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Here s one of my pet peeves: allowing Null values 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 Null values 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.
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As a more general solution, CSLA .NET 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. Starting in .NET 2.0, you can use the 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 such as 0 is different from Null.
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In 20, you will see how to implement an ASP.NET Web Forms UI on top of business objects. This chapter uses the data binding capabilities introduced in Web Forms 2.0. In this technology, the Insert and Update operations provide the data from the form in IDictionary objects (name/value pairs). The values in these name/value pairs must be loaded into corresponding properties in the business object. You end up writing code much like this: cust.Name = e.Values("Name").ToString() cust.Address1 = e.Values("Address1").ToString() cust.City = e.Values("City").ToString() Similarly, in 21, you ll see how to implement a WCF service interface on top of business objects. When data is sent or received through a web service, it goes through a proxy object, an object
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with properties containing the data but no other logic or code. Since the goal is to get the data into or out of a business object, this means copying the data from one object s properties to the other. You end up writing code much like this: cust.Name = message.Name cust.Address1 = message.Address1 cust.City = message.City In both cases, this is repetitive, boring code to write. One alternative, though it does incur a performance hit, is to use reflection to automate the copy process. This is the purpose of the DataMapper class: to automate the copying of data to reduce all those lines of code to one simple line. It is up to you whether to use DataMapper in your applications.
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Dates are a perennial development problem. Of course, there s the DateTime data type, which provides powerful support for manipulating dates, but it has no concept of an empty date. The trouble is that many applications allow the user to leave date fields empty, so you need to deal with the concept of an empty date within the application. On top of this, date formatting is problematic rather, formatting an ordinary date value is easy, but again you re faced with the special case whereby an empty date must be represented by an empty string value for display purposes. In fact, for the purposes of data binding, you often want any date properties on the objects to be of type String so that the user has full access to the various data formats as well as the ability to enter a blank date into the field. Dates are also a challenge when it comes to the database; the date data types in the database don t recognize an empty date any more than .NET does. To resolve this, date columns in a database typically do allow Null values, so a Null can indicate an empty date.
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Note Technically, this is a misuse of the Null value, which is intended to differentiate between a value that was never entered and one that s empty. Unfortunately, you re typically left with no choice because there s no way to put an empty date value into a date data type.
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You may be able to use DateTime (Nullable(Of DateTime)) as a workable data type for your date values. But even that isn t always perfect because DateTime doesn t offer specialized formatting and parsing capabilities for working with dates, nor does it really acknowledge an empty date; it isn t possible to compare actual dates with empty dates, yet that is often a business requirement. The SmartDate type is an attempt to resolve this issue. Repeating the problem with SqlDataReader, the DateTime data type isn t inheritable, so SmartDate can t just subclass DateTime to create a more powerful data type. Instead, it uses containment and delegation to create a new type that provides the capabilities of the DateTime data type while also supporting the concept of an empty date. This isn t as easy at it might at first appear, as you ll see when the SmartDate class is implemented in 16. Much of the complexity flows from the fact that applications often need to compare an empty date to a real date, but an empty date might be considered very small or very large. You ll see an example of both cases in the sample application in 17. The SmartDate class is designed to support these concepts and to integrate with the SafeDataReader so that it can properly interpret a Null database value as an empty date. Additionally, SmartDate is a robust data type, supporting numerous operator overloads, casting, and type conversion. Better still, it works with both DateTime and the new DateTimeOffset type.
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