Rijndael encrypt function syntax

Can anyone help me
I am trying to create a function that encrypts using Rijndael. I am using the following code
But I get the error message "This is obsolete" when I use
Dim keyBytes As Byte()
 keyBytes = password.GetBytes(keySize / 8)
 Can anyone tell me the correct syntax.
 
Many thanks
 
Martin
This is the section that throws the error.

' First, we must create a password, from which the key will be derived.
        ' This password will be generated from the specified passphrase and
        ' salt value. The password will be created using the specified hash
        ' algorithm. Password creation can be done in several iterations.
     ...