Drawing on Windows controls in Visual Studio .NET

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Listing 133 Drawing on Windows controls
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private void TheButtonPaintEventHandler(object sender, SystemWindowsFormsPaintEventArgs btnArgs) { btnArgsGraphicsFillEllipse( BrushesBlue, 10, 10, 100, 100); } private void TheDataGridPaintEventHandler(object sender, SystemWindowsFormsPaintEventArgs dtGridArgs) { dtGridArgsGraphicsFillEllipse( BrushesBlue, 10, 10, 100, 100); } Figure 133 shows the output of Listing 133 As you can see, a button or a data grid can function as a drawing canvas The top left-hand corner of a control is the (0, 0) coordinate of the canvas associated with that control
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Figure 133 Drawing on Windows controls
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At this stage it is worth pointing out another big advantage that GDI+ has over GDI: the flexibility to have a Graphics object associated with a control
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1314 Overriding the OnPaint Method of a Form
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We have already seen this in previous chapters We can override the OnPaint method by defining it as follows:
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protected override void OnPaint( PaintEventArgs args) { // Add your drawing code here } Then we can use the Graphics property of PaintEventArgs to draw lines, shapes, text, and images Listing 134 draws a few graphics shapes and text on our form's OnPaint method To test this code, create a Windows application and add the code to it
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Listing 134 Using OnPaint to draw
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protected override void OnPaint( PaintEventArgs args ) { // Get the Graphics object from // PaintEventArgs Graphics g = argsGraphics; // Draw rectangle gDrawRectangle( new Pen(ColorBlue, 3), new Rectangle(10, 10, 50, 50)); // Fill ellipse gFillEllipse( BrushesRed, new Rectangle(60, 60, 100, 100)); // Draw text gDrawString("Text", new Font("Verdana", 14), new SolidBrush(ColorGreen), 200, 200) ; }
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1315 Using Visual Studio NET to Add the Paint Event Handler
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If you are using Visual Studio NET, the easiest way to add a paint event handler is to use the Properties windows of a form or control and add a paint event handler We have seen examples of this in previous chapters
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1316 Disposing of Graphics Objects
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It is usually good programming practice to dispose of objects when you're finished using them But it may not always be the best practice A Graphics object must always be disposed of if it was created via the CreateGraphics method or other "CreateFrom" methods If we use a Graphics object on a paint event or theOnPaint method from the PaintEventArgsGraphics property, we do not have to dispose of it
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Note
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Do not dispose of Graphics objects associated with Windows controls such asButton, ListBox, or DataGrid
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If you create objects such as pens and brushes, always dispose of them Although it is acceptable practice to rely on the garbage collector, doing so may often be at the expense of application performance Garbage collection can be a costly affair because the garbage collector checks the memory for objects that haven't been disposed of, and this process absorbs processor time However, the Dispose method of an object tells the garbage collector that the object is finished and ready to be disposed of Calling the Dispose method eliminates the need to have the garbage collector check memory, and thus saves processor time In Web pages, it is always good practice to dispose of objects as soon as they are done being used
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1317 The OnPaintBackground Method
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The OnPaintBackground method paints the background of a control This method is usually overridden in the derived classes to handle the event without attaching a delegate Calling the OnPaintBackground method calls OnPaintBackground of the base class automatically, so we do not need to call it explicitly
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1318 Scope and Type of Variables and Performance
One of the best programming practices is the efficient use of variables and their scope Before adding a new variable to a program, think for a second and ask yourself, "Do I really need this variable " If you need a variable, do you really need it right now The scope of variables and use of complex calculations can easily degrade the performance of your applications Using global scope for pens, brushes, paths, and other objects may be useful instead of defining variables in the OnPaint or OnPaintBackground methods Let's look at a practical example: Listing 135 is written on a form's paint event handler, which creates pens and brushes, and draws rectangles and polygons