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{ public ReadWriteAuthorization(IContainer container) { container.Add(this); } } The ProvideProperty attribute is 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 10-2 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.
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Figure 10-2. ApplyAuthorization property added to NameTextBox 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.
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The IExtenderProvider Interface
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The IExtenderProvider interface defines just one method: CanExtend().Windows Forms calls this method to ask the extender control whether it wishes to extend any given control. Windows Forms calls CanExtend()automatically for every control on the form. public bool CanExtend(object extendee) { if (IsPropertyImplemented(extendee, "ReadOnly") || IsPropertyImplemented(extendee, "Enabled")) return true; else return false; }
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The ReadWriteAuthorization control can extend any control that implements either a ReadOnly or Enabled property. This covers most controls, making ReadWriteAuthorization broadly useful. If the potential target control implements either of these properties, a true result will be 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 static bool IsPropertyImplemented( object obj, string propertyName) { if (obj.GetType().GetProperty(propertyName, BindingFlags.FlattenHierarchy | BindingFlags.Instance | BindingFlags.Public) != null) return true; else return false; }
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The ApplyAuthorization Property
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The ProvideProperty attribute on ReadWriteAuthorization specified that an ApplyAuthorization property would be dynamically added to all controls extended by ReadWriteAuthorization. Of course, the controls being extended really have no knowledge of this new property or what to do with it. All the behavior associated with the property is contained within the extender control itself. The extender control manages the ApplyAuthorization property by implementing both the GetApplyAuthorization() and SetApplyAuthorization() methods. Windows Forms calls these methods to get and set the property value for each control that has been extended. Windows Forms prepends Get and Set automatically to call these methods. To manage a list of the controls that have been extended, a Dictionary object is used. private Dictionary<Control, bool> _sources = new Dictionary<Control, bool>(); public bool GetApplyAuthorization(Control source) { bool result; if (_sources.TryGetValue(source, out result)) return result; else return false; } public void SetApplyAuthorization(Control source, bool value) { if (_sources.ContainsKey(source)) _sources[source] = value; else _sources.Add(source, value); } When Windows Forms indicates that the ApplyAuthorization property has been set for a particular extended control, the SetApplyAuthorization() method is called. This method records the value of the ApplyAuthorization property for that particular control, using the control itself as the key value within the Dictionary.
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Conversely, when Windows Forms needs to know the property value of ApplyAuthorization for a particular control, it calls GetApplyAuthorization(). The value for that control is retrieved from the Dictionary object and returned. If the control can t be found in the Dictionary, then false is returned, since that control is obviously not being extended. The end result here is that the ReadWriteAuthorization control maintains a list of all the controls it extends, along with their ApplyAuthorization property values. In short, it knows about all the controls it will affect, and whether it should be affecting them or not.
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