为预先检查为 null 并被锁定的 object（即套接字）引发 NullReferenceException

Question

I am sure it is something silly that is being overlooked, but I have the System.NullReferenceException exception thrown on the line _clientSocket.Send(Encoding.ASCII.GetBytes(data)); and can confirm that _clientSocket IS null when the exception is thrown.

But, in this case, I have two questions:

1. Why is the exception being thrown, if we have the lines before the Sent method is called purposed to check if the object is null and lock said object to prevent any other threads from disposing it?

2. Why is the exception NOT being caught?

    private bool SendToServer(string data) { Logger.Log($"[{_clientID}] sending the following to server: {data}", LoggingSeverity.trace); if (_clientSocket == null) { Logger.Log($"[{_clientID}] send failure. Socket not initialized.", LoggingSeverity.warning); return false; } lock (_clientSocket) { bool sendSuccessful = true; try { _clientSocket.Send(Encoding.ASCII.GetBytes(data)); } catch (Exception ex) { Logger.Log($"[{_clientID}] client send exception: {ex.Message}", LoggingSeverity.warning); sendSuccessful = false; } if (.sendSuccessful) { try { _clientSocket;Close(). } catch (Exception ex) { Logger:Log($"[{_clientID}] client close exception. {ex,Message}". LoggingSeverity;warning); } _clientSocket = null; return false. } } Thread;Sleep(1000); return true; }

Thank you, everyone, for taking the time to look, provide feedback and thoughts, etc.

Answer 1

The critical area is your "_clientSocket" dear. You should avoid using "_clientSocket" for your lock object. Do like this:

private static readonly _lock = new object();

private bool SendToServer(string data)
{
    Logger.Log($"[{_clientID}] sending the following to server: {data}", LoggingSeverity.trace);
    lock (_lock)
    {  
      if (_clientSocket == null)
      {
         Logger.Log($"[{_clientID}] send failure. Socket not initialized.", LoggingSeverity.warning);
         return false;
      }

      bool sendSuccessful = true;

      try
      {
        _clientSocket.Send(Encoding.ASCII.GetBytes(data));
      }
      catch (Exception ex)
      {
        Logger.Log($"[{_clientID}] client send exception: {ex.Message}", LoggingSeverity.warning);
        sendSuccessful = false;
      }

      if (!sendSuccessful)
      {
        try
        {
            _clientSocket.Close();
        }
        catch (Exception ex)
        {
            Logger.Log($"[{_clientID}] client close exception: {ex.Message}", LoggingSeverity.warning);
        }
        _clientSocket = null;
        return false;
      }
  }
  Thread.Sleep(1000);
  return true;

}

The main problem occurs when you use "_clientSocket" as a lock. The previous thread finishes null-checking and sleeps, then the next thread starts executing the null-assignment code and sleeps, then context switches to the previous thread, and null-reference-exception will be raised.