使用tcp客户端和streamreader阻塞while循环

Question

This method is freezing my GUI and I don't understand why.

Specifically it's getting stuck in while ((feedback = reader.ReadLine()) != null)

I've tried breaking out of the loop by checking reader.EndOfStream() with the same result. The printer is behaving as expected and printing the label, and the expected feedback is also being returned. It just sits in the loop, blinking a cursor in the console.

I can get rid of the loop, as I don't really need the feedback for this program, but I would still like to understand what's happening.

It's being called like this:

private void printBarcodeButton_Click(object sender, RoutedEventArgs e)
    {
        ...
        code to get id and name
        ...

        messageTextBlock.Text = "Printing Barcode.";

        using (var printer = new BarcodePrinter())
        {
            printer.printBarcode(employeeID, employeeName);
        }

        messageTextBlock.Text = "Barcode printed.";
    }

Here is the method:

public void printBarcode(string employeeID, string employeeName)
    {
        var projectFolder = Directory.GetParent(Directory.GetCurrentDirectory()).Parent.FullName;
        var printerScript = Path.Combine(projectFolder, @"PrinterScript.txt");

        // Printer IP Address and communication port
        string ipAddress = "10.1.10.23";
        int port = 9100;

        // Fingerprint Commands read from text and sent to printer
        using (TcpClient client = new TcpClient())
        {
            client.Connect(ipAddress, port);
            using (var stream = client.GetStream())
            {
                // open the script file
                using (var file = new StreamReader(printerScript))
                {
                    // open a stream to get feedback from printer
                    string feedback = "none yet";
                    using (var reader = new StreamReader(stream))
                    {
                        // open stream to send commands to printer
                        using (var writer = new StreamWriter(stream))
                        {
                            string command, script = null;
                            while ((command = file.ReadLine()) != null)
                            {
                                // send all non-empty lines in script 
                                if (command != string.Empty && !command.Contains("**"))
                                {
                                    command = command.Replace("_employeeID_", employeeID);
                                    command = command.Replace("_employeeName_", employeeName);
                                    script += command + "\n";
                                }
                            }
                            writer.Write(script);
                            writer.Flush();

                            // display data sent back from printer (including echo of commands)
                            while ((feedback = reader.ReadLine()) != null)
                            {
                                Console.WriteLine(feedback);                                    
                            }
                        }
                    }
                }
            }
        }
    }

Answer 1

Excerpt from MSDN :

A line is defined as a sequence of characters followed by a line feed ("\\n"), a carriage return ("\\r"), or a carriage return immediately followed by a line feed ("\\r\\n"). The string that is returned does not contain the terminating carriage return or line feed. The returned value is null if the end of the input stream is reached.

You probably haven't reached a \\n , \\r or \\r\\n nor the end of the input. The caveat here is that normally "end of the input" means that the other point (the printer in your scenario) closes the channel and most probably it is not doing that.

I recommend using timeouts on the reception as those devices normally send all the data at once. There are other options if this one is not valid, just ask, but it should work most of the times.

Answer 2

Simply has to add this:

while ((feedback = reader.ReadLine()) != null)
                        {
                            Console.WriteLine(feedback);
                            if (feedback == "Ok") break;
                        }

It even sends the Ok message after an error, so that covers everything.

Answer 3

Instead of having the assignment to the command within the while condition, I would try putting it as the last command in the while statement instead:

string command, script = null;
command = file.ReadLine();
while (command != null)
{
// send all non-empty lines in script 
     if (command != string.Empty && !command.Contains("**"))
     {
     command = command.Replace("_employeeID_", employeeID);
     command = command.Replace("_employeeName_", employeeName);
     script += command + "\n";
     }
     command = file.ReadLine();
}

This change would merely ensure explicit reading before checking the while-loop condition, and assures that the assignment within the while-loop is not the condition.