C# – Serial port communications in C#

The API help says,

By default, the ReadLine method will block until a line is received. If this behavior is undesirable, set the ReadTimeout property to any non-zero value to force the ReadLine method to throw a TimeoutException if a line is not available on the port.

... so, do that if you want to avoid it hanging.

Anyway:

• How do you know that the two devices/ports are connected to each other (via Bluetooth)?
• Are you setting the SerialPort properties (e.g. BaudRate etc.) before you call the Open method?
• Does it make any difference which you invoke first, the ReadLine or the WriteLine? Instead of doing open/write/close and open/read/close, how about doing open/open/read/write/close/close instead?
• What the receivers BytesToRead property return after the sender has invoked WriteLine?
• Have you looked at the serial port sample code, which exists (according to the Stonestreet One Bluetooth SDK FAQ)

If their SDK does have a sample program, then I suggest you use it (unaltered) to verify your test setup (e.g. to verify that your devices are connecting properly), before you alter the sample program and/or before you test your own software (using the same devices/test setup which you've already tested with their sample software).

To get local Ip Address:

public static string GetLocalIPAddress()
{
    var host = Dns.GetHostEntry(Dns.GetHostName());
    foreach (var ip in host.AddressList)
    {
        if (ip.AddressFamily == AddressFamily.InterNetwork)
        {
            return ip.ToString();
        }
    }
    throw new Exception("No network adapters with an IPv4 address in the system!");
}

To check if you're connected or not:

System.Net.NetworkInformation.NetworkInterface.GetIsNetworkAvailable();