同步调用异步方法

Question

我有一个async 方法：

public async Task<string> GenerateCodeAsync()
{
    string code = await GenerateCodeService.GenerateCodeAsync();
    return code;
}

我需要从同步方法中调用此方法。

我怎样才能做到这一点而不必复制GenerateCodeAsync 方法以使其同步工作？

更新

尚未找到合理的解决方案。

不过，我看到HttpClient 已经实现了这个模式

using (HttpClient client = new HttpClient())
{
    // async
    HttpResponseMessage responseAsync = await client.GetAsync(url);

    // sync
    HttpResponseMessage responseSync = client.GetAsync(url).Result;
}

Answer 1

您可以访问任务的Result 属性，这将导致您的线程阻塞，直到结果可用：

string code = GenerateCodeAsync().Result;

---

注意：在某些情况下，这可能会导致死锁：您对Result 的调用会阻塞主线程，从而阻止异步代码的其余部分执行。您可以通过以下选项来确保不会发生这种情况：

• Add .ConfigureAwait(false) to your library method或

• 在线程池线程中显式执行您的异步方法并等待它完成：

    string code = Task.Run(() => GenerateCodeAsync).Result;
  

这并不意味着您应该在所有异步调用之后无意识地添加.ConfigureAwait(false)！有关为什么以及何时应该使用.ConfigureAwait(false) 的详细分析，请参阅以下博客文章：

• .NET Blog: ConfigureAwait FAQ

Answer 2

你应该得到等待者（GetAwaiter()）并结束等待异步任务完成（GetResult()）。

string code = GenerateCodeAsync().GetAwaiter().GetResult();

Answer 3

您应该能够使用委托、lambda 表达式来完成这项工作

private void button2_Click(object sender, EventArgs e)
    {

        label1.Text = "waiting....";

        Task<string> sCode = Task.Run(async () =>
        {
            string msg =await GenerateCodeAsync();
            return msg;
        });

        label1.Text += sCode.Result;

    }

    private Task<string> GenerateCodeAsync()
    {
        return Task.Run<string>(() => GenerateCode());
    }

    private string GenerateCode()
    {
        Thread.Sleep(2000);
        return "I m back" ;
    }

Answer 4

我需要从同步方法中调用此方法。

正如另一个答案所暗示的那样，GenerateCodeAsync().Result 或 GenerateCodeAsync().Wait() 是可能的。这将阻塞当前线程，直到 GenerateCodeAsync 完成。

但是，您的问题被标记为asp.net，并且您还留下了评论：

我希望有一个更简单的解决方案，认为 asp.net 可以处理 这比写这么多行代码要容易得多

我的意思是，您不应该阻塞 ASP.NET 中的异步方法。这将降低您的 Web 应用程序的可扩展性，并可能造成死锁（当 GenerateCodeAsync 中的 await 延续发布到 AspNetSynchronizationContext 时）。使用 Task.Run(...).Result 将某些内容卸载到池线程然后阻塞将进一步损害可伸缩性，因为它会导致 +1 个线程来处理给定的 HTTP 请求。

ASP.NET 内置了对异步方法的支持，可以通过异步控制器（在 ASP.NET MVC 和 Web API 中）或直接通过经典 ASP.NET 中的AsyncManager 和PageAsyncTask。你应该使用它。更多详情，请查看this answer。

Answer 5

Microsoft Identity 具有同步调用异步方法的扩展方法。 例如有 GenerateUserIdentityAsync() 方法和等于 CreateIdentity()

如果您查看 UserManagerExtensions.CreateIdentity() 它看起来像这样：

 public static ClaimsIdentity CreateIdentity<TUser, TKey>(this UserManager<TUser, TKey> manager, TUser user,
        string authenticationType)
        where TKey : IEquatable<TKey>
        where TUser : class, IUser<TKey>
    {
        if (manager == null)
        {
            throw new ArgumentNullException("manager");
        }
        return AsyncHelper.RunSync(() => manager.CreateIdentityAsync(user, authenticationType));
    }

现在让我们看看 AsyncHelper.RunSync 做了什么

  public static TResult RunSync<TResult>(Func<Task<TResult>> func)
    {
        var cultureUi = CultureInfo.CurrentUICulture;
        var culture = CultureInfo.CurrentCulture;
        return _myTaskFactory.StartNew(() =>
        {
            Thread.CurrentThread.CurrentCulture = culture;
            Thread.CurrentThread.CurrentUICulture = cultureUi;
            return func();
        }).Unwrap().GetAwaiter().GetResult();
    }

所以，这是你对异步方法的封装。 请不要从 Result 中读取数据 - 它可能会阻止您在 ASP 中的代码。

还有一种方式——我觉得很可疑，但你也可以考虑一下

  Result r = null;

            YourAsyncMethod()
                .ContinueWith(t =>
                {
                    r = t.Result;
                })
                .Wait();

Answer 6

为了防止死锁，当我必须同步调用@Heinzi 提到的异步方法时，我总是尝试使用Task.Run()。

但是，如果异步方法使用参数，则必须修改该方法。例如Task.Run(GenerateCodeAsync("test")).Result 给出错误：

参数 1：无法从 'System.Threading.Tasks.Task<string>' 转换 到'System.Action'

可以这样调用：

string code = Task.Run(() => GenerateCodeAsync("test")).Result;

Answer 7

此线程上的大多数答案要么很复杂，要么会导致死锁。

下面的方法很简单，它可以避免死锁，因为我们正在等待任务完成，然后才得到它的结果-

var task = Task.Run(() => GenerateCodeAsync()); 
task.Wait();
string code = task.Result;

此外，这是对 MSDN 文章的参考，其中谈到了完全相同的事情- https://blogs.msdn.microsoft.com/jpsanders/2017/08/28/asp-net-do-not-use-task-result-in-main-context/

Answer 8

我正在使用这种方法，它还将处理和传播来自底层异步任务的异常。

    private string RunSync()
    {
        var task = Task.Run(async () => await GenerateCodeService.GenerateCodeAsync());
        if (task.IsFaulted && task.Exception != null)
        {
            throw task.Exception;
        }

        return task.Result;
    }

Answer 9

一些扩展方法如何异步等待异步操作完成，然后设置一个ManualResetEvent来表示完成。

注意：您可以使用 Task.Run()，但扩展方法是表达您真正想要的更简洁的接口。

展示如何使用扩展的测试：

    [TestClass]
    public class TaskExtensionsTests
    {
        [TestMethod]
        public void AsynchronousOperationWithNoResult()
        {
            SampleAsynchronousOperationWithNoResult().AwaitResult();
        }

        [TestMethod]
        public void AsynchronousOperationWithResult()
        {
            Assert.AreEqual(3, SampleAsynchronousOperationWithResult(3).AwaitResult());
        }

        [TestMethod]
        [ExpectedException(typeof(Exception))]
        public void AsynchronousOperationWithNoResultThrows()
        {
            SampleAsynchronousOperationWithNoResultThrows().AwaitResult();
        }

        [TestMethod]
        [ExpectedException(typeof(Exception))]
        public void AsynchronousOperationWithResultThrows()
        {
            SampleAsynchronousOperationWithResultThrows(3).AwaitResult();
        }

        private static async Task SampleAsynchronousOperationWithNoResult()
        {
            await Task.Yield();
        }

        private static async Task<T> SampleAsynchronousOperationWithResult<T>(T result)
        {
            await Task.Yield();
            return result;
        }

        private static async Task SampleAsynchronousOperationWithNoResultThrows()
        {
            await Task.Yield();
            throw new Exception();
        }

        private static async Task<T> SampleAsynchronousOperationWithResultThrows<T>(T result)
        {
            await Task.Yield();
            throw new Exception();
        }

        [TestMethod]
        public void AsynchronousValueOperationWithNoResult()
        {
            SampleAsynchronousValueOperationWithNoResult().AwaitResult();
        }

        [TestMethod]
        public void AsynchronousValueOperationWithResult()
        {
            Assert.AreEqual(3, SampleAsynchronousValueOperationWithResult(3).AwaitResult());
        }

        [TestMethod]
        [ExpectedException(typeof(Exception))]
        public void AsynchronousValueOperationWithNoResultThrows()
        {
            SampleAsynchronousValueOperationWithNoResultThrows().AwaitResult();
        }

        [TestMethod]
        [ExpectedException(typeof(Exception))]
        public void AsynchronousValueOperationWithResultThrows()
        {
            SampleAsynchronousValueOperationWithResultThrows(3).AwaitResult();
        }

        private static async ValueTask SampleAsynchronousValueOperationWithNoResult()
        {
            await Task.Yield();
        }

        private static async ValueTask<T> SampleAsynchronousValueOperationWithResult<T>(T result)
        {
            await Task.Yield();
            return result;
        }

        private static async ValueTask SampleAsynchronousValueOperationWithNoResultThrows()
        {
            await Task.Yield();
            throw new Exception();
        }

        private static async ValueTask<T> SampleAsynchronousValueOperationWithResultThrows<T>(T result)
        {
            await Task.Yield();
            throw new Exception();
        }
    }

扩展

    /// <summary>
    /// Defines extension methods for <see cref="Task"/> and <see cref="ValueTask"/>.
    /// </summary>
    public static class TaskExtensions
    {
        /// <summary>
        /// Synchronously await the results of an asynchronous operation without deadlocking; ignoring cancellation.
        /// </summary>
        /// <param name="task">
        /// The <see cref="Task"/> representing the pending operation.
        /// </param>
        public static void AwaitCompletion(this ValueTask task)
        {
            new SynchronousAwaiter(task, true).GetResult();
        }

        /// <summary>
        /// Synchronously await the results of an asynchronous operation without deadlocking; ignoring cancellation.
        /// </summary>
        /// <param name="task">
        /// The <see cref="Task"/> representing the pending operation.
        /// </param>
        public static void AwaitCompletion(this Task task)
        {
            new SynchronousAwaiter(task, true).GetResult();
        }

        /// <summary>
        /// Synchronously await the results of an asynchronous operation without deadlocking.
        /// </summary>
        /// <param name="task">
        /// The <see cref="Task"/> representing the pending operation.
        /// </param>
        /// <typeparam name="T">
        /// The result type of the operation.
        /// </typeparam>
        /// <returns>
        /// The result of the operation.
        /// </returns>
        public static T AwaitResult<T>(this Task<T> task)
        {
            return new SynchronousAwaiter<T>(task).GetResult();
        }

        /// <summary>
        /// Synchronously await the results of an asynchronous operation without deadlocking.
        /// </summary>
        /// <param name="task">
        /// The <see cref="Task"/> representing the pending operation.
        /// </param>
        public static void AwaitResult(this Task task)
        {
            new SynchronousAwaiter(task).GetResult();
        }

        /// <summary>
        /// Synchronously await the results of an asynchronous operation without deadlocking.
        /// </summary>
        /// <param name="task">
        /// The <see cref="ValueTask"/> representing the pending operation.
        /// </param>
        /// <typeparam name="T">
        /// The result type of the operation.
        /// </typeparam>
        /// <returns>
        /// The result of the operation.
        /// </returns>
        public static T AwaitResult<T>(this ValueTask<T> task)
        {
            return new SynchronousAwaiter<T>(task).GetResult();
        }

        /// <summary>
        /// Synchronously await the results of an asynchronous operation without deadlocking.
        /// </summary>
        /// <param name="task">
        /// The <see cref="ValueTask"/> representing the pending operation.
        /// </param>
        public static void AwaitResult(this ValueTask task)
        {
            new SynchronousAwaiter(task).GetResult();
        }

        /// <summary>
        /// Ignore the <see cref="OperationCanceledException"/> if the operation is cancelled.
        /// </summary>
        /// <param name="task">
        /// The <see cref="Task"/> representing the asynchronous operation whose cancellation is to be ignored.
        /// </param>
        /// <returns>
        /// The <see cref="Task"/> representing the asynchronous operation whose cancellation is ignored.
        /// </returns>
        public static async Task IgnoreCancellationResult(this Task task)
        {
            try
            {
                await task.ConfigureAwait(false);
            }
            catch (OperationCanceledException)
            {
            }
        }

        /// <summary>
        /// Ignore the <see cref="OperationCanceledException"/> if the operation is cancelled.
        /// </summary>
        /// <param name="task">
        /// The <see cref="ValueTask"/> representing the asynchronous operation whose cancellation is to be ignored.
        /// </param>
        /// <returns>
        /// The <see cref="ValueTask"/> representing the asynchronous operation whose cancellation is ignored.
        /// </returns>
        public static async ValueTask IgnoreCancellationResult(this ValueTask task)
        {
            try
            {
                await task.ConfigureAwait(false);
            }
            catch (OperationCanceledException)
            {
            }
        }

        /// <summary>
        /// Ignore the results of an asynchronous operation allowing it to run and die silently in the background.
        /// </summary>
        /// <param name="task">
        /// The <see cref="Task"/> representing the asynchronous operation whose results are to be ignored.
        /// </param>
        public static async void IgnoreResult(this Task task)
        {
            try
            {
                await task.ConfigureAwait(false);
            }
            catch
            {
                // ignore exceptions
            }
        }

        /// <summary>
        /// Ignore the results of an asynchronous operation allowing it to run and die silently in the background.
        /// </summary>
        /// <param name="task">
        /// The <see cref="ValueTask"/> representing the asynchronous operation whose results are to be ignored.
        /// </param>
        public static async void IgnoreResult(this ValueTask task)
        {
            try
            {
                await task.ConfigureAwait(false);
            }
            catch
            {
                // ignore exceptions
            }
        }
    }

    /// <summary>
    /// Internal class for waiting for asynchronous operations that have a result.
    /// </summary>
    /// <typeparam name="TResult">
    /// The result type.
    /// </typeparam>
    public class SynchronousAwaiter<TResult>
    {
        /// <summary>
        /// The manual reset event signaling completion.
        /// </summary>
        private readonly ManualResetEvent manualResetEvent;

        /// <summary>
        /// The exception thrown by the asynchronous operation.
        /// </summary>
        private Exception exception;

        /// <summary>
        /// The result of the asynchronous operation.
        /// </summary>
        private TResult result;

        /// <summary>
        /// Initializes a new instance of the <see cref="SynchronousAwaiter{TResult}"/> class.
        /// </summary>
        /// <param name="task">
        /// The task representing an asynchronous operation.
        /// </param>
        public SynchronousAwaiter(Task<TResult> task)
        {
            this.manualResetEvent = new ManualResetEvent(false);
            this.WaitFor(task);
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="SynchronousAwaiter{TResult}"/> class.
        /// </summary>
        /// <param name="task">
        /// The task representing an asynchronous operation.
        /// </param>
        public SynchronousAwaiter(ValueTask<TResult> task)
        {
            this.manualResetEvent = new ManualResetEvent(false);
            this.WaitFor(task);
        }

        /// <summary>
        /// Gets a value indicating whether the operation is complete.
        /// </summary>
        public bool IsComplete => this.manualResetEvent.WaitOne(0);

        /// <summary>
        /// Synchronously get the result of an asynchronous operation.
        /// </summary>
        /// <returns>
        /// The result of the asynchronous operation.
        /// </returns>
        public TResult GetResult()
        {
            this.manualResetEvent.WaitOne();
            return this.exception != null ? throw this.exception : this.result;
        }

        /// <summary>
        /// Tries to synchronously get the result of an asynchronous operation.
        /// </summary>
        /// <param name="operationResult">
        /// The result of the operation.
        /// </param>
        /// <returns>
        /// The result of the asynchronous operation.
        /// </returns>
        public bool TryGetResult(out TResult operationResult)
        {
            if (this.IsComplete)
            {
                operationResult = this.exception != null ? throw this.exception : this.result;
                return true;
            }

            operationResult = default;
            return false;
        }

        /// <summary>
        /// Background "thread" which waits for the specified asynchronous operation to complete.
        /// </summary>
        /// <param name="task">
        /// The task.
        /// </param>
        private async void WaitFor(Task<TResult> task)
        {
            try
            {
                this.result = await task.ConfigureAwait(false);
            }
            catch (Exception exception)
            {
                this.exception = exception;
            }
            finally
            {
                this.manualResetEvent.Set();
            }
        }

        /// <summary>
        /// Background "thread" which waits for the specified asynchronous operation to complete.
        /// </summary>
        /// <param name="task">
        /// The task.
        /// </param>
        private async void WaitFor(ValueTask<TResult> task)
        {
            try
            {
                this.result = await task.ConfigureAwait(false);
            }
            catch (Exception exception)
            {
                this.exception = exception;
            }
            finally
            {
                this.manualResetEvent.Set();
            }
        }
    }

    /// <summary>
    /// Internal class for  waiting for  asynchronous operations that have no result.
    /// </summary>
    public class SynchronousAwaiter
    {
        /// <summary>
        /// The manual reset event signaling completion.
        /// </summary>
        private readonly ManualResetEvent manualResetEvent = new ManualResetEvent(false);

        /// <summary>
        /// The exception thrown by the asynchronous operation.
        /// </summary>
        private Exception exception;

        /// <summary>
        /// Initializes a new instance of the <see cref="SynchronousAwaiter{TResult}"/> class.
        /// </summary>
        /// <param name="task">
        /// The task representing an asynchronous operation.
        /// </param>
        /// <param name="ignoreCancellation">
        /// Indicates whether to ignore cancellation. Default is false.
        /// </param>
        public SynchronousAwaiter(Task task, bool ignoreCancellation = false)
        {
            this.manualResetEvent = new ManualResetEvent(false);
            this.WaitFor(task, ignoreCancellation);
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="SynchronousAwaiter{TResult}"/> class.
        /// </summary>
        /// <param name="task">
        /// The task representing an asynchronous operation.
        /// </param>
        /// <param name="ignoreCancellation">
        /// Indicates whether to ignore cancellation. Default is false.
        /// </param>
        public SynchronousAwaiter(ValueTask task, bool ignoreCancellation = false)
        {
            this.manualResetEvent = new ManualResetEvent(false);
            this.WaitFor(task, ignoreCancellation);
        }

        /// <summary>
        /// Gets a value indicating whether the operation is complete.
        /// </summary>
        public bool IsComplete => this.manualResetEvent.WaitOne(0);

        /// <summary>
        /// Synchronously get the result of an asynchronous operation.
        /// </summary>
        public void GetResult()
        {
            this.manualResetEvent.WaitOne();
            if (this.exception != null)
            {
                throw this.exception;
            }
        }

        /// <summary>
        /// Background "thread" which waits for the specified asynchronous operation to complete.
        /// </summary>
        /// <param name="task">
        /// The task.
        /// </param>
        /// <param name="ignoreCancellation">
        /// Indicates whether to ignore cancellation. Default is false.
        /// </param>
        private async void WaitFor(Task task, bool ignoreCancellation)
        {
            try
            {
                await task.ConfigureAwait(false);
            }
            catch (OperationCanceledException)
            {
            }
            catch (Exception exception)
            {
                this.exception = exception;
            }
            finally
            {
                this.manualResetEvent.Set();
            }
        }

        /// <summary>
        /// Background "thread" which waits for the specified asynchronous operation to complete.
        /// </summary>
        /// <param name="task">
        ///     The task.
        /// </param>
        /// <param name="ignoreCancellation">
        /// Indicates whether to ignore cancellation. Default is false.
        /// </param>
        private async void WaitFor(ValueTask task, bool ignoreCancellation)
        {
            try
            {
                await task.ConfigureAwait(false);
            }
            catch (OperationCanceledException)
            {
            }
            catch (Exception exception)
            {
                this.exception = exception;
            }
            finally
            {
                this.manualResetEvent.Set();
            }
        }
    }
}

Answer 10

编辑：

Task 有 Wait 方法，Task.Wait()，它等待“promise”解决然后继续，从而使其同步。 示例：

async Task<String> MyAsyncMethod() { ... }

String mySyncMethod() {

    return MyAsyncMethod().Wait();
}

Answer 11

我更喜欢非阻塞方法：

            Dim aw1=GenerateCodeAsync().GetAwaiter()
            While Not aw1.IsCompleted
                Application.DoEvents()
            End While

Answer 12

如果您有一个名为“RefreshList”的异步方法，那么您可以从如下所示的非异步方法中调用该异步方法。

Task.Run(async () => { await RefreshList(); });