C# – How to set the ReadBufferSize on .NET serial port

So, I am creating a VB.NET program that listens to a serial port. I was using this

Dim WithEvents ptr_SerialPort As System.IO.Ports.SerialPort
...
 ptr_SerialPort = My.Computer.Ports.OpenSerialPort(serialportname, baudrate,parity, databits, stopbits)

which worked fine until i realized that I am overrunning my SerialPort.ReadBufferSize property, which is 4096, and losing data:

https://msdn.microsoft.com/en-us/library/system.io.ports.serialport.readbuffersize(v=vs.110).aspx

So I tried to increase the size of the buffer as follows

ptr_SerialPort = My.Computer.Ports.OpenSerialPort(serialportname, baudrate, parity, databits, stopbits)
ptr_SerialPort.ReadBufferSize = 2000000

But I get the error "can't set the buffer size while it's open". Which is reasonable, so I tried this:

ptr_SerialPort = New System.IO.Ports.SerialPort(serialportname, baudrate, parity, databits, stopbits)
ptr_SerialPort.ReadBufferSize = 2000000
ptr_SerialPort.Open()

Well, this results in a Serial port that does NOT receive data… I guess that My.Computer.Ports.OpenSerialPort is doing more than just instantiating it, and what I have above is a serial port object that is not connected to my hardware. So, my question is… how do I open a serial port on my PC, and specify the ReadBufferSize property??

Thanks very much!