Question

我正在使用多线程数据库查询一次检索多个数据表。我使用了一个被锁定的列表...一旦返回所有回调，我想返回列表。我想知道如何等待所有回调完成。

private Object TableLock = new Object();
private List<DataTable> tables_from_callback = new List<DataTable>();

private void ExecReaderHandleCallback(IAsyncResult result)
{
    SqlCommand command = (SqlCommand)result.AsyncState;
    SqlDataReader reader = command.EndExecuteReader(result);
    DataTable dt = new DataTable();
    dt.Load(reader);
    lock (TableLock)
    {
        tables_from_callback.Add(dt);
    }

}

public List<DataTable> BegExecReader(String query, int query)
{
    conn = new SqlConnection(connectionString);
    try
    {
        conn.Open();
        WaitHandle[] waitHandles = new WaitHandle[query];
        IAsyncResult[] results = new IAsyncResult[query];
        SqlCommand[] cmds = new SqlCommand[query];
        for (int i = 0; i < query; i++ )
        {
            SqlCommand cmd = new SqlCommand();
            cmd.Connection = conn;
            cmd.CommandText = query;
            cmd.CommandType = CommandType.Text;
            cmd.CommandTimeout = queryTIMEOUT;

            AsyncCallback callback = new AsyncCallback(ExecReaderHandleCallback);
            cmd.BeginExecuteReader(callback, cmd);
            cmds[i] = cmd;
        }

        /* ????
        here I need to wait for the callbacks to finish
        HOW?
        ??????? */
        return tables_from_callback;
    }
    finally
    {
        conn.Close();
    }
}

我已经做了类似的事情而没有使用回调

IAsyncResult result = cmd.BeginExecuteReader();
results[i] = result;
waitHandles[i] = result.AsyncWaitHandle;
...
WaitHandle.WaitAll(waitHandles);

但是现在我必须使用回调，所以我没有等待句柄。任何帮助将不胜感激。

Answer 1

这样做的一种简洁方法是使用两个上下文都可以访问的变量。根据上下文，我的意思是调用者传递回调和回调本身。

    static void Main(string[] args)
    {
        int x = 1;
        Task.Run(() =>
        {
            Thread.Sleep(5000);
            Console.WriteLine("Yay!");
            x = 0;
        });

        while (x != 0) { Thread.Yield(); }

        Console.WriteLine("Done");
        Console.ReadKey(true);
    }

我正在使用Task.Run来模拟你的回调情况;但是，该原则仍适用于回调的情况......区别在于您将使用类级变量或（yikes）静态变量。只要信号量没有立即发出信号，就像信号量一样，你也可以像信号量一样......就像这样。

    static void Main(string[] args)
    {
        Semaphore s = new Semaphore(1, 2);
        Task.Run(() =>
        {
            s.WaitOne();
            Thread.Sleep(5000);
            Console.WriteLine("Yay!");
            s.Release();
        });

        Thread.Sleep(100);
        s.WaitOne();

        Console.WriteLine("Done");
        Console.ReadKey(true);
    }

如何等待所有AsyncCallback在asp.net中返回

1 个答案: