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);
}