.
6. When DoLongTaskAsync() completes, DoStuffAsync() resumes, and jumps to PerformAnotherCalc().

All of that can happen on the same thread.

So to answer your questions:

1. Yes. If it is an async method, it might end up going out and doing things on other threads. But it will start synchronously on the same thread.
2. DoLongTaskAsync() will be called asynchronously, but PerformAnotherCalc() will not be called before DoLongTaskAsync() finishes, because you used await.
3. Yes. This is how await works. It will return an incomplete Task (that is, only if DoLongTaskAsync() is truly asynchronous and returns an incomplete Task). Then once DoLongTaskAsync() finishes, execution of DoStuffAsync() resumes where it left off.

C#: calling [async] method without [await] will not catch its thrown exception?

Within an async method, any exceptions are caught by the runtime and placed on the returned Task. If your code ignores the Task returned by an async method, then it will not observe those exceptions. Most tasks should be awaited at some point to observe their results (including exceptions).

The easiest solution is to make your Main asynchronous:

public static async Task Main(string[] args)
{
  try
  {
    await ProcessAsync(null);
  }
  catch(Exception e)
  {
    Console.WriteLine(e.Message);
  }
}

C# Async Task Method Without Await or Return

public Task DoSomething()
{
    return Task.CompletedTask;
}

No need for the async.

If you're using an older version of .NET, use this:

public Task DoSomething()
{
    return Task.FromResult(0);
}

If you find you need to return a result but you still dont need to await anything, try;

public Task<Result> DoSomething()
{
    return Task.FromResult(new Result())
}

or, if you really want to use async (not recommended);

public async Task<Result> DoSomething()
{
    return new Result();
}

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