.NET base 64 encoder. When will i have to write custom encode for it?

The .net provides its own implementation of converting to/from base64 encoding but sometime i see people writing their own encodes. Just wondering what could be the real life applications where you will have 开发者_如何学编程to write your custom encoder rather using the one that .net provides

These generally occur in code ported from another environment.

.Net has sufficient Bas64 capabilities in Convert.ToBase64String and System.Security.Cryptography.ToBase64Transform; there is no need to write your own.