为什么这个 AsyncCallback 测试有时会失败？

Question

我有以下类尝试充当简单的异步操作：

public class AsyncLineWriter
{
    private delegate void SynchronousWriteLineDelegate(string message);
    private SynchronousWriteLineDelegate DoWriteLine;
    private void SynchronousWriteLine(string message)
    {
        Console.WriteLine(message);
    }
    public AsyncLineWriter()
    {
        DoWriteLine = new SynchronousWriteLineDelegate(SynchronousWriteLine);

    public IAsyncResult BeginWriteLine(string message, AsyncCallback callback, object state)
    {
        return DoWriteLine.BeginInvoke(message,callback,state);
    }
    public void EndWriteLine(IAsyncResult asyncResult)
    {
        DoWriteLine.EndInvoke(asyncResult);
    }
}

以下单元测试间歇性失败，但我不明白竞争条件在哪里：

[TestMethod]
public void Callback_is_called()
{
    // Arrange
    AsyncLineWriter lineWriter = new AsyncLineWriter();
    object state = new object();
    object callbackState = null;
    AsyncCallback callback = (r) =>
        {
            callbackState = r.AsyncState;
        };

    // Act
    IAsyncResult asyncResult = lineWriter.BeginWriteLine("test", callback, state);
    lineWriter.EndWriteLine(asyncResult);

    // Assert
    Assert.AreSame(state, callbackState);
}

Answer 1

在此模式中，回调在线程池线程上运行，您应该从回调中调用 EndInvoke。

EndInvoke 不会等待回调完成（因为这会导致死锁），因此回调和测试方法之间存在竞争。

---

编辑：等待句柄也可以在回调完成之前设置。试试这个：

[TestMethod]
public void Callback_is_called()
{
    // Arrange
    var lw = new AsyncLineWriter();

    object state = new object();
    object callbackState = null;

    var mre = new ManualResetEvent( false );

    AsyncCallback callback = r =>
        {
            callbackState = r.AsyncState;

            lw.EndWriteLine( r );

            mre.Set();
        };

    // Act
    var ar = lw.BeginWriteLine( "test", callback, state );
    mre.WaitOne();

    // Assert
    Assert.AreSame( state, callbackState );
}

Answer 2

如前所述，在测试成功的情况下，您只是幸运，线程以这样的方式交错，即在对 EndInvoke 的调用发生之前调用回调。正确的 APM 模式是在回调中调用 EndWriteLine，这意味着您必须将 AsyncLineWriter 作为状态的一部分传递给 BeginInvoke 方法。

编辑：有一个额外的复杂性，因为回调可能发生在 IAsyncResult WaitHandle 发出信号之后。因此，并不是回调没有被调用，而是在检查发生后被调用。这修复了它：

AsyncLineWriter lineWriter = new AsyncLineWriter();
Object myState = new Object();
object[] state = new object[2];
state[0] = lineWriter;
state[1] = myState;
object callbackState = null;

ManualResetEvent evnt = new ManualResetEvent(false);

AsyncCallback callback = (r) =>
    {  
        Object[] arr = (Object[])r.AsyncState;
        LineWriter lw = (LineWriter)arr[0];
        Object st = arr[1];
        callbackState = st;
        lw.EndWriteLine(r);
        evnt.Set();
    };

// Act
IAsyncResult asyncResult = lineWriter.BeginWriteLine("test", callback, state);

//asyncResult.AsyncWaitHandle.WaitOne(); -- callback can still happen after this!

evnt.WaitOne();

//Assert
Assert.AreSame(myState, callbackState);