public static void AddCellListValidation(this Range cell, IList <string> allowedValues, string initialValue = null) { var flatList = String.Join(System.Globalization.CultureInfo.CurrentCulture.TextInfo.ListSeparator, allowedValues.Select(x => x.ToString()).ToArray()); // var flatList = allowedValues.Aggregate((x, y) => "{x},{y}"); //if (flatList.Length > 255) //{ // throw new ArgumentException("Combined number of chars in the list of allowedValues can't exceed 255 characters"); //} cell.AddCellListValidation(flatList, initialValue); }