IT

Nullable Types (C# Programming Guide)
Nullable types are instances of the System.Nullable struct. A nullable type can represent the correct range of values for its underlying value type, plus an additional null value.
Examples:

• a Nullable int32, pronounced "Nullable of Int32," can be assigned any value from -2147483648 to 2147483647, or it can be assigned the null value.
• a Nullable bool can be assigned the values truefalse, or null.

The AddProduct method take in as parameters the values for the various product fields and add a new product. Since many of the Product table's columns can accept NULL values (CategoryID, SupplierID, and UnitPrice, to name a few), those input parameters for AddProduct that map to such columns use nullable types [2]:

[System.ComponentModel.DataObjectMethodAttribute]
    (System.ComponentModel.DataObjectMethodType.Insert, true)]
public bool AddProduct(string productName, int? supplierID,
    int? categoryID, string quantityPerUnit, decimal? unitPrice,
    short? unitsInStock, short? unitsOnOrder, short? reorderLevel,
    bool discontinued)
    {  
          (....)
    }