岁月留声QtAsp

变量定义中含有一个问号，意思是这个数据类型是NullAble类型的。
*变量定义中含有两个问号，意思是取所赋值??左边的，如果左边为null，取所赋值??右边的。

Nullable Types (C# Programming Guide)
http://msdn.microsoft.com/en-us/library/1t3y8s4s(VS.80).aspx

C# 2.0 Nullable Types
By Edward Moemeka

General instructions on the use of nullable types in C#.
http://www.codeproject.com/KB/cs/c__20_nullable_types.aspx

数据库例子
[System.ComponentModel.DataObjectMethodAttribute(System.ComponentModel.DataObjectMethodType.Select, false)]
    public Northwind.ProductsDataTable GetProductsBySupplierID(int supplierID)
    {
        return Adapter.GetProductsBySupplierID(supplierID);
    }
    [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)
    {
        // Create a new ProductRow instance
        Northwind.ProductsDataTable products = new Northwind.ProductsDataTable();
        Northwind.ProductsRow product = products.NewProductsRow();

        product.ProductName = productName;
        if (supplierID == null) product.SetSupplierIDNull(); else product.SupplierID = supplierID.Value;
        if (categoryID == null) product.SetCategoryIDNull(); else product.CategoryID = categoryID.Value;
        if (quantityPerUnit == null) product.SetQuantityPerUnitNull(); else product.QuantityPerUnit = quantityPerUnit;
        if (unitPrice == null) product.SetUnitPriceNull(); else product.UnitPrice = unitPrice.Value;
        if (unitsInStock == null) product.SetUnitsInStockNull(); else product.UnitsInStock = unitsInStock.Value;
        if (unitsOnOrder == null) product.SetUnitsOnOrderNull(); else product.UnitsOnOrder = unitsOnOrder.Value;
        if (reorderLevel == null) product.SetReorderLevelNull(); else product.ReorderLevel = reorderLevel.Value;
        product.Discontinued = discontinued;

        // Add the new product
        products.AddProductsRow(product);
        int rowsAffected = Adapter.Update(products);

        // Return true if precisely one row was inserted, otherwise false
        return rowsAffected == 1;
    }