how to handle if async process returns before await.

Question

ifGetStringAsync (and therefore getStringTask) completes before GetUrlContentLengthAsync awaits it, control remains in GetUrlContentLengthAsync. The expense of suspending and then returning to GetUrlContentLengthAsync would be wasted if the called asynchronous process getStringTask has already completed and GetUrlContentLengthAsync doesn't have to wait for the final result.

q1) so if getStringTask complets before await then how could/should i write this code ?

https://learn.microsoft.com/en-us/dotnet/csharp/asynchronous-programming/task-asynchronous-programming-model#BKMK_ReturnTypesandParameters

public async Task<int> GetUrlContentLengthAsync()
{
    using var client = new HttpClient();

    Task<string> getStringTask =
        client.GetStringAsync("https://learn.microsoft.com/dotnet");

    DoIndependentWork();

    string contents = await getStringTask;

    return contents.Length;
}

void DoIndependentWork()
{
    Console.WriteLine("Working...");
}

Answer 1

q1) so if getStringTask complets before await then how could/should i write this code ?

You've asked a great question about the timing of the HTTP request and the independent work!

If I understand correctly, you're curious about what happens if the HTTP request finishes before the DoIndependentWork() method is done.

Here's the key: with async and await, you simply don't know the exact moment an asynchronous task (like getStringTask) will complete. It could finish very quickly, even before your independent work is done, or it could take longer.

The line string contents = await getStringTask; is crucial. It acts as a synchronization point. Regardless of when getStringTask actually finishes, the await keyword ensures that:

Your DoIndependentWork() can run concurrently while getStringTask is fetching data.

The code will pause execution at await getStringTask only if getStringTask hasn't completed yet.

Once getStringTask does complete (whether it was before or after DoIndependentWork()), the value of contents will be assigned, and the method will resume execution from that point.

Think of await as setting a "bookmark" in your code. You tell the program, "Start this task, go do other things (DoIndependentWork()), but when this specific task is done, come back to this bookmark and pick up right here."

Therefore, the original code structure handles this scenario correctly. There's no need to change the reference code because the await keyword inherently manages the dependency on the getStringTask's completion.

Answer 2

in your code, client.GetStringAsync() is running of a different thread. It will complete before or after DoIndependentWork() completes. the await after DoIndependentWork() will return from the function the task (which may or may not have completed).

the caller of GetUrlContentLengthAsync() when it executes await, will find the task completed, or need to await the results. without await the code would be (and is what the code looks without the compiler sugar of await):

public Task<int> GetUrlContentLengthAsync()
{
    using var client = new HttpClient();

    // start async thread
    Task<string> getStringTask =
        client.GetStringAsync("https://learn.microsoft.com/dotnet");

    // run sync code on this thread
    DoIndependentWork();

    // ContinueWith() returns a new task of type Task<int>
    return getStringTask.ContinueWith(s => s.Length);
}

again when the caller of GetUrlContentLengthAsync() awaits task completion, it will get the result the Continue Task, which may or may not have completed yet.

note: all awaits are converted to Continue / ContinueWith by the compiler. so the the code after an await, is always a callback.

Answer 3

Aren't you talking about ASP.NET as tagged "Developer technologies | ASP.NET | ASP.NET Core"? Why does you code include Console.WriteLine("Working...");?

Assuming that you are talking about the console app ...

q1) so if getStringTask complets before await then how could/should i write this code ?

Your code is OK. Completion timing of the GetStringAsync method does not matter at all regardless of whether it is after or before the DoIndependentWork method.

You need the line string contents = await getStringTask; in the GetUrlContentLengthAsync anyway to obtain the "contents.Length" as the returned value of GetUrlContentLengthAsync method regardless of the completion timing of GetStringAsync.

The "await" occurs at the line string contents = await getStringTask; in the GetUrlContentLengthAsync and continues until getStringTask.Status has changed to RanToCompletion.

If the GetStringAsync method has completed before the DoIndependentWork method, getStringTask.Status becomes RanToCompletion before the line string contents = await getStringTask;. Therefore, the "await" does NOT occur. That's only difference. No difference in the result.

Try the following sample to understand the above mentioned matters:

using System.Threading.Tasks;

namespace ConsoleAppAsync
{
    internal class Program
    {
        static async Task Main(string[] args)
        {
            var sample = new SampleClass();
            int length = await sample.GetUrlContentLengthAsync();
            Console.WriteLine($"Length: {length}");
        }
    }

    public class SampleClass
    {
        public async Task<int> GetUrlContentLengthAsync()
        {
            Task<string> getStringTask = GetStringAsync();
            Console.WriteLine($"1: {getStringTask.Status}");

            DoIndependentWork();
            Console.WriteLine($"2: {getStringTask.Status}");

            string contents = await getStringTask;
            Console.WriteLine($"3: {getStringTask.Status}");

            return contents.Length;
        }

        void DoIndependentWork()
        {
            Console.WriteLine("Working...");
        }

        public async Task<string> GetStringAsync()
        {
            await Task.Delay(10);
            return "abcdef";
        }
    }
}

/*
Result of Task.Delay(10)

1: WaitingForActivation
Working...
2: WaitingForActivation
3: RanToCompletion
Length: 6

Result of Task.Delay(0)

1: RanToCompletion
Working...
2: RanToCompletion
3: RanToCompletion
Length: 6
*/

Answer 4

Hi Rajesh,

As other users here have pointed out, your code is already working as intended. I can guide you through the behavior of await in both scenarios.

Scenario 1: getStringTask completes BEFORE await

Task<string> getStringTask = client.GetStringAsync("https://learn.microsoft.com/dotnet");
DoIndependentWork(); // HTTP request completes during this 
string contents = await getStringTask; // Task is already complete and no suspension occurs

• The await sees the task is already in RanToCompletion state
• No context switching or suspension happens
• Execution continues synchronously
• You get the result immediately

Scenario 2: getStringTask is still running when you hit await

Task<string> getStringTask = client.GetStringAsync("https://learn.microsoft.com/dotnet");
DoIndependentWork(); // HTTP request still running 
string contents = await getStringTask; // Task not complete, suspension occurs

• The method suspends at the await
• Control returns to the caller
• When the HTTP request completes, execution resumes
• You get the result when it's ready

The pattern you're using is exactly what Microsoft recommends for concurrent operations

However, if you want to be more explicit about this pattern, you can check completion explicitly:

public async Task<int> GetUrlContentLengthAsync() 
{
    using var client = new HttpClient();
	
    Task<string> getStringTask = client.GetStringAsync("https://learn.microsoft.com/dotnet");
	
    DoIndependentWork();
	
    string contents;
	
    if (getStringTask.IsCompleted)
    {
        contents = getStringTask.Result; // No await needed
    }
    else
    {
        contents = await getStringTask;
    }
	
    return contents.Length;
}

Answer 5

You took the description in your question from the note in the Microsoft document What happens in an async method.

You have to understand that the above note does not apply to the sample code because the operation form (1) to (8) below must complete to obtain the "contents.Length" (8) regardless of the completion timing of GetStringAsync.

Be aware that the lines string contents = await getStringTask; and return contents.Length; cannot be skipped and must exit as is in the GetUrlContentLengthAsync method to obtain the "contents.Length" (8).