Check out
System.Text.AsciiEncoding.GetBytes[
^]
and
Convert.ToBase64String[
^]
The latter of which converts it to a base-64 number that can be converted back to the text. However there really isn't a way to convert the text to a number, have that number fit inside a numerical structure, and be unique to the text. BigInteger isn't big enough to handle that much data.
You could write your own hashing function, but even then its hard to guarantee a unique string.