ReadWriteAuthorization in Visual Basic .NET

Making PDF-417 2d barcode in Visual Basic .NET ReadWriteAuthorization

3 added authorization code to business objects, making them aware of whether each property can be read or changed. The CanReadProperty() and CanWriteProperty() methods were made Public so that code outside the object could easily determine whether the current user is allowed to get or set each property on the object. One primary user of this functionality is the UI, which can decide to alter its appearance to give users clues as to whether they are able to view or alter each piece of data. While this could be done by hand for each control on every form, the ReadWriteAuthorization control helps automate the process of building a UI that enables or disables controls based on whether properties can be read or changed. If a control is bound to a property, and the user does not have read access to that property due to authorization rules, the ReadWriteAuthorization control will disable that control. It also adds a handler for the control s Format event to intercept the value coming from the data source, substituting an empty value instead. The result is that data binding is prevented from displaying the data to the user.

Similarly, if the user doesn t have write access to a property, ReadWriteAuthorization will attempt to mark any controls bound to that property as being read-only (or failing that, disabled); ensuring that the user can t attempt to alter the property value. Like all Windows Forms components, extender controls inherit from System.ComponentModel. Component. Additionally, to act as an extender control, the ReadWriteAuthorization control must implement the IExtenderProvider interface: <DesignerCategory("")> _ <ProvideProperty("ApplyAuthorization", GetType(Control))> _ Public Class ReadWriteAuthorization Inherits System.ComponentModel.Component Implements IExtenderProvider Public Sub New(ByVal container As System.ComponentModel.IContainer) container.Add(Me) End Sub The <ProvideProperty()> attribute is quite important. It specifies that ReadWriteAuthorization extends components of type Control by adding an ApplyAuthorization property to them. In other words, when a ReadWriteAuthorization control is on a form, all other controls on the form get a dynamically added ApplyAuthorization property. Figure 5-6 shows a text box control s Properties window with the dynamically added ApplyAuthorization property.

The UI developer can set this property to True or False to indicate whether the ReadWriteAuthorization control should apply authorization rules to that particular control. You ll see how this works as the control is implemented. The <DesignerCategory()> attribute is just used to help Visual Studio decide what kind of visual designer to use when editing the control. The value used here specifies that the default designer should be used. The class also implements a constructor that accepts an IContainer parameter. This constructor is required for extender controls, and is called by Windows Forms when the control is instantiated. Notice that the control adds itself to the container as required by the Windows Forms infrastructure.

The IExtenderProvider interface defines just one method: CanExtend(). This method is called by Windows Forms to ask the extender control whether it wishes to extend any given control. Windows Forms automatically calls CanExtend() for every control on the form: Public Function CanExtend( _ ByVal extendee As Object) As Boolean _ Implements IExtenderProvider.CanExtend If IsPropertyImplemented(extendee, "ReadOnly") OrElse _ IsPropertyImplemented(extendee, "Enabled") Then Return True Else Return False End If End Function The ReadWriteAuthorization control can extend any control that implements either a ReadOnly or an Enabled property. This covers most controls, making ReadWriteAuthorization broadly useful. If the potential target control implements either of these properties, a True result is returned to indicate that the control will be extended. The IsPropertyImplemented() method is a helper that uses reflection to check for the existence of the specified properties on the target control: Private Shared Function IsPropertyImplemented(ByVal obj As Object, _ ByVal propertyName As String) As Boolean If obj.GetType.GetProperty(propertyName, _ BindingFlags.FlattenHierarchy Or _ BindingFlags.Instance Or _ BindingFlags.Public) IsNot Nothing Then Return True Else Return False End If End Function