Deriving Directly from the Control Class in VS .NET

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Deriving Directly from the Control Class
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In VSNET, if you right-click on your project in Solution Explorer and choose Add | Add New Item | Custom Control, you'll get the following skeleton: using using using using using using System; SystemCollections; SystemComponentModel; SystemDrawing; SystemData; SystemWindowsForms;
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namespace MyCustomControls { /// <summary> /// Summary description for CustomControl1 /// </summary> public class CustomControl1 : SystemWindowsFormsControl { public CustomControl1() { } protected override void OnPaint(PaintEventArgs pe) { // TODO: Add custom paint code here // Calling the base class OnPaint baseOnPaint(pe); } } } This skeleton derives from the Control base class and provides a handler for the Paint event It even provides a helpful comment letting you know where to add your custom code to render your custom control's state
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After you've worked with your custom control for a while, you'll want it to show up on the Toolbox so that you can use it in various places To do this, right-click on the Toolbox and choose Add/Remove Items[3] To choose a NET assembly, click on the NET Framework Component tab and press the Browse button When you do that, you will get the Customize Toolbox dialog showing the NET components that VSNET knows about, as shown in Figure 810
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In VSNET 2002, "Add/Remove Items" is called "Customize Toolbox"
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Figure 810 Customizing the Toolbox
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Choose the assembly that your control lives in and press OK If you are writing a Windows Forms application and writing your custom control in the same assembly, select the application's EXE file as the assembly Even controls from applications are available for reuse, although DLLs are the preferred vehicle for distributing reusable controls After you've chosen the assembly to add, the custom controls will be added to the Toolbox, as shown in Figure 811
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Figure 811 Custom Controls Added to the Toolbox in VSNET
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Although it's possible to customize any of the tabs on the Toolbox, it's handy to have custom tabs for custom controls so that they don't get lost among the standard controls and components Figure 811 shows custom controls organized on the My Custom Controls tab
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When your control is available on the Toolbox, you can drop it onto a Form and use the Property Browser to set all public properties and handle all public events Because your custom controls inherit from the Control base, all this comes essentially for free For the details of how to customize your control's interaction with the Designer and the Property Browser, see 9: Design-Time Integration
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Looking back at the skeleton code generated by the Designer for a custom control, remember that it handles the Paint event by deriving from the Control base class and overriding the OnPaint method Because we're deriving from the Control class, we have two options when deciding how to handle a method The first option is to add a delegate and handle the event This is the only option available when you're handling a control's event from a container The second option is to override the virtual method that the base class provides that actually fires the methods By convention, these methods are named On<EventName> and take an object of the EventArgs (or EventArgs-derived) class When you override an event method, remember to call to the base class's implementation of the method so that all the event subscribers will be notified For example, here's how to implement OnPaint for a custom label like control: public class EllipseLabel : Control { public EllipseLabel() { // Required for Designer support InitializeComponent(); } protected override void OnPaint(PaintEventArgs pe) { // Custom paint code Graphics g = peGraphics; using( Brush foreBrush = new SolidBrush(thisForeColor) ) using( Brush backBrush = new SolidBrush(thisBackColor) ) { gFillEllipse(foreBrush, thisClientRectangle); StringFormat fmt = new StringFormat(); fmtAlignment = StringAlignmentCenter; fmtLineAlignment = StringAlignmentCenter; gDrawString( thisText, thisFont, backBrush, thisClientRectangle, fmt);
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} // Calling the base class OnPaint baseOnPaint(pe); } }
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In this code, notice how much functionality is available from the base class without the need to add any new properties, methods, or events In fact, the sheer amount of functionality in the base Control class is too large to list here
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Many of the properties have corresponding <PropertyName>Changed events to track when they change For example, the state of our custom label like control depends on the state of the BackColor, ForeColor, Text, Font, and ClientRectangle properties; so when any of these properties changes, we must apply the principles of drawing and invalidation from 4: Drawing Basics to keep the control visually up-todate: public EllipseLabel() { // Required for Designer support InitializeComponent(); // Automatically redraw when resized // (See 6: Advanced Drawing for ControlStyles details) thisSetStyle(ControlStylesResizeRedraw, true); } void InitializeComponent() { thisTextChanged += new EventHandler(thisEllipseLabel_TextChanged); } void EllipseLabel_TextChanged(object sender, EventArgs e) { thisInvalidate(); }
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In this case, we track when the Text property has changed by using the Designer[4] to set up an event handler for the TextChanged event (saving us from typing in the event handler skeleton or remembering to call the base class) When the text changes, we invalidate our control's client area However, we don't need to track any of the BackColorChanged, FontChanged, or ForeColorChanged events because the base class knows to invalidate the client area of the control in those cases for us Those properties are special, as explained next
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Be careful when using the Designer with custom controls It adds an InitializeComponent method if there isn't already one in the class, but it doesn't add a call from your control's constructor to InitializeComponent, so you must do that manually
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