barcode printing vb.net Part IV in C#.NET

Printing PDF-417 2d barcode in C#.NET Part IV

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Automatic Memory Management (Garbage Collection)
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internal sealed class DoNotLiveJustForTheEvent { public void Clicked(Object sender, EventArgs e) { MessageBox.Show("Test got notified of button click."); } }
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Now, here is the code that creates a Form and two Button controls . The first Button control fills the left half of the Form s client area and the second Button control fills the right half of the Form s client area . Then, on the first Button control, I construct an instance of the DoNotLiveJustForTheEvent class and register this object s Clicked method as the event handler for this Button control s Click event . However, I do this by using my WeakEventHandler class, which turns an EventHandler delegate into a weak version of itself . I ll show how this class is implemented shortly . On the second Button control s Click event, I register a callback that will force a garbage collection to occur; I click this button to test that everything is working OK . Finally, I add the controls to the form s control collection, resize the form s client area, and then show the form:
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public static void Go() { var form = new Form() { Text = "Weak Delegate Test", FormBorderStyle = FormBorderStyle.FixedSingle }; var btnTest = new Button() { Text = "Click me", Width = form.Width / 2 }; var btnGC = new Button() { Text = "Force GC", Left = btnTest.Width, Width = btnTest.Width }; // WeakEventHandler turns an EventHandler delegate into a weak version of itself btnTest.Click += new WeakEventHandler(new DoNotLiveJustForTheEvent().Clicked) { RemoveDelegateCode = eh => btnTest.Click -= eh }; btnGC.Click += (sender, e) => { GC.Collect(); MessageBox.Show("GC complete."); }; form.Controls.Add(btnTest); form.Controls.Add(btnGC); form.ClientSize = new Size(btnTest.Width * 2, btnTest.Height); form.ShowDialog(); }
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Since I do not store the reference to the DoNotLiveJustForEvent object in a root variable, the object will be considered garbage when the next garbage collection runs . But, until then, I can click the left button multiple times and see that the DoNotLiveJustForEvent object s Clicked method is getting called . However, once I click the right button in the form, the DoNotLiveJustForEvent object is garbage collected . Now, when I click the left button, the
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Part IV
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Core Facilities
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WeakEventHandler object determines that the DoNotLiveJustForEvent object is gone,
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and it unregisters itself with the Button s Click event so that it never gets called again . Of course, the WeakEventHandler object will have its memory reclaimed during the next garbage collection . To understand my WeakEventHandler class, you need to first understand its base class . WeakEventHandler is derived from my abstract generic WeakDelegate class:
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public abstract class WeakDelegate<TDelegate> where TDelegate : class /* MulticastDelegate */ { private WeakReference<TDelegate> m_weakDelegate; private Action<TDelegate> m_removeDelegateCode; public WeakDelegate(TDelegate @delegate) { var md = (MulticastDelegate)(Object)@delegate; if (md.Target == null) throw new ArgumentException( "There is no reason to make a WeakDelegate to a static method."); // Save a WeakReference to the delegate m_weakDelegate = new WeakReference<TDelegate>(@delegate); } public Action<TDelegate> RemoveDelegateCode { set { // Save the delegate that refers to code that knows how to remove the // WeakDelegate object when the non-weak delegate object is GC d m_removeDelegateCode = value; } } protected TDelegate GetRealDelegate() { // If the real delegate hasn't been GC'd yet, just return it TDelegate realDelegate = m_weakDelegate.Target; if (realDelegate != null) return realDelegate; // The real delegate was GC'd, we don't need our // WeakReference to it anymore (it can be GC'd) m_weakDelegate.Dispose(); // Remove the delegate from the chain (if the user told us how) if (m_removeDelegateCode != null) { m_removeDelegateCode(GetDelegate()); m_removeDelegateCode = null; // Let the remove handler delegate be GC'd } return null; // The real delegate was GC'd and can t be called } // All derived classes must return a delegate to // a private method matching the TDelegate type public abstract TDelegate GetDelegate(); // Implicit conversion operator to convert a WeakDelegate object to an actual delegate public static implicit operator TDelegate(WeakDelegate<TDelegate> @delegate) {
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