Overriding the OnPaint Method of a Form in GDI+

This article has been excerpted from book "GraphicsProgramming with GDI+".

We have already seen this in previous articles. We can override the onPaint method by defining it as follows:

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 13.4 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.

LISTING 13.4: Using OnPaint to draw

        protected override void OnPaint(PaintEventArgs args)
        {
            //Get the Graphics object from PaintEventArgs
            Graphics g = args.Graphics;
            //Draw rectangle
            g.DrawRectangle(
            new Pen(Color.Blue, 3),
            new Rectangle(10, 10, 50, 50));
            //Fill ellipse
            g.FillEllipse(
            Brushes.Red,
            new Rectangle(60, 60, 100, 100));
            //Draw text
            g.DrawString("Text",
            new Font("Verdana", 14),
            new SolidBrush(Color.Green),
            200, 200);
        }

Using Visual Studio .NET to add the Paint Event Handler

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 ad add a paint event handler.

Disposing of Graphics Objects

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 it was created via the CreateGraphics method or other "CreateFrom" methods. If we use a Graphics object on a paint event or the OnPaint method from the PaintEventArgs.Graphics property, we do not have to dispose of it.

NOTE

Do not dispose of Graphics objects associated with Windows controls such as Button, ListBox, or DataGrid.

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 food practice to dispose of objects as soon as they are done being used.

The OnPaintBackground Method

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 automatically, so we do not need to call it explicitly.