Attribute in Visual Basic .NET

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Attribute
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DataMember Association XmlIgnore RoundtripOriginal Key
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Seeing the attributes in place provides a little insight into how Silverlight keeps track of various properties. For example, you now know how the client knows that a certain field is the primary key for the entity. Although the Entity class provides extensibility points on the client, it s rare for an application to use them for validation or anything remotely like a business function. Extensions provided on the client can t be used back at the server and so can become a disconnect between the two models. In order to keep the two in sync, the RIA Services team provided a server-side model for extending the entity: metadata.
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17.4.4 Using validation and display metadata
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When you first created the domain service on the server, the wizard offered an option to generate the associated metadata class. This metadata class is a partial class that exists on the server and relates to a single entity. If you open the EmployeeService.metadata.cs file in the server project, you ll see both the Contact and Employee partial classes. These partial classes include nested classes with the same public properties that are also defined in the entity classes. Those are just placeholders, providing a location on which you can define metadata to control the display and validation of the fields. But wait why am I covering metadata in this section Because this metadata is useful only if the client understands it. Silverlight and parts of ASP.NET are currently the only clients that can make sense of attribute-based annotation metadata for validation and display.
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Updating data
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CONTROLLING DISPLAY
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The DataForm labels and the DataGrid column headers have that ugly PascalCase text formatting. It d be nicer to introduce actual spaces to make the fields more humanreadable. You may even want to provide some tooltip descriptive information for certain fields. In the EmployeeService.metadata.cs class on the server, scroll down to the Employee partial class and the nested EmployeeMetadata class within it. Find the BirthDate field, and add this attribute:
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[Display(Name="Birth Date", Description="The date this person was born.")] public DateTime BirthDate { get; set; }
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That says to use the string Birth Date for column headers and field labels; and if a tooltip or other description approach is available, use this description. Figure 17.14 shows how this looks at runtime.
Figure 17.14 The Display annotation in use on the DataGrid on the left and the DataForm on the right. At lower right is the Description property in a tooltip.
As you learned in chapter 13, annotations can be used for more than display. One of the more powerful uses is for validation.
ADDING VALIDATION
You get data type validation and the inferred validation (string length, required, and so forth) from the database for free. But you ll typically want to add your own validation to make the UI more bulletproof. In the EmployeeService.metadata.cs class, scroll down to the Employee partial class and the nested EmployeeMetadata class within it. Find the Gender field, and add this attribute:
[RegularExpression("[MmFf]", ErrorMessage="Specify (M)ale or (F)emale, please")] public string Gender { get; set; }
Run the application, and attempt to type something else into the Gender field. The regular expression restricts the valid input choices to M, m, F, and f. The metadata
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WCF RIA Services
entered on the server was automatically carried over to the client. If you open the 17.Web.g.cs file on the client and navigate to the generated Gender property, you ll see the addition of the new attribute:
[DataMember()] [RegularExpression("[MFmf]", ErrorMessage="Specify (M)ale or (F)emale, please")] [Required()] [StringLength(1)] public string Gender ...
The StringLength, Required, and DataMember attributes were previously there as part of the inferred metadata coming from the data model.9 Annotation for display and validation is a nice, easy way to add significant robustness to your classes. Because the information goes into metadata buddy classes, you don t have to worry about the autogeneration process stepping on them. What you ve seen so far is a model where the entity generated by the data access layer, typically based directly on tables or views on the database, makes its way from the database through the service to the client and into the UI. That s okay sometimes, especially when you have good mapping at the data access layer, but an additional layer of abstraction could help protect the UI from changes in the database. That layer is called a presentation model.
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