public string base64Decode(string data)
{
    try
    {
        System.Text.UTF8Encoding encoder = new System.Text.UTF8Encoding();  
        System.Text.Decoder utf8Decode = encoder.GetDecoder();
    
        byte[] bt= Convert.FromBase64String(data);
        int charCount = utf8Decode.GetCharCount(bt, 0, bt.Length);    
        char[] ch = new char[charCount];
        utf8Decode.GetChars(bt, 0, bt.Length, ch , 0);                   
        string result = new String(ch );
        return result;
    }
    catch(Exception e)
    {
        throw new Exception("Error in base64Decode" + e.Message);
    }
}