How to extend an existing control (Windows Forms .NET)

If you want to add more features to an existing control, you can create a control that inherits from an existing control. The new control contains all of the capabilities and visual aspect of the base control, but gives you opportunity to extend it. For example, if you created a control that inherits Button, your new control would look and act exactly like a button. You could create new methods and properties to customize the behavior of the control. Some controls allow you to override the OnPaint method to change the way the control looks.

Add a custom control to a project

After creating a new project, use the Visual Studio templates to create a user control. The following steps demonstrate how to add a user control to your project:

1. In Visual Studio, find the Project Explorer pane. Right-click on the project and choose Add > Class.

   

2. In the Name box, type a name for your user control. Visual Studio provides a default and unique name that you may use. Next, press Add.

   

3. In Design mode of the control, press F7 or click the switch to code view link.

   Tip

   You can also right-click the file in the Solution Explorer window and select View Code.

Change the custom control to a button

In this section, you learn how to change a custom control into a button that counts and displays the number of times it's clicked.

After you add a custom control to your project named CustomControl1, the control designer should be opened. If it's not, double-click on the control in the Solution Explorer. Follow these steps to convert the custom control into a control that inherits from Button and extends it:

1. With the control designer opened, press F7 or right-click on the designer window and select View Code.

2. In the code-editor, you should see a class definition:

   C#

   namespace CustomControlProject
   {
       public partial class CustomControl2 : Control
       {
           public CustomControl2()
           {
               InitializeComponent();
           }
   
           protected override void OnPaint(PaintEventArgs pe)
           {
               base.OnPaint(pe);
           }
       }
   }
   

3. Change the base class from Control to Button.

   Important

   If you're using Visual Basic, the base class is defined in the *.designer.vb file of your control. The base class to use in Visual Basic is System.Windows.Forms.Button.

4. Add a class-scoped variable named _counter.

   C#

   private int _counter = 0;
   

5. Override the OnPaint method. This method draws the control. The control should draw a string on top of the button, so you must call the base class' OnPaint method first, then draw a string.

   C#

   protected override void OnPaint(PaintEventArgs pe)
   {
       // Draw the control
       base.OnPaint(pe);
   
       // Paint our string on top of it
       pe.Graphics.DrawString($"Clicked {_counter} times", Font, Brushes.Purple, new PointF(3, 3));
   }
   

6. Lastly, override the OnClick method. This method is called every time the control is pressed. The code is going to increase the counter, and then call the Invalidate method, which forces the control to redraw itself.

   C#

   protected override void OnClick(EventArgs e)
   {
       // Increase the counter and redraw the control
       _counter++;
       Invalidate();
   
       // Call the base method to invoke the Click event
       base.OnClick(e);
   }
   

   The final code should look like the following snippet:

   C#

   public partial class CustomControl1 : Button
   {
       private int _counter = 0;
   
       public CustomControl1()
       {
           InitializeComponent();
       }
   
       protected override void OnPaint(PaintEventArgs pe)
       {
           // Draw the control
           base.OnPaint(pe);
   
           // Paint our string on top of it
           pe.Graphics.DrawString($"Clicked {_counter} times", Font, Brushes.Purple, new PointF(3, 3));
       }
   
       protected override void OnClick(EventArgs e)
       {
           // Increase the counter and redraw the control
           _counter++;
           Invalidate();
   
           // Call the base method to invoke the Click event
           base.OnClick(e);
       }
   }
   

Now that the control is created, compile the project to populate the Toolbox window with the new control. Open a form designer and drag the control to the form. Run the project and press the button. Each press increases the number of clicks by one. The total clicks are printed as text on top of the button.