How to convert system.string to uint8?

I'm using .NET to read FTDI usb uart serial data.

The following method:

 str     = System.String('create a System.String');
 [r str] = Read( fo, BytesToRead, BytesReceived);
 arr     = (uint8(char(str)))

results to 'arr' maximum value is 127.

How to pass system.string values between 127..255 (uint8)? Or is there better conversion method for system.string variables to uint8?