PRB: TransparentPaint Backstyle Option Unavailable (174216)

SYMPTOMS

The Visual Basic 5.0 documentation discusses a TransparentPaint option for the BackStyle property of the UserControl. However, this feature was not implemented in the product. Without this option, it is not possible to create a completely transparent ActiveX Control using Visual Basic. If you need a transparent ActiveX Control, develop the control using Microsoft Visual C++.

This presents two problems when working with ActiveX controls created using Visual Basic. Transparent controls cannot be drawn on and they do not respond to events that occur on the control; these events are passed on to the container.

NOTE: This documentation error has been corrected in Visual Basic version 6.0.

MORE INFORMATION

Background Color Matching with the Container

If the container has a solid color background, the backcolor of your UserControl can be set to match the backcolor of the container. This has the same effect as transparency. To accomplish this, add the following code to your UserControl's Paint Event:

   BackColor = Parent.Backcolor

Using a "Hot Spot" Bitmap

The UserControl has two properties that can be used to create transparent areas on the control: MaskColor and MaskPicture. These properties allow you to have non-transparent areas on a transparent control that will respond to events and can be drawn on. The transparent areas on the control, however, will still not respond to events and cannot be drawn on.

Step-by-Step Example

Create a bitmap with a white background and containing a filled red circle.
Start a new ActiveX Control project.

Set the following properties for the UserControl:

   BackStyle:      0 - Transparent
   MaskColor:      White
   MaskPicture:    The bitmap created in Step 1.
   BackColor:      &H00FF0000&

Add the following code to the UserControl:

      Private Sub UserControl_Click()
         MsgBox "UserControl Click"
      End Sub

      Private Sub UserControl_Paint()
         'Test to see if background of control is same color as container
         'If it is, change it, or the unmasked area will be invisible
         If BackColor = Parent.BackColor Then BackColor = vbGreen
      End Sub

Add a Standard EXE project.

Add the following code to Form1:

      Private Sub Form_Click()
         MsgBox "Form Click"
      End Sub

Close the UserControl and add it to Form1.
Run the project.
Clicking on the circle should result in the "UserControl Click" message box.
Clicking on the transparent area of the UserControl should display the "Form Click" message box.

It is important that the MaskColor be set to the same color as the background of the bitmap. To prevent problems using your control under different versions of Windows, use White as the color. Notice also that in this example the "Red" circle on the bitmap was blue or green when the UserControl was displayed on the form. The actual color of the unmasked area is determined by the BackColor property of the UserControl, not the colors in the bitmap.

Although it is not demonstrated here, the non-transparent area of the UserControl can also be drawn on.