How to use custom function in XlsIO

CalcEngine engine;
private void button1_Click(object sender, EventArgs e)
{
    using (ExcelEngine excelEngine = new ExcelEngine())
    {
        IApplication application = excelEngine.Excel;
        IWorkbook workbook = application.Workbooks.Open("../../Data/Sample.xlsx");
        IWorksheet worksheet = workbook.Worksheets[0];
 
        // Class derived from ICalcData,
        CalcData calcData = new CalcData();
        // Set value from A1 cell to variable "A1"
        calcData.SetValueRowCol(worksheet["A1"].Value, 1, 1);
        // Set value from B1 cell to variable "B1"
        calcData.SetValueRowCol(worksheet["B1"].Value, 1, 2);
 
        engine = new CalcEngine(calcData);
        engine.AddFunction("CustomSum", new CalcEngine.LibraryFunction(CustomSum));
 
        worksheet["C1"].Value = engine.ParseAndComputeFormula("=CustomSum(A1,A1)");
        worksheet["D1"].Value = engine.ParseAndComputeFormula("=CustomSum(A1,B1)");
 
        MessageBox.Show(worksheet["C1"].Value);
        MessageBox.Show(worksheet["D1"].Value);
 
        workbook.SaveAs("../../Output/Output.xlsx");
    }
}
 
public string CustomSum(string args)
{
    double sum = 0;
    double d;
    var splitArgs = args.Split(new char[] { CalcEngine.ParseArgumentSeparator });
    foreach (string s in splitArgs)
    {
        var s1 = engine.GetValueFromArg(s);
        if (double.TryParse(s1, NumberStyles.Number | NumberStyles.AllowExponent, null, out d))
            sum += d;
    }
    return sum.ToString();
}
 
public class CalcData : ICalcData
{
    public event ValueChangedEventHandler ValueChanged;
 
    Dictionary<string, object> values = new Dictionary<string, object>();
    public object GetValueRowCol(int row, int col)
    {
        object value = null;
        var key = RangeInfo.GetAlphaLabel(col) + row;
        this.values.TryGetValue(key, out value);
        return value;
    }
 
    public void SetValueRowCol(object value, int row, int col)
    {
        var key = RangeInfo.GetAlphaLabel(col) + row;
        if (!values.ContainsKey(key))
            values.Add(key, value);
        else if (values.ContainsKey(key) && values[key] != value)
            values[key] = value;
    }
 
    public void WireParentObject() { }
 
    private void OnValueChanged(int row, int col, string value)
    {
        if (ValueChanged != null)
            ValueChanged(this, new ValueChangedEventArgs(row, col, value));
    }
}