Wrong COM Port Number

[C# Programming]

Hello...

I just installed USB-RS232 Device and I see in DeviceManager, that the COM Port ist 7:
"Prolific USB-COM Bridged (COM7)"

But if I try to use the COM7, the SerialPort Class throw an exception "COM Port unknown".
The COM7 is now COM1, but the text in DeviceManager is still COM7.

Has anybody the same problem ?
How to solve the problem ?

Thanks...