Hi i'm using ClosedXML in one of my projects and replaced only this library and got this exception Method not found 'System.String ExcelNumberFormat.NumberFormat.Format(System.Object, System.Globalization.CultureInfo)' StackTrace: at ClosedXML.Extensions.FormatExtensions.ToExcelFormat(Object o, String format) at ClosedXML.Excel.XLCell.GetFormattedString() ... Looks like you changed the function signature and do not provide the original function any longer ?? 1.0.10 public string Format(object value, CultureInfo culture) 1.1.0 public string Format(object value, CultureInfo culture, bool isDate1904 = false) Better would be to provide the original public function as well ?? Thanks Peter