解释 NetCore 上 C# TcpClient/TcpListener 的奇怪行为

Question

我在 Windows 10 上运行 NetCore，我有两个程序 - 我在本地运行的服务器和客户端。

执行顺序如下：

• 运行服务器 - 有一个处理客户端的循环
• 运行客户端 - 客户端完成其工作并终止进程
• 第二次运行客户端

此序列使用以下代码从服务器生成以下输出：

Waiting for client.
Reading message.
Incoming message: This message is longer than what
Sending message.
Closing connection.
Waiting for client.
Reading message.
Incoming message: This message is longer than what
Sending message.
Closing connection.
Waiting for client.

以及来自客户端的以下输出：

Connecting to server.
Sending message.
Reading message.
Incoming message: Thank you!


Connecting to server.
Sending message.
Reading message.

Unhandled Exception: System.AggregateException: One or more errors occurred. (Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host.) ---> System.IO.IOException: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host. ---> System.Net.Sockets.SocketException: An existing connection was forcibly closed by the remote host

换句话说 - 第二次尝试运行客户端以异常结束。那是奇怪的部分。

这是我重现问题的最小代码示例。

服务器代码：

public static async Task StartServerAsync()
{
  TcpListener listener = new TcpListener(IPAddress.Any, 1234);
  listener.Server.NoDelay = true;
  listener.Server.LingerState = new LingerOption(true, 0);
  listener.Start();

  while (true)
  {
    Console.WriteLine("Waiting for client.");
    using (TcpClient client = await listener.AcceptTcpClientAsync())
    {
      client.NoDelay = true;
      client.LingerState = new LingerOption(true, 0);

      Console.WriteLine("Reading message.");
      using (NetworkStream stream = client.GetStream())
      {
        byte[] buffer = new byte[32];
        int len = await stream.ReadAsync(buffer, 0, buffer.Length);
        string incomingMessage = Encoding.UTF8.GetString(buffer, 0, len);
        Console.WriteLine("Incoming message: {0}", incomingMessage);


        Console.WriteLine("Sending message.");
        byte[] message = Encoding.UTF8.GetBytes("Thank you!");
        await stream.WriteAsync(message, 0, message.Length);
        Console.WriteLine("Closing connection.");
      }
    }
  }
}

客户端代码：

public static async Task StartClientAsync()
{
  using (TcpClient client = new TcpClient())
  {
    client.NoDelay = true;
    client.LingerState = new LingerOption(true, 0);

    Console.WriteLine("Connecting to server.");
    await client.ConnectAsync("127.0.0.1", 1234);

    Console.WriteLine("Sending message.");
    using (NetworkStream stream = client.GetStream())
    {
      byte[] buffer = Encoding.UTF8.GetBytes("This message is longer than what the server is willing to read.");
      await stream.WriteAsync(buffer, 0, buffer.Length);

      Console.WriteLine("Reading message.");
      int len = await stream.ReadAsync(buffer, 0, buffer.Length);
      string message = Encoding.UTF8.GetString(buffer, 0, len);

      Console.WriteLine("Incoming message: {0}", message);
    }
  }
}

奇怪的是，如果我们将客户端的消息从

"This message is longer than what the server is willing to read."

到

"This message is short."

客户端的两个实例都在没有崩溃的情况下完成并产生相同的输出。

请注意，如果我省略了设置 LingerState 的所有三行，则没有区别。如果我将 LingerState 设置为

，则行为也没有区别

new LingerState(true, 5)

如果我将 NoDelay 设置为 false，也没有区别。换句话说，在任一侧为 LingerState 和 NoDelay 设置任何值似乎对崩溃没有任何影响。防止崩溃的唯一方法是在服务器端从客户端读取整个输入。

这很奇怪，我想知道是否有人可以解释一下。我不确定它是否也适用于 .NET Framework，仅在 NetCore 1.0.0 和 Windows 10 上进行了测试。

Answer 1

.NET Core 团队终于在 GitHub 上回答了这个问题。

长答案-见https://github.com/dotnet/corefx/issues/13114的最后评论

简答——

行为是预期的，是设计使然。

Answer 2

我已使用您的代码 sn-ps 并创建了一个实际可运行的 demo solution，但仍处于未应答状态。

只需稍作改动即可获得预期的行为。在您的服务器端代码中，您需要确保实际读取整个字符串。调整以下几行：

byte[] buffer = new byte[32];
int len = await stream.ReadAsync(buffer, 0, buffer.Length);
string incomingMessage = Encoding.UTF8.GetString(buffer, 0, len);
Console.WriteLine("Incoming message: {0}", incomingMessage);

与：

StringBuilder sb = new StringBuilder();
byte[] buffer = new byte[32];
int len;
do
{
    len = await stream.ReadAsync(buffer, 0, buffer.Length);
    sb.Append(Encoding.UTF8.GetString(buffer, 0, len));
} while (len == buffer.Length);
Console.WriteLine("Incoming message: {0}", sb.ToString());

只要看看the demo solution 的历史，你就会发现所有的变化都是微乎其微的。