.net UrlEncode - lowercase problem

I think you're stuck with what C# gives you, and getting errors suggests a poorly implemented UrlDecode function on the other end.

With that said, you should just need to loop through the string and uppercase only the two characters following a % sign. That'll keep your base64 data intact while massaging the encoded characters into the right format:

public static string UpperCaseUrlEncode(string s)
{
  char[] temp = HttpUtility.UrlEncode(s).ToCharArray();
  for (int i = 0; i < temp.Length - 2; i++)
  {
    if (temp[i] == '%')
    {
      temp[i + 1] = char.ToUpper(temp[i + 1]);
      temp[i + 2] = char.ToUpper(temp[i + 2]);
    }
  }
  return new string(temp);
}

Replace the lowercase percent encoding from HttpUtility.UrlEncode with a Regex:

static string UrlEncodeUpperCase(string value) {
    value = HttpUtility.UrlEncode(value);
    return Regex.Replace(value, "(%[0-9a-f][0-9a-f])", c => c.Value.ToUpper());
}

var value = "SomeWords 123 #=/ äöü";

var encodedValue = HttpUtility.UrlEncode(value);
// SomeWords+123+%23%3d%2f+%c3%a4%c3%b6%c3%bc

var encodedValueUpperCase = UrlEncodeUpperCase(value);
// now the hex chars after % are uppercase:
// SomeWords+123+%23%3D%2F+%C3%A4%C3%B6%C3%BC

This is very easy

Regex.Replace( encodedString, @"%[a-f\d]{2}", m => m.Value.ToUpper() )

I.e. replace all hex letter-digit combinations to upper case

I know this is very old and maybe this solution didn't exist, but this was one of the top pages I found on google when trying to solve this.

System.Net.WebUtility.UrlEncode(posResult);

This encodes to uppercase.