Encode

public static string Base64Encode(string plainText) {
            string myRes = "";

            try

            {

                var base64EncodedBytes = System.Convert.FromBase64String(base64EncodedData);

                myRes = System.Text.Encoding.UTF8.GetString(base64EncodedBytes);

            }

            catch

            {

                myRes = base64EncodedData;

            }

            return myRes;

}

Decode

public static string Base64Decode(string base64EncodedData) {

            string myRes = "";

            try

            {

                var plainTextBytes = System.Text.Encoding.UTF8.GetBytes(plainText);

                myRes = System.Convert.ToBase64String(plainTextBytes);

            }

            catch

            {

                myRes = plainText;

            }

            return myRes;

}