Question

I appear to have a named pipes 101 issue. I have a very simple set up to connect a simplex named pipe transmitting from a C++ unmanaged app to a C# managed app. The pipe connects, but I cannot send a “message” through the pipe unless I close the handle which appears to flush the buffer and pass the message through. It’s like the message is blocked. I have tried reversing the roles of client/server and invoking them with different Flag combinations without any luck. I can easily send messages in the other direction from C# managed to C++ unmanaged. Does anyone have any insight. Can any of you guys successfully send messages from C++ unmanaged to C# managed? I can find plenty of examples of intra amanged or unmanaged pipes but not inter managed to/from unamanged – just claims to be able to do it.

In the listings, I have omitted much of the wrapper stuff for clarity. The key bits I believe that are relevant are the pipe connection/creation/read and write methods. Don’t worry too much about blocking/threading here.

C# Server side

    // This runs in its own thread and so it is OK to block
    private void ConnectToClient()
    {
        // This server will listen to the sending client
        if (m_InPipeStream == null)
        {
            m_InPipeStream =
                new NamedPipeServerStream("TestPipe", PipeDirection.In, 1);
        }

        // Wait for client to connect to our server
        m_InPipeStream.WaitForConnection();

        // Verify client is running
        if (!m_InPipeStream.IsConnected)
        {
            return;
        }

        // Start listening for messages on the client stream
        if (m_InPipeStream != null && m_InPipeStream.CanRead)
        {
            ReadThread = new Thread(new ParameterizedThreadStart(Read));
            ReadThread.Start(m_InPipeStream);
        }
    }


    // This runs in its own thread and so it is OK to block
    private void Read(object serverObj)
    {
        NamedPipeServerStream pipeStream = (NamedPipeServerStream)serverObj;
        using (StreamReader sr = new StreamReader(pipeStream))
        {
            while (true)
            {
                string buffer = "" ;
                try
                {
                    // Blocks here until the handle is closed by the client-side!!
                    buffer = sr.ReadLine();   // <<<<<<<<<<<<<<  Sticks here
                }
                catch
                {
                    // Read error
                    break;
                }

                // Client has disconnected?
                if (buffer == null || buffer.Length == 0)
                    break;

                // Fire message received event if message is non-empty
                if (MessageReceived != null && buffer != "")
                {
                    MessageReceived(buffer);
                }
            }
        }
    }

C++ client side

    // Static - running in its own thread.
    DWORD CNamedPipe::ListenForServer(LPVOID arg)
    {
        // The calling app (this) is passed as the parameter
        CNamedPipe* app = (CNamedPipe*)arg;

        // Out-Pipe: connect as a client to a waiting server
        app->m_hOutPipeHandle =
        CreateFile("\\\\.\\pipe\\TestPipe",
               GENERIC_WRITE,
               0,
               NULL,
               OPEN_EXISTING,
               FILE_ATTRIBUTE_NORMAL,
               NULL);
        // Could not create handle
        if (app->m_hInPipeHandle == NULL ||
            app->m_hInPipeHandle == INVALID_HANDLE_VALUE)
        {
            return 1;
        }

        return 0;
    }


    // Sends a message to the server
    BOOL CNamedPipe::SendMessage(CString message)
    {
    DWORD dwSent;

        if (m_hOutPipeHandle == NULL ||
            m_hOutPipeHandle == INVALID_HANDLE_VALUE)
        {
            return FALSE;
        }
        else
        {
            BOOL bOK =
                WriteFile(m_hOutPipeHandle,
                          message, message.GetLength()+1, &dwSent, NULL);
            //FlushFileBuffers(m_hOutPipeHandle);             // <<<<<<< Tried this
            return (!bOK || (message.GetLength()+1) != dwSent) ? FALSE : TRUE;
        }
    }


    // Somewhere in the Windows C++/MFC code...
    ...
    // This write is non-blocking. It just passes through having loaded the pipe.
    m_pNamedPipe->SendMessage("Hi de hi");
    ...

Answer 1

sr.ReadLine() expects to see a newline character(s) to know the end of the line. Because it receives neither new-line nor end-of-stream, it waits for more.
Try:

m_pNamedPipe->SendMessage("Hi de hi\n")

or some of the sr.Read() methods.