The problem about serial port connect of Yun

Hello, I want ask some problem about the YUN.
I make a C# code by visual studio, and I used the YUN and UNO to do the serial port connect test.
Then only the UNO can work normaly.
The Yun only can receive the char from PC interface. But it can not send the char to the PC.
Then I try to use the CMD to receive the YUN's char, then it can work.
I want to ask what the YUN problem.
Thank you very much.