s DATA ACCESS AND SECURITY in VB.NET

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CHAPTER 4 s DATA ACCESS AND SECURITY
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This MethodInfo object is immediately used to determine whether the <RunLocal()> attribute has been applied to the method on the business class. This value is used as a parameter to the GetDataPortalProxy() method, which returns the appropriate proxy object for server communication: proxy = GetDataPortalProxy(RunLocal(method)) Next, a DataPortalContext object is created and initialized. The details of this object and the means of dealing with context information are discussed later in the chapter. Dim dpContext As New Server.DataPortalContext( _ GetPrincipal, proxy.IsServerRemote) Then the DataPortalInvoke event is raised, notifying client-side business or UI logic that a data portal call is about to take place: OnDataPortalInvoke(New DataPortalEventArgs(dpContext)) Finally, the Fetch() call itself is delegated to the proxy object: result = proxy.Fetch(criteria, dpContext) All a proxy object does is relay the method call across the network to Csla.Server.DataPortal, so you can almost think of this as delegating the call directly to Csla.Server.DataPortal, which in turn delegates to Csla.Server.SimpleDataPortal. The ultimate result is that the business object s DataPortal_XYZ methods are invoked on the server.
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Remember that the default is that the server-side code actually runs in the client process on the client workstation (or web server). Even so, the full sequence of events described here occur just much faster than if network communication were involved.
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An exception could occur while calling the server. The most likely cause of such an exception is that an exception occurred in the business logic running on the server, though exceptions can also occur because of network issues or similar problems. When an exception does occur in business code on the server, it will be reflected here as a Csla.Server.DataPortalException, which is caught and handled: result = ex.Result If proxy.IsServerRemote Then ApplicationContext.SetGlobalContext(result.GlobalContext) End If Throw New DataPortalException("DataPortal.Fetch " & _ My.Resources.Failed, ex.InnerException, result.ReturnObject) The Csla.Server.DataPortalException returns the business object from the server exactly as it was when the exception occurred. It also returns the GlobalContext information from the server so that it can be used to update the client s context data. Ultimately, the data from the server is used to create a Csla.DataPortalException that is thrown back to the business object. It can be handled by the business object or the UI code as appropriate. Notice that the Csla.DataPortalException object contains not only all the exception details from the server, but also the business object from the server. This object can be useful when debugging server-side exceptions. More commonly, an exception won t occur. In that case, the result returned from the server includes the GlobalContext data from the server, which is used to update the context on the client:
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CHAPTER 4 s DATA ACCESS AND SECURITY
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If proxy.IsServerRemote Then ApplicationContext.SetGlobalContext(result.GlobalContext) End If The details around context are discussed later in the chapter. With the server call complete, the DataPortalInvokeComplete event is raised: OnDataPortalInvokeComplete(New DataPortalEventArgs(dpContext)) Finally, the business object created and loaded with data on the server is returned to the factory method that called DataPortal.Fetch() in the first place. Remember that in a physical n-tier scenario, this is a copy of the object that was created on the server. .NET serialized the object on the server, transferred its data to the client, and deserialized it on the client. This object being returned as a result of the Fetch() method exists on the client workstation and so can be used by other client-side objects and UI components in a very efficient manner.
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The Create() method works in virtually the same manner as Fetch(). The only difference is in how the type of business object is managed. When retrieving an existing object, some criteria information is virtually always required. But when creating a new object that is to be loaded with default values, a criteria object may or may not be useful. In many cases, there s no need for criteria at all when creating a new object. However, the criteria object is central to the MethodCaller.GetObjectType() method and the determination of the type of business object to be created. To make the criteria object optional, Create() takes a slightly different approach. The Public methods look like this: Public Function Create(Of T)(ByVal criteria As Object) As T Return DirectCast(Create(GetType(T), criteria), T) End Function Public Function Create(Of T)() As T Return DirectCast(Create(GetType(T), Nothing), T) End Function Public Function Create(ByVal criteria As Object) As Object Return Create(MethodCaller.GetObjectType(criteria), criteria) End Function Again, there s the generic version that returns a casted value. But there s also a version that doesn t require a criteria object as a parameter. Finally, there s a loosely typed version that returns a value of type Object. All three implementations delegate to a Private version of the method that accepts not only the criteria object, but also a Type object specifying the type of business object to be created. The generic versions of the method get this by calling GetType(T), while the loosely typed version uses the same GetObjectType() method used in the Fetch() method earlier. The private implementation of Create() follows the same structure as Fetch(), with the exception of how it calls GetMethod() in the first step. That code is bolded here: Private Function Create( _ ByVal objectType As Type, ByVal criteria As Object) As Object Dim result As Server.DataPortalResult Dim method As MethodInfo = _ MethodCaller.GetMethod(objectType, "DataPortal_Create", criteria)
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