Error 1 The type or namespace name 'ShortMessageCollection' could not be found (are you missing a using directive or an assembly reference?) C:\Users\prashant\Documents\Visual Studio...

public ShortMessageCollection ReadSMS(SerialPort port)
       {
           // Set up the phone and read the messages
           ShortMessageCollection messages = null;
           try
           {
               #region Execute Command
               // Check connection
               ExecCommand(port, "AT", 300, "No phone connected");
               // Use message format "Text mode"
               ExecCommand(port, "AT+CMGF=1", 300, "Failed to set message format.");
               // Use character set "PCCP437"
               ExecCommand(port, "AT+CSCS=\"PCCP437\"", 300,
               "Failed to set character set.");
               // Select SIM storage
               ExecCommand(port, "AT+CPMS=\"SM\"", 300,
               "Failed to select message storage.");
               // Read the messages
               string input = ExecCommand(port, "AT+CMGL=\"ALL\"", 5000,
                   "Failed to read the messages.");
               #endregion

               #region Parse messages
               messages = ParseMessages(input);
               #endregion
           }
           catch (Exception ex)
           {
               throw new Exception(ex.Message);
           }

           if (messages != null)
               return messages;
           else
               return null;
       }