C# version of the javascript function btoa

Question

I need to recode something from js to c# that utilises the btoa method in js on a string of unicode chars to convert them to base64. However, as far as I know the encoding used by javascrpt is different to all those available in c#. I need the encoding to be exactly the same and not return different values across these languages. I have tried setting up a nodejs server and making get requests to it, in order to run the encoding that way, but this is far too slow and unstable. I am under the impression I would need to make my own encoding table but I have no idea where to start or how to implement this. If anyone could help that would be greatly appreciated.

tl;dr: javascript's btoa returns different value than base 64 encoding in c# does. I need it to be the same values.

code and outputs:

c#:
String fromChar = new 
String(247,71,186,8,125,72,2,1.0078125,0.003936767578125);
var plainTextBytes = System.Text.Encoding.UTF8.GetBytes(fromChar);
Console.WriteLine(System.Convert.ToBase64String(plainTextBytes));

output = w7dHwroIfUgCAQA=


javascript:
var x = btoa(String.fromCharCode(247,71,186,8,125,72,2,1.0078125,0.003936767578125);
console.log(x)

output = 
90e6CH1IAgEA

I am aware the former example uses utf8 encoding which js does not, the problem is there is no encoding in .net that matches the javascript encoding.

Edit: Tried to compare the byte arrays of both c# and javascript but the problem is the btoa function uses an unnamed encoding, so I can't actually get the bytes to print the byte array for it without assuming it is something like utf8 which it is not.

Answer 1

Worked it out. For anyone wondering the encoding used is iso encoding. The btoa function in javascript can be replicated by using the following c# method:

public string encoding(string toEncode)
{
byte[] bytes= Encoding.GetEncoding(28591).GetBytes(toEncode);
string toReturn = System.Convert.ToBase64String(bytes);
return toReturn;
}

Answer 2

The decoding will be the following:

string base64EncodedString = "6Q==";
Encoding.GetEncoding(28591).GetString(Convert.FromBase64String(base64EncodedString));
// "é"