[]
Gets or sets the format code for the object as a string in the language of the user.
Use this property to define or retrieve the number format code stored in the style.
string NumberFormat { get; set; }
Property NumberFormat As String
IStyle style = workbook.Styles.Add("currencyStyle");
style.NumberFormat = "$#,##0.00";
worksheet.Range["A1"].Style = style;
string numberFormat = style.NumberFormat;