如何将同步上下文添加到异步测试方法

    [TestMethod]
    public async Task MyTest()
    {
        Assert.IsNotNull( SynchronizationContext.Current );
        await MyTestAsync();
        DoSomethingOnTheSameThread();
    }

你可以使用一个单线程的SynchronizationContext在我的AsyncEx库称为AsyncContext:

[TestMethod]
public void MyTest()
{
  AsyncContext.Run(async () =>
  {
    Assert.IsNotNull( SynchronizationContext.Current );
    await MyTestAsync();
    DoSomethingOnTheSameThread();
  });
}

然而，这并不能完全模拟一个特定的UI环境，例如，Dispatcher.CurrentDispatcher仍然是null。如果您需要这种级别的伪造，您应该使用原始异步CTP中的SynchronizationContext实现。它附带了三个可用于测试的SynchronizationContext实现:一个通用的(类似于我的AsyncContext)，一个用于WinForms，一个用于WPF。

使用Panagiotis Kanavos和Stephen Cleary提供的信息，我可以这样编写测试方法:

    [TestMethod]
    public void MyTest()
    {
      Helper.RunInWpfSyncContext( async () =>
      {
        Assert.IsNotNull( SynchronizationContext.Current );
        await MyTestAsync();
        DoSomethingOnTheSameThread();
      });
    }

内部代码现在在WPF同步上下文中运行，并像MSTest一样处理所有异常。Helper方法来自Stephen Toub:

using System.Windows.Threading; // WPF Dispatcher from assembly 'WindowsBase'
public static void RunInWpfSyncContext( Func<Task> function )
{
  if (function == null) throw new ArgumentNullException("function");
  var prevCtx = SynchronizationContext.Current;
  try
  {
    var syncCtx = new DispatcherSynchronizationContext();
    SynchronizationContext.SetSynchronizationContext(syncCtx);
    var task = function();
    if (task == null) throw new InvalidOperationException();
    var frame = new DispatcherFrame();
    var t2 = task.ContinueWith(x=>{frame.Continue = false;}, TaskScheduler.Default);
    Dispatcher.PushFrame(frame);   // execute all tasks until frame.Continue == false
    task.GetAwaiter().GetResult(); // rethrow exception when task has failed 
  }
  finally
  { 
    SynchronizationContext.SetSynchronizationContext(prevCtx);
  }
}

您可以创建自定义的synchronizationcontext派生类，并使用SynchronizationContext.SetSynchronizationContext将其注册为当前上下文。阅读Stephen Toub关于"Await, SynchronizationContext, and Console Apps"answers"Await, SynchronizationContext, and Console Apps: Part 2"的文章。

您的自定义SynchronizationContext只需要覆盖接收异步执行回调的Post方法。你怎么执行就看你自己了。

第一个post提供了一个同步上下文，该上下文将所有发布的动作存储在队列中，并提供了一个阻塞循环，该循环从队列中获取动作并在单个线程中执行它们。

可以声明您自己的测试方法属性，您可以在测试运行中注入自定义代码。使用它，您可以用自己的[SynchronizationContextTestMethod]替换[TestMethod]属性，该属性会自动使用上下文集运行测试(仅在VS2019中测试):

public class SynchronizationContextTestMethodAttribute : TestMethodAttribute
{
    public override TestResult[] Execute(ITestMethod testMethod)
    {
        Func<Task> function = async () =>
        {
            var declaringType = testMethod.MethodInfo.DeclaringType;
            var instance = Activator.CreateInstance(declaringType);
            await InvokeMethodsWithAttribute<TestInitializeAttribute>(instance, declaringType);
            await (Task)testMethod.MethodInfo.Invoke(instance, null);
            await InvokeMethodsWithAttribute<TestCleanupAttribute>(instance, declaringType);
        };
        var result = new TestResult();
        result.Outcome = UnitTestOutcome.Passed;
        var stopwatch = Stopwatch.StartNew();
        try
        {
            RunInSyncContext(function);
        }
        catch (Exception ex)
        {
            result.Outcome = UnitTestOutcome.Failed;
            result.TestFailureException = ex;
        }
        result.Duration = stopwatch.Elapsed;
        return new[] { result };
    }
    private static async Task InvokeMethodsWithAttribute<A>(object instance, Type declaringType) where A : Attribute
    {
        if (declaringType.BaseType != typeof(object))
            await InvokeMethodsWithAttribute<A>(instance, declaringType.BaseType);
        var methods = declaringType.GetMethods(BindingFlags.Instance | BindingFlags.Public);
        foreach (var methodInfo in methods)
            if (methodInfo.DeclaringType == declaringType && methodInfo.GetCustomAttribute<A>() != null)
            {
                if (methodInfo.ReturnType == typeof(Task))
                {
                    var task = (Task)methodInfo.Invoke(instance, null);
                    if (task != null)
                        await task;
                }
                else
                    methodInfo.Invoke(instance, null);
            }
    }
    public static void RunInSyncContext(Func<Task> function)
    {
        if (function == null)
            throw new ArgumentNullException(nameof(function));
        var prevContext = SynchronizationContext.Current;
        try
        {
            var syncContext = new DispatcherSynchronizationContext();
            SynchronizationContext.SetSynchronizationContext(syncContext);
            var task = function();
            if (task == null)
                throw new InvalidOperationException();
            var frame = new DispatcherFrame();
            var t2 = task.ContinueWith(x => { frame.Continue = false; }, TaskScheduler.Default);
            Dispatcher.PushFrame(frame);   // execute all tasks until frame.Continue == false
            task.GetAwaiter().GetResult(); // rethrow exception when task has failed
        }
        finally
        {
            SynchronizationContext.SetSynchronizationContext(prevContext);
        }
    }
}