Message Objects in VB.NET

Encoding PDF 417 in VB.NET Message Objects

When the client-side DataPortal calls the server-side data portal, several types of information are passed from client to server. Obviously, the data method call (Create, Update, Insert, etc.) itself is transferred from client to server. But other information is also included, as follows: Client-side context data (such as the client machine s culture setting) Application-wide context data (as defined by the application) The user s principal and identity security objects (if using custom security)

Client-side context data is passed one way, from the client to the server. This information may include things like the client workstation s culture setting thus allowing the server-side code to also use that context when servicing requests for that user. This can be important for localization of an application when a server may be used by workstations in different nations. Application-wide context data is passed both from client to server and from server back to client. You may use this context data to pass arbitrary application-specific data between client and server on each data portal operation. This can be useful for debugging, as it allows you to build up a trace log of the call as it goes from client to server and back again. If the application is using custom authentication, then the custom principal and identity objects representing the user are passed from client to server. This means the code on the server will run under the same security context as the client. If you are using Windows integrated or AD security, then Windows itself can be configured to handle the impersonation. When the server-side data portal has completed its work, the results are returned to the client. Other information is also included, as follows: Application-wide context data (as defined by the application) Details about any server-side exception that may have occurred Again, the application-wide context data is passed from client to server and from server to client. If an exception occurs on the server, the details about that exception are returned to the client. This is important for debugging, as it means you get the full details about any issues on the server. It is also important at runtime, since it allows you to write exception handling code on the client to gracefully handle server-side exceptions including data-oriented exceptions such as duplicate key or concurrency exceptions. All the preceding bulleted items are passed to and from the server on each data portal operation. Keeping in mind that the data portal supports several verbs, it is important to understand what information is passed to and from the server to support each verb. This is listed in Table 2-4. Table 2-4. Data Passed to and from the Server for Data Portal Operations

Type of object to create and (optional) criteria about new object Criteria for desired object Object to be updated Criteria for object to be deleted Object to be executed (must derive from CommandBase)

New object loaded with default values Object loaded with data Object after update (possibly containing changed data) Nothing Object after execution (possibly containing changed data)

Notice that the Create, Fetch, and Delete operations all require criteria information about the object to be created, retrieved, or removed. At a minimum, the criteria object must specify the type of object you are trying to create, retrieve, or delete. It may also contain any other data you need to describe your particular business object. A criteria object can be created one of two ways: By creating a nested class within your business class By creating a class that inherits from CriteriaBase

When a criteria class is nested within a business class, the .NET type system can be used to easily determine the type of class in which the criteria is nested. The CriteriaBase class, on the other hand, directly includes a property you must set, indicating the type of the business object. In either case, your criteria class should include properties containing any specific information you need in order to identify the specific object to be created, retrieved, or removed.