Handling Control Events in .NET framework

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Handling Control Events
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As mentioned earlier, most of the lifecycle of a Windows Forms application is determined by handling control events All controls inherit a large set of events from the Control base class These events correspond to the common Windows messages that can get passed to any window by the operating system This includes events like mouse clicks, keyboard events, Windows timer events, and repainting events Many of these events are of the same delegate typethe EventHandler delegate type introduced earlier If you need to know more about delegates and events, I would suggest picking up a good book on general NET programming (Programming NET Components, second edition, by Juval L wy includes great coverage of NET events and delegates) Some events are defined using more specialized delegate types that include specialized event arguments These arguments carry additional information about the event that can be used by subscribers to perform processing based on that event in their event handlers For example, the Control class defines events forMouseDown, MouseUp, MouseMove, and MouseWheel to name a few These events are declared to be of type MouseEventHandler The MouseEventHandler delegate type takes a MouseEventArgs object as the second parameter in its signature The MouseEventArgs class defines several properties that can be used to perform conditional processing based on the specifics of the event These properties include the X and Y locations in screen-based coordinates of where the mouse event occurred, which button was pressed and how many times if the event was a mouse click, and how many detents (notches) a mouse wheel was rotated if the event was caused by moving the mouse wheel The way you handle control events is no different than how you handle any NET event You need to subscribe to the event using the built-in mechanisms for your NET language of choice For C#, you subscribe to an event using the += operator, passing an instance of the appropriate delegate type initialized to point to the event handler method: eventPublisherMyEvent += new EventHandler(mySubscriberHandlerMethod);
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Another notation to be aware of (and which I use a lot in this book) is the new C# 20 syntax for delegate inference (new in 20) In this code, I explicitly created a new instance of the delegate type for the event, EventHandler, and passed that to the += operator In C# 20, the compiler is smart enough to infer what the appropriate delegate type is based on the type of the event you are setting up a subscription on So in C#, you can compact the notation to just the name of the handler method as follows: eventPublisherMyEvent += mySubscriberHandlerMethod;
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Even though I don't include any samples in Visual Basic in the text, all the sample code is available in Visual Basic for download Because event handling syntax is one of the few places that Visual Basic code looks distinctly different than the C# equivalent, here's a Visual Basic example In Visual Basic you can use the AddHandler keyword, passing the event and the handler method: AddHandler publisherMyEvent, _ AddressOf subscriberHandlerMethod
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In Visual Basic, you also have the option of declaring the instance of the control using theWithEvents keyword, then using the Handles keyword modifier on the end of a method declaration to hook it up as an event handler: Public Class MyForm Inherits Form Private WithEvents m_LoadButton As Button Private Sub MyButtonHandler(ByVal sender As Object, _
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This document was created by an unregistered ChmMagic, please go to http://wwwbisentercom to register it Thanks ByVal e As EventArgs) Handles m_LoadButtonClick End Sub End Class
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This document was created by an unregistered ChmMagic, please go to http://wwwbisentercom to register it Thanks
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