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Async return types (C#)

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Async methods can have the following return types:

For more information about async methods, see Asynchronous programming with async and await (C#).

Several other types also exist that are specific to Windows workloads:

Task return type

Async methods that don't contain a return statement or that contain a return statement that doesn't return an operand usually have a return type of Task. Such methods return void if they run synchronously. If you use a Task return type for an async method, a calling method can use an await operator to suspend the caller's completion until the called async method has finished.

In the following example, the WaitAndApologizeAsync method doesn't contain a return statement, so the method returns a Task object. Returning a Task enables WaitAndApologizeAsync to be awaited. The Task type doesn't include a Result property because it has no return value.

public static async Task DisplayCurrentInfoAsync()
{
    await WaitAndApologizeAsync();

    Console.WriteLine($"Today is {DateTime.Now:D}");
    Console.WriteLine($"The current time is {DateTime.Now.TimeOfDay:t}");
    Console.WriteLine("The current temperature is 76 degrees.");
}

static async Task WaitAndApologizeAsync()
{
    await Task.Delay(2000);

    Console.WriteLine("Sorry for the delay...\n");
}
// Example output:
//    Sorry for the delay...
//
// Today is Monday, August 17, 2020
// The current time is 12:59:24.2183304
// The current temperature is 76 degrees.

WaitAndApologizeAsync is awaited by using an await statement instead of an await expression, similar to the calling statement for a synchronous void-returning method. The application of an await operator in this case doesn't produce a value. When the right operand of an await is a Task<TResult>, the await expression produces a result of T. When the right operand of an await is a Task, the await and its operand are a statement.

You can separate the call to WaitAndApologizeAsync from the application of an await operator, as the following code shows. However, remember that a Task doesn't have a Result property, and that no value is produced when an await operator is applied to a Task.

The following code separates calling the WaitAndApologizeAsync method from awaiting the task that the method returns.

Task waitAndApologizeTask = WaitAndApologizeAsync();

string output =
    $"Today is {DateTime.Now:D}\n" +
    $"The current time is {DateTime.Now.TimeOfDay:t}\n" +
    "The current temperature is 76 degrees.\n";

await waitAndApologizeTask;
Console.WriteLine(output);

Task<TResult> return type

The Task<TResult> return type is used for an async method that contains a return statement in which the operand is TResult.

In the following example, the GetLeisureHoursAsync method contains a return statement that returns an integer. The method declaration must specify a return type of Task<int>. The FromResult async method is a placeholder for an operation that returns a DayOfWeek.

public static async Task ShowTodaysInfoAsync()
{
    string message =
        $"Today is {DateTime.Today:D}\n" +
        "Today's hours of leisure: " +
        $"{await GetLeisureHoursAsync()}";

    Console.WriteLine(message);
}

static async Task<int> GetLeisureHoursAsync()
{
    DayOfWeek today = await Task.FromResult(DateTime.Now.DayOfWeek);

    int leisureHours =
        today is DayOfWeek.Saturday || today is DayOfWeek.Sunday
        ? 16 : 5;

    return leisureHours;
}
// Example output:
//    Today is Wednesday, May 24, 2017
//    Today's hours of leisure: 5

When GetLeisureHoursAsync is called from within an await expression in the ShowTodaysInfo method, the await expression retrieves the integer value (the value of leisureHours) that's stored in the task returned by the GetLeisureHours method. For more information about await expressions, see await.

You can better understand how await retrieves the result from a Task<T> by separating the call to GetLeisureHoursAsync from the application of await, as the following code shows. A call to method GetLeisureHoursAsync that isn't immediately awaited returns a Task<int>, as you would expect from the declaration of the method. The task is assigned to the getLeisureHoursTask variable in the example. Because getLeisureHoursTask is a Task<TResult>, it contains a Result property of type TResult. In this case, TResult represents an integer type. When await is applied to getLeisureHoursTask, the await expression evaluates to the contents of the Result property of getLeisureHoursTask. The value is assigned to the ret variable.

Important

The Result property is a blocking property. If you try to access it before its task is finished, the thread that's currently active is blocked until the task completes and the value is available. In most cases, you should access the value by using await instead of accessing the property directly.

The previous example retrieved the value of the Result property to block the main thread so that the Main method could print the message to the console before the application ended.

var getLeisureHoursTask = GetLeisureHoursAsync();

string message =
    $"Today is {DateTime.Today:D}\n" +
    "Today's hours of leisure: " +
    $"{await getLeisureHoursTask}";

Console.WriteLine(message);

Void return type

You use the void return type in asynchronous event handlers, which require a void return type. For methods other than event handlers that don't return a value, you should return a Task instead, because an async method that returns void can't be awaited. Any caller of such a method must continue to completion without waiting for the called async method to finish. The caller must be independent of any values or exceptions that the async method generates.

The caller of a void-returning async method can't catch exceptions thrown from the method. Such unhandled exceptions are likely to cause your application to fail. If a method that returns a Task or Task<TResult> throws an exception, the exception is stored in the returned task. The exception is rethrown when the task is awaited. Make sure that any async method that can produce an exception has a return type of Task or Task<TResult> and that calls to the method are awaited.

The following example shows the behavior of an async event handler. In the example code, an async event handler must let the main thread know when it finishes. Then the main thread can wait for an async event handler to complete before exiting the program.

public class NaiveButton
{
    public event EventHandler? Clicked;

    public void Click()
    {
        Console.WriteLine("Somebody has clicked a button. Let's raise the event...");
        Clicked?.Invoke(this, EventArgs.Empty);
        Console.WriteLine("All listeners are notified.");
    }
}

public class AsyncVoidExample
{
    static readonly TaskCompletionSource<bool> s_tcs = new TaskCompletionSource<bool>();

    public static async Task MultipleEventHandlersAsync()
    {
        Task<bool> secondHandlerFinished = s_tcs.Task;

        var button = new NaiveButton();

        button.Clicked += OnButtonClicked1;
        button.Clicked += OnButtonClicked2Async;
        button.Clicked += OnButtonClicked3;

        Console.WriteLine("Before button.Click() is called...");
        button.Click();
        Console.WriteLine("After button.Click() is called...");

        await secondHandlerFinished;
    }

    private static void OnButtonClicked1(object? sender, EventArgs e)
    {
        Console.WriteLine("   Handler 1 is starting...");
        Task.Delay(100).Wait();
        Console.WriteLine("   Handler 1 is done.");
    }

    private static async void OnButtonClicked2Async(object? sender, EventArgs e)
    {
        Console.WriteLine("   Handler 2 is starting...");
        Task.Delay(100).Wait();
        Console.WriteLine("   Handler 2 is about to go async...");
        await Task.Delay(500);
        Console.WriteLine("   Handler 2 is done.");
        s_tcs.SetResult(true);
    }

    private static void OnButtonClicked3(object? sender, EventArgs e)
    {
        Console.WriteLine("   Handler 3 is starting...");
        Task.Delay(100).Wait();
        Console.WriteLine("   Handler 3 is done.");
    }
}
// Example output:
//
// Before button.Click() is called...
// Somebody has clicked a button. Let's raise the event...
//    Handler 1 is starting...
//    Handler 1 is done.
//    Handler 2 is starting...
//    Handler 2 is about to go async...
//    Handler 3 is starting...
//    Handler 3 is done.
// All listeners are notified.
// After button.Click() is called...
//    Handler 2 is done.

Generalized async return types and ValueTask<TResult>

An async method can return any type that has an accessible GetAwaiter method that returns an instance of an awaiter type. In addition, the type returned from the GetAwaiter method must have the System.Runtime.CompilerServices.AsyncMethodBuilderAttribute attribute. You can learn more in the article on Attributes read by the compiler or the C# spec for the Task type builder pattern.

This feature is the complement to awaitable expressions, which describes the requirements for the operand of await. Generalized async return types enable the compiler to generate async methods that return different types. Generalized async return types enabled performance improvements in the .NET libraries. Because Task and Task<TResult> are reference types, memory allocation in performance-critical paths, particularly when allocations occur in tight loops, can adversely affect performance. Support for generalized return types means that you can return a lightweight value type instead of a reference type to avoid additional memory allocations.

.NET provides the System.Threading.Tasks.ValueTask<TResult> structure as a lightweight implementation of a generalized task-returning value. The following example uses the ValueTask<TResult> structure to retrieve the value of two dice rolls.

class Program
{
    static readonly Random s_rnd = new Random();

    static async Task Main() =>
        Console.WriteLine($"You rolled {await GetDiceRollAsync()}");

    static async ValueTask<int> GetDiceRollAsync()
    {
        Console.WriteLine("Shaking dice...");

        int roll1 = await RollAsync();
        int roll2 = await RollAsync();

        return roll1 + roll2;
    }

    static async ValueTask<int> RollAsync()
    {
        await Task.Delay(500);

        int diceRoll = s_rnd.Next(1, 7);
        return diceRoll;
    }
}
// Example output:
//    Shaking dice...
//    You rolled 8

Writing a generalized async return type is an advanced scenario, and is targeted for use in specialized environments. Consider using the Task, Task<T>, and ValueTask<T> types instead, which cover most scenarios for asynchronous code.

In C# 10 and later, you can apply the AsyncMethodBuilder attribute to an async method (instead of the async return type declaration) to override the builder for that type. Typically you'd apply this attribute to use a different builder provided in the .NET runtime.

Async streams with IAsyncEnumerable<T>

An async method may return an async stream, represented by IAsyncEnumerable<T>. An async stream provides a way to enumerate items read from a stream when elements are generated in chunks with repeated asynchronous calls. The following example shows an async method that generates an async stream:

static async IAsyncEnumerable<string> ReadWordsFromStreamAsync()
{
    string data =
        @"This is a line of text.
              Here is the second line of text.
              And there is one more for good measure.
              Wait, that was the penultimate line.";

    using var readStream = new StringReader(data);

    string? line = await readStream.ReadLineAsync();
    while (line != null)
    {
        foreach (string word in line.Split(' ', StringSplitOptions.RemoveEmptyEntries))
        {
            yield return word;
        }

        line = await readStream.ReadLineAsync();
    }
}

The preceding example reads lines from a string asynchronously. Once each line is read, the code enumerates each word in the string. Callers would enumerate each word using the await foreach statement. The method awaits when it needs to asynchronously read the next line from the source string.

See also