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5 BUILDING THE BUSINESS LOGIC LAYER
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Notice the Private keyword on the setter This allows the getter to be public but restricts the setter to be private The code within the class can then get or set the property, and code outside the class can only get the property
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Some restrictions and rules apply when you use accessibility on the accessors:
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The accessibility on the Property statement must be less restrictive than the accessibility on the accessor For example, you cannot define the Property statement to be private and then make the getter public You can add accessibility to the getter or setter, but not both If the getter needs to be friend and the setter needs to be private, for example, make the Property statement friend (the least restrictive), and make the setter private If you use the ReadOnly or WriteOnly keywords, you cannot add accessibility on the accessor
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De ne accessibility appropriately to ensure that your properties are accessed only as they should be Most properties are public, but for some properties, such as IDs, de ne private setters to allow reading but not setting of the property
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For the Purchase Tracker sample application: Open the Product class in the Code Editor Add a ProductID property with a private setter, as defined in this section Add code to declare a backing variable, and get and set its value in the property Add documentation for the ProductID property using XML comments This new property is used later in this chapter
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A data type is said to be nullable if it can be assigned a value or a null reference Reference types, such as strings and class types, are nullable; they can be set to a null reference, and the result is a null value Value types, such as integers, Booleans, and dates, are not nullable If you set a value type to a null reference, the result is a default value, such as 0 or
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false A value type can express only the values appropriate to its type; there is no easy way for a value type to understand that it is null The NET Framework 20 introduces a Nullable class and an associated Nullable structure The Nullable structure includes the value type itself and a eld identifying whether the value is null A variable of a Nullable type can represent all the values of the underlying type, plus an additional null value The Nullable structure supports only value types because reference types are nullable by design For example, say you have a Product class with a ProductID property de ned as an integer, a ProductName property de ned as a string, and an InventoryDate property de ned as a date The following code sets each property to Nothing to assign a null reference:
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Dim prod as Product prod = New Product prodProductID = Nothing prodProductName = Nothing prodInventoryDate = Nothing DebugWriteLine(prodProductID) ' Displays 0 DebugWriteLine(prodProductName) ' Displays (Nothing) DebugWriteLine(prodInventoryDate) ' Displays 1/1/0001 12:00:00 AM
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5 BUILDING THE BUSINESS LOGIC LAYER
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erties are value types and therefore cannot store a null Instead, they store a default value when they are assigned a null reference There may be cases, however, when you need your code to really handle a null as a null and not as a default value It would be odd, for example, to handle a null date by hard-coding a check for the 1/1/0001 date To make a value type property nullable, you need to declare it using the Nullable structure However, you still want your property to be strongly typed as an integer, date, Boolean, or the appropriate underlying type The ability to use a class or structure for only a speci c type of data is the purpose of generics Generics allow you to tailor a class, structure, method, or interface to a speci c data type So you can create a class, structure, method, or interface with generalized code When you use it, you de ne that it can work only on a particular data type This gives you greater code reusability and type safety
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If you view these values, they are 0, Nothing, and 1/1/0001 12:00:00 AM, respectively The ProductID and InventoryDate prop-
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