[C#] base64 encoding / decoding

public string Encoding(string EncodingText, System.Text.Encoding oEncoding = null)
{
    if (oEncoding == null)
        oEncoding = System.Text.Encoding.UTF8;

    byte[] arr = oEncoding.GetBytes(EncodingText);
    return System.Convert.ToBase64String(arr);
}

        public string Decoding(string DecodingText, System.Text.Encoding oEncoding = null)
        {
            if (oEncoding == null)
                oEncoding = System.Text.Encoding.UTF8;
            try
            {
                byte[] arr = System.Convert.FromBase64String(DecodingText);
                return oEncoding.GetString(arr);
            }
            catch
            {
                return "err" + DecodingText;
            }
        }