c# - 为什么这个 AsyncCallback 测试有时会失败？

Question

我有以下类试图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 不等待回调完成(因为这会导致死锁)，因此您在回调和测试方法之间进行竞争。

---

编辑:WAITING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 );
}