Task.ContinueWith混乱 [英] Task.ContinueWith confusion

问题描述

使用ASP.NET 4.5我试图用新的异步播放/等待玩具。我有一个的IDataReader，实现类，包装了一个供应商特定的读者（如SqlDataReader的）。我有一个同步运行，像这样一个简单的ExecuteSQL（）方法：

With ASP.NET 4.5 I'm trying to play with the new async/await toys. I have a IDataReader-implementing class that wraps a vendor-specific reader (like SqlDatareader). I have a simple ExecuteSql() method that operates synchronously like so:

public IDataReader ReaderForSql(string sql)
{
    var cmd = NewCommand(sql, CommandType.Text);
    return DBReader.ReaderFactory(cmd.ExecuteReader());
}

我要的就是这样的一个异步版本。这是我第一次尝试：

What I want is an Async version of this. Here's my first try:

public Task<IDataReader> ReaderForSqlAsync(string sql, CancellationToken ct)
{
    var cmd = NewCommand(sql, CommandType.Text);
    return cmd.ExecuteReaderAsync(ct)
              .ContinueWith(t => DBReader.ReaderFactory(t.Result));
}

和我使用它：

using (var r = await connection.ReaderForSqlAsync("SELECT ...", cancellationToken))
{
    ...
}

这在我有限的测试的伟大工程至今。但看着这Cloud9视频几次之后：<一href=\"http://channel9.msdn.com/Events/aspConf/aspConf/Async-in-ASP-NET\">http://channel9.msdn.com/Events/aspConf/aspConf/Async-in-ASP-NET我得到了worreid关于他们给了有关警告：

This works great in my limited testing so far. But after watching this Cloud9 video a few times: http://channel9.msdn.com/Events/aspConf/aspConf/Async-in-ASP-NET I got worreid about warnings they gave regarding:

• ContinueWith摄入额外的线程池资源 - ！Readerfactory很轻


• Task.Result阻塞

和因为我传递一个ContinuationToken到ExecuteReaderAsync（）似乎取消仅仅是另一个原因ExecuteReaderAsync（）可能失败（它的SQL毕竟！）

and since I am passing a ContinuationToken to ExecuteReaderAsync() it seems cancellation is just yet another reason ExecuteReaderAsync() could fail (it's SQL after all!)

会有什么任务的状态，当我尝试ContinueWith呢？将t.Result块？扔？做错事？

What will be the state of the task when I try to ContinueWith it? Will t.Result block? throw? do the wrong thing?

推荐答案

ContinueWith 将使用当前的任务调度程序（线程池线程）在默认情况下，但你可以改变通过传递 TaskContinuationOptions.ExecuteSynchronously 和明确的的TaskScheduler 。

ContinueWith will use the current task scheduler (a thread pool thread) by default, but you can change that by passing TaskContinuationOptions.ExecuteSynchronously and an explicit TaskScheduler.

这是说，我会做这个作为第一次尝试：

That said, I would make this as a first effort:

public async Task<IDataReader> ReaderForSqlAsync(string sql, CancellationToken ct)
{
  var cmd = NewCommand(sql, CommandType.Text);
  var readerResult = await cmd.ExecuteReaderAsync(ct).ConfigureAwait(false);
  return DBReader.ReaderFactory(readerResult);
}

异步和等待处理所有的 ContinueWith 美食和边缘条件为您提供了一致的方式。它可能会为这个code复杂化要快如果性能测试表明这是一个严重的问题。

async and await handle all the ContinueWith delicacies and edge conditions in a consistent manner for you. It may be possible to complicate this code to be faster if performance testing indicates it's a serious problem.

这篇关于Task.ContinueWith混乱的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！