Changes online value of the Scaling Factor Numerator through the CurrentConfig.xml in TwinCAT 3.1

Tc3_CurrentConfig.cs

using System;
using System.Linq;
using System.Xml.Linq;

namespace Tc3_CurrentConfig
{
    public class CurrentConfig
    {
        const string CURRENTCONFIG_PATH = @"C:\TwinCAT\3.1\Boot\CurrentConfig.xml";

        /// <summary>
        /// Writes Scaling Factor Numerator
        /// </summary>
        /// <param name="newValue">New value of Scaling Factor Numerator</param>
        /// <param name="axisNo">NC axis number. Count begins from 1. 1 is a first axis.</param>
        /// <returns>Current value of Scaling Factor Numerator</returns>
        public static double WriteScalingFactorNumerator(double newValue, int axisNo)
        {
            if (axisNo < 1 || axisNo > 255)
                throw new ArgumentOutOfRangeException( "axisNo", axisNo,
                    "1 >= axisNo <= 255. Count begins from 1. 1 is a first axis.");

            const int HEX_START_INDEX = 96;
            const int HEX_VALUE_LENGTH = 16; // double = 8 bytes length

            string hexAxisNo = axisNo.ToString("X2");

            /// Loading ///////////////////////////////////////////////////////

            XDocument doc = XDocument.Load(CURRENTCONFIG_PATH);

            var xEncAxis1 = doc.Descendants()
                .Single(e =>
                    e.Name.LocalName == "InitCmd"
                    && e.Elements().Where(
                        ee =>  (ee.Name.LocalName == "port" && ee.Value == "500" )
                            || (ee.Name.LocalName == "iGrp" && ee.Value == "5124")
                            || (ee.Name.LocalName == "iOffs" && ee.Value == "0"  )
                            || (ee.Name.LocalName == "data" && ee.Value.StartsWith(hexAxisNo)) )
                        .Count() == 4 );

            var xData = xEncAxis1.Elements().Single(e => e.Name.LocalName == "data");                
            string data = xData.Value;

            /// Current Value /////////////////////////////////////////////////

            string hexValue = data.Substring(HEX_START_INDEX, HEX_VALUE_LENGTH);
            double currentDoubleValue = BitConverter.ToDouble(HexToByteArray(hexValue));

            /// Write New Value ///////////////////////////////////////////////

            hexValue = BitConverter.ToString(BitConverter.GetBytes(newValue))
                .Replace("-","")    // BitConverter returns like this AA-BB-CC-...
                .ToLower();         // Tc3 writes hex in lower case

            xData.Value = data.Substring(0, HEX_START_INDEX) + hexValue + data.Substring(HEX_START_INDEX + HEX_VALUE_LENGTH);
            doc.Save(CURRENTCONFIG_PATH);

            return currentDoubleValue;
        }

        // inefficient but fun, see: https://stackoverflow.com/a/321404
        public static byte[] HexToByteArray(string hexValue)
        {
            return Enumerable.Range(0, hexValue.Length / 2)
                .Select(x => Convert.ToByte(hexValue.Substring(x * 2, 2), 16))
                .ToArray();
        }
    }
}