s DATA ACCESS AND SECURITY in Visual Basic .NET

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CHAPTER 4 s DATA ACCESS AND SECURITY
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When forceLocal is False, the real work begins. First, the proxy string is retrieved from the CslaDataPortalProxy key in the config file by calling the ApplicationContext.DataPortalProxy property. The ApplicationContext class is covered later in the chapter, but this property reads the config file and returns the value associated with the CslaDataPortalProxy key. If that key value is "Local", then again an instance of the LocalProxy class is created and returned. The ApplicationContext.DataPortalProxy method also returns a LocalProxy object if the key is not found in the config file. This makes LocalProxy the default proxy. If some other config value is returned, then it is parsed and used to create an instance of the appropriate proxy class: Dim typeName As String = _ proxyTypeName.Substring(0, proxyTypeName.IndexOf(",")).Trim Dim assemblyName As String = _ proxyTypeName.Substring(proxyTypeName.IndexOf(",") + 1).Trim mPortal = DirectCast(Activator.CreateInstance(assemblyName, _ typeName).Unwrap, DataPortalClient.IDataPortalProxy) In the preceding <appSettings> example, notice that the value is a comma-separated value with the full class name on the left and the assembly name on the right. This follows the .NET standard for describing classes that are to be dynamically loaded. The config value is parsed to pull out the full type name and assembly name. Then Activator.CreateInstance() is called to create an instance of the object. The .NET runtime automatically loads the assembly if needed. The object returned from Activator.CreateInstance() isn t the actual proxy object. Instead, it is an internal .NET object representing the underlying object. The Unwrap() method returns the real proxy object that was dynamically loaded. The final result is that the appropriate proxy object is loaded into memory and returned for use by the code in Csla.DataPortal.
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The next step is to implement the five primary methods in the client-side DataPortal. Most of the hard work is handled by the code implemented thus far in the Channel Adapter section and in the MethodCaller class, so implementing these will be pretty straightforward. All five will follow the same basic flow: Get the MethodInfo for the business method to be ultimately invoked. Get the data portal proxy object. Create a DataPortalContext object. Raise the DataPortalInvoke event. Delegate the call to the proxy object (and thus to the server). Handle and throw any exceptions. Restore the GlobalContext returned from the server. Raise the DataPortalInvokeComplete event. Return the resulting business object (if appropriate). Let s look at the Fetch() method in detail, followed by the minor differences required to implement the other four methods.
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CHAPTER 4 s DATA ACCESS AND SECURITY
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There are two Fetch() methods: a generic one to provide a strongly typed result, and the actual implementation: Public Function Fetch(Of T)(ByVal criteria As Object) As T Return DirectCast(Fetch(criteria), T) End Function Public Function Fetch(ByVal criteria As Object) As Object Dim result As Server.DataPortalResult Dim method As MethodInfo = _ MethodCaller.GetMethod( _ MethodCaller.GetObjectType(criteria), "DataPortal_Fetch", criteria) Dim proxy As DataPortalClient.IDataPortalProxy proxy = GetDataPortalProxy(RunLocal(method)) Dim dpContext As New Server.DataPortalContext( _ GetPrincipal, proxy.IsServerRemote) OnDataPortalInvoke(New DataPortalEventArgs(dpContext)) Try result = proxy.Fetch(criteria, dpContext) Catch ex As Server.DataPortalException 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) End Try If proxy.IsServerRemote Then ApplicationContext.SetGlobalContext(result.GlobalContext) End If OnDataPortalInvokeComplete(New DataPortalEventArgs(dpContext)) Return result.ReturnObject End Function The generic method simply casts the result so that the calling code doesn t have to. Remember that the data portal can return virtually any type of object, and so the actual Fetch() method implementation must deal with results of type Object. Looking at the code, you should see all the steps listed in the preceding bulleted list. The first is to retrieve the MethodInfo for the business method that will be ultimately invoked on the server: Dim method As MethodInfo = _ MethodCaller.GetMethod( _ MethodCaller.GetObjectType(criteria), "DataPortal_Fetch", criteria)
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