Component Resource Management in .NET framework

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Component Resource Management
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Although components and controls are similar as far as their design-time interaction is concerned, they are not identical The most obvious difference lies in the way they are drawn on the design surface A less obvious difference is that the Designer does not generate the same hosting code for components that it does for controls Specifically, a component gets extra code so that it can add itself to the container's list of components When the container shuts down, it uses this list of components to notify all the components that they can release any resources that they're holding Controls don't need this extra code because they already get the Closed event, which is an equivalent notification for most purposes To let the Designer know that it would like to be notified when its container goes away, a component can implement a public constructor that takes a single argument of type IContainer: public AlarmComponent(IContainer container) { // Add object to container's list so that // we get notified when the container goes away containerAdd(this); InitializeComponent(); } Notice that the constructor uses the passed container interface to add itself as a container component In the presence of this constructor, the Designer generates code that uses this constructor, passing it a container for the component to add itself to Recall that the code to create the AlarmComponent uses this special constructor: public class AlarmForm : Form { IContainer components; AlarmComponent alarmComponent1; void InitializeComponent() { thiscomponents = new Container(); thisalarmComponent1 = new AlarmComponent(thiscomponents); } } By default, most of the VSNET-generated classes that contain components will notify each component in the container as part of the Dispose method implementation: public class AlarmForm : Form { IContainer components; // Overridden from the base class ComponentDispose method protected override void Dispose( bool disposing ) { if( disposing ) { if (components != null) { // Call each component's Dispose method componentsDispose(); } } baseDispose( disposing ); } } As you may recall from 4: Drawing Basics, the client is responsible for calling the
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Dispose method from the IDisposable interface The IContainer interface derives from IDisposable, and the Container implementation of Dispose walks the list of components, calling IDisposable Dispose on each one A component that has added itself to the container can override the Component base class's Dispose method to catch the notification that is being disposed of: public class AlarmComponent : Component { Timer timer1; IContainer components; void InitializeComponent() { thiscomponents = new Container(); thistimer1 = new Timer(thiscomponents); } protected override void Dispose(bool disposing) { if( disposing ) { // Release managed resources // Let contained components know to release their resources if( components != null ) { componentsDispose(); } } // Release unmanaged resources } } Notice that, unlike the method that the client container is calling, the alarm component's Dispose method takes an argument The Component base class routes the implementation of IDisposableDispose() to call its own Dispose(bool) method, with the Boolean argument disposing set to true This is done to provide optimized, centralized resource management A disposing argument of true means that Dispose was called by a client that remembered to properly dispose of the component In the case of our alarm component, the only resources we have to reclaim are those of the timer component we're using to provide our implementation, so we ask our own container to dispose of the components it's holding on our behalf Because the Designer-generated code added the timer to our container, that's all we need to do A disposing argument of false means that the client forgot to properly dispose of the object and that the NET Garbage Collector (GC) is calling our object's finalizer A finalizer is a method that the GC calls when it's about to reclaim the memory associated with the object Because the GC calls the finalizer at some indeterminate time potentially long after the component is no longer needed (perhaps hours or days later) the finalizer is a bad place to reclaim resources, but it's better than not reclaiming them at all The Component base class's finalizer implementation calls the Dispose method, passing a disposing argument of false, which indicates that the component shouldn't touch any of the managed objects it may contain The other managed objects should remain untouched because the GC may have already disposed of them, and their state is undefined Any component that contains other objects that implement IDisposable, or handles to unmanaged resources, should implement the Dispose(bool) method to properly release those objects' resources when the component itself is being released by its container
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