Well, on the one hand, it makes no sense to create a byte array and then ignore it - so your code will be simpler:
byte[] byteEncString = Encoding.UTF8.GetBytes(_strToConvert);
return Convert.ToBase64String(byteEncString);
.NET char 3 , UTF-8 1 45- . base64 3 4 , 60 base64.
1 , , , . 4 , 2 , " char" 2.