I want to convert this function to C#

A quick test of taking your input string and encoding it in Base64 using [^], then converting it to Hexadecimal format suggests that what the ActionScript 'encode64 function is doing appears to be not a standard Base64 conversion.

Until you understand what the code in ActionScript is doing ... which could be influenced by:

1. OS in use and its character encoding

2. ActionScript character encoding

3. whatever Character Encoding set is "in play"

and you will need to understand how operators/api methods in ActionScript that appear similar to C# actually work.

So, start doing some basic research into ActionScript; ask questions on ActionScript forums. And, of course, use Google and search for similar questions, for code identical to the ActionScript 'encode64 function.

You just might be lucky and have someone here turn up who has dealt with the same problem but, most likely, you will have to puzzle it out yourself.

Here's an example in C# that takes a string, converts it to Base64 and then converts that to a string in Hex format:

// required
using System.Linq;

var plainTextBytes = System.Text.Encoding.UTF8.GetBytes(plainText);
string strBase64 = System.Convert.ToBase64String(plainTextBytes);
string strBase64ToHex = string.Join("", str.Select(c => ((int)c).ToString("X2")));

as base 64:

QUJDREVGR0hJSktMTU5PUFFSU1RVVldYWVphYmNkZWZnaGlqa2xtbm9wcXJzdHV2d3h5ejAxMjM0NTY3ODkrLz0=

as Hexadecimal representation of base 64:

51554A44524556475230684A536B744D545535505546465355315256566C645957567068596D4E6B5A575A6E61476C7161327874626D397763584A7A6448563264336835656A41784D6A4D304E5459334F446B724C7A303D