AT command for sending sms through code

Question

Hello

Actually i am doing a computer program in C# to send and receive SMS through gsm mode(serial port)
i am using the below code

port = new SerialPort(cmbport.Text, 115200, Parity.None, 8, StopBits.One);
port.Open();
port.WriteLine("AT\r");

after opening the port the program is halt, it does not execute the api
writeline

if any body have idea please suggest me
waiting your reply
thanks
sadaba lam