Convert a String to Base64 in C#

I keep looking for a function like
public static string StringToBase64(string s) in System.Text, etc.
So far, the easiest way I've found to accomplish this is:

string sAscii = "my text";
System.Text.ASCIIEncoding encoding = new System.Text.ASCIIEncoding();
byte[] bytes = encoding.GetBytes(sAscii);
string sBase64 = System.Convert.ToBase64String(bytes)

And how about just doing string -> byte[] and back? (no base 64?)
System.Text.ASCIIEncoding encoding = new System.Text.ASCIIEncoding();
byte[] bytes = encoding .GetBytes(sAscii);
string sAscii = encoding.GetString(bytes);