ToHexEncodedString(string s) - Umlaute beim Dateidownload

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/// <summary>
    /// Encodiert Sonderzeichen
    /// </summary>
    /// <param name="s"></param>
    /// <returns></returns>
    public static string ToHexEncodedString(string s)
    {
        System.Text.StringBuilder returnValue = new System.Text.StringBuilder();
        System.Text.UTF8Encoding utf8 = new System.Text.UTF8Encoding();
        string reservedChars = "_+-=.,!'()@&$";
        char[] chr = s.ToCharArray();
        for (int i=0;i!=chr.Length; i++)
        {
            if (chr[i] <= 127 && (reservedChars.IndexOf(chr[i]) != -1 || char.IsLetterOrDigit(chr[i])))
                returnValue.Append(chr[i]);
            else
            {
                System.Text.StringBuilder encodedValue = new System.Text.StringBuilder();
                byte[] encBytes = utf8.GetBytes(chr[i].ToString());
                for (int j = 0; j != encBytes.Length; j++)
                {
                    encodedValue.AppendFormat("%{0}", Convert.ToString(encBytes[j], 16));
                }
                returnValue.Append(encodedValue.ToString());
            }
        }
        return returnValue.ToString();
    }