how to read the data from serial port in c#

Question

how to read data (at every 1 millisecond)from serial port in c# windows applications . i have read the the data from serial port  Continuosly at that  time  not able to send  data onto the serial port . at that time again connect the serial port then able to send the data.  so at that time how to clear the  serial port receive buffer(how to clear the data on port)
Please help me anyone