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CHAPTER 2 s FRAMEWORK DESIGN
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Public Property Name() As String Get CanReadProperty(True) return mName End Get Set(ByVal value As String) CanWriteProperty(True) If mName <> value Then mName = value PropertyHasChanged() End If End Set End Property The CanReadProperty() and CanWriteProperty() methods check the current user s roles against the list of roles allowed and denied read and write access to this particular property. If the authorization rules are violated, a security exception is thrown; otherwise, the user is allowed to read or write the property. There are other overloads of these methods as well, offering variation in coding simplicity, control, and performance. These will be fully explored in 3. The CanReadProperty() and CanWriteProperty() methods are Public in scope. This is important because it allows code in the UI layer to ask the object about the user s permissions to read and write each property. The UI can use this information to alter its display to give the user visual cues as appropriate. In 9, you ll see how this capability can be exploited by an extender control in Windows Forms to eliminate most authorization code in a typical application. While the story isn t quite as compelling in Web Forms, 10 will demonstrate how to leverage this capability in a similar manner.
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Helper Types and Classes
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Most business applications require a set of common behaviors not covered by the concepts discussed thus far. These behaviors are a grab bag of capabilities that can be used to simplify common tasks that would otherwise be complex. These include the items listed in Table 2-6. Table 2-6. Helper Types and Classes
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Type or Class
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SafeDataReader ObjectAdapter DataMapper SmartDate
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Description
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Wraps any IDataReader (such as SqlDataReader) and converts all null values from the database into non-null empty or default values Fills a DataSet or DataTable with information from an object or a collection of objects Maps data from an IDictionary to an object s properties, or from one object s properties to another object s properties Implements a DateTime data type that understands how to translate values transparently between DateTime and string representations, and also understands the concept of an empty date Provides a sorted view of any IList(Of T); if the underlying collection is editable, then the view will also be editable
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SortedBindingList
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Let s discuss each of these in turn.
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CHAPTER 2 s FRAMEWORK DESIGN
SafeDataReader
Most of the time, applications don t care about the difference between a null value and an empty value (such as an empty string or a zero) but databases often do. When retrieving data from a database, an application needs to handle the occurrence of unexpected null values with code such as the following: If dr.IsDBNull(idx) Then myValue = "" Else myValue = dr.GetString(idx) End If Clearly, doing this over and over again throughout the application can get very tiresome. One solution is to fix the database so that it doesn t allow nulls when they provide no value, but this is often impractical for various reasons.
s Note
Here s one of my pet peeves: allowing nulls in a column in which you care about the difference between a value that was never entered and the empty value ("", or 0, or whatever) is fine. Allowing nulls in a column where you don t care about the difference merely complicates your code for no good purpose, thereby decreasing developer productivity and increasing maintenance costs.
As a more general solution, the framework includes a utility class that uses SqlDataReader (or any IDataReader implementation) in such a way that you never have to worry about null values again. Unfortunately, the SqlDataReader class isn t inheritable it can t be subclassed directly. Instead, it is wrapped using containment and delegation. The result is that your data access code works the same as always, except that you never need to write checks for null values. If a null value shows up, SafeDataReader will automatically convert it to an appropriate empty value. Obviously, if you do care about the difference between a null and an empty value, you can just use a regular SqlDataReader to retrieve the data. In this case, .NET 2.0 includes the new Nullable(Of T) generic type that helps manage null database values. This new type is very valuable when you do care about null values: when business rules dictate that an empty value like 0 is different from null.
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