Did you configure your port properly?
Somthing close to this should work:
SerialPort^ port = gcnew SerialPort();
port->PortName = "COM1";
port->BaudRate = 9600;
port->DataBits = 8;
port->Parity = Parity::None;
port->StopBits = StopBits::One;
port->ReadTimeout = 500;
port->WriteTimeout = 500;
port->Encoding = System::Text::Encoding::GetEncoding("Latin1");
String^ txt = port->ReadExisting();
StreamWriter^ sw = gcnew StreamWriter("TestFile.txt");
sw->WriteLine(txt);
sw->Flush();
sw->Close();