Draw over other controls

[Phil]

I would like to draw some text which will be in front of any other controls,
but without obscuring them completely. If I use DrawString in the form's
Paint handler the text is always behind, not in front of the other controls.
In VB6 I would have simply used a label control and set it to transparent,
but this doesn't work in .NET, the background of a label can be set to
'transparent', but this does not show the other controls through, instead it
fills in the background with the form background. I'm sure there must be a
simple way to acheive this, but I can't find it at the moment.
Any ideas appreciated.
Thanks
Phil.
(VB2005Express)

[Phil]

Thanks, I'll give this a go.
Cheers,
Phil

Unfortunately because the form is TopMost, this means not only does it
display in front of the controls on my existing form, it also displays in
front of all other windows, including other applications that might be
running.
Also setting ShowInTaskbar = False stops an icon showing in the task bar,
but an icon for the form is still shown when pressing Alt+Tab to switch
windows.

[Linda Liu [MSFT]]

Hi Phil,

Thank you for your prompt reply!

I work out another solution to your problem, i.e. draw the text to the
screen. To do this, we need to get the handle of the screen and then draw a
text to the screen.

I will illustrate my new solution with an example. It requires you to add
two Buttons on the form. When you click the Button1, a text "hello world"
appears on the screen. If the form is covered by another form and this form
is shown again, the text "hello world" previously drawn on the screen will
be erased. If you click the Button2, the text "hello world" is erased.

You can modify the sample code to accomplish what you want, if necessary.

The following is the sample code.

Public Class Form1
Declare Auto Function GetDC Lib "user32.dll" (ByVal hwnd As IntPtr) As
IntPtr
Declare Auto Function ReleaseDC Lib "user32.dll" (ByVal hwnd As IntPtr,
ByVal hdc As IntPtr) As Integer

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button1.Click
Dim hScreenDC As IntPtr = GetDC(IntPtr.Zero)
Dim g As Graphics = Graphics.FromHdc(hScreenDC)
Using (g)
Dim screenPt As Point = Me.PointToScreen(New Point(30, 50))
g.DrawString("hello world", Me.Font, Brushes.Thistle,
screenPt.X, screenPt.Y)
End Using

ReleaseDC(IntPtr.Zero, hScreenDC)
End Sub

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button2.Click
Me.Refresh()
End Sub
End Class

Please try my new solution to see if it meets your requirement and let me
know the result.

Sincerely,
Linda Liu
Microsoft Online Community Support

[Phil]

Please try my new solution to see if it meets your requirement and let me
know the result.

Thanks, I tried it. It looks like this approach may be promising, but there
are still a couple of problems.
If I switch away from the form and back again, the text dissappears,
although I can get around this by placing the code in the Form Paint event.
Also any control that redraws itself (e.g. a button when you hover over with
the mouse) will draw itself over the text. Another problem is if there is
another window partly obscuring the form, the text can still get written
over the top of this other window. I wonder if it is possible to clip the
output of drawstring to the currently visible area of the form?

Thanks again for your help. It is very much appreciated.
Phil.

[Linda Liu [MSFT]]

Hi Phil,

Thank you for your reply!

Also any control that redraws itself (e.g. a button when you hover over

with the mouse) will draw itself over the text.

We can solve this problem by handling the child control's Invalidated event
and draw the text on the screen again.

Another problem is if there is another window partly obscuring the form,

the text can still get written over the top of this other window.

I think we could redraw the text in the Activated event handler of the
form.

The following is the modified sample code.

Public Class Form1
Declare Auto Function GetDC Lib "user32.dll" (ByVal hwnd As IntPtr) As
IntPtr
Declare Auto Function ReleaseDC Lib "user32.dll" (ByVal hwnd As IntPtr,
ByVal hdc As IntPtr) As Integer

Private showFnt As Font
Private showText As String
Private showPt As Point
Private invalidateChildControls As New List(Of Control)
Private textRect As New Rectangle

Private Sub Form1_Activated(ByVal sender As Object, ByVal e As
System.EventArgs) Handles Me.Activated
If (showText <"") Then
DrawText(showText, showFnt, showPt)
End If
End Sub

Private Function GetChildControls(ByVal rect As Rectangle) As List(Of
Control)
Dim ctrls As New List(Of Control)
For Each ctrl As Control In Me.Controls
Dim result As Rectangle = Rectangle.Intersect(ctrl.Bounds, rect)
If (Not result.IsEmpty) Then
ctrls.Add(ctrl)
End If
Next
Return ctrls
End Function

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button1.Click
DrawText("hello world", Me.Font, New Point(30, 50))
End Sub

Private Sub DrawText(ByVal text As String, ByVal fnt As Font, ByVal pt
As Point)
Me.Refresh()
If (showText = "") Then
Dim textSize As Size = TextRenderer.MeasureText(text, fnt)
textRect.X = pt.X
textRect.Y = pt.Y
textRect.Width = textSize.Width
textRect.Height = textSize.Height
showText = text
showFnt = fnt
showPt = pt
invalidateChildControls = GetChildControls(textRect)
For Each ctrl As Control In invalidateChildControls
AddHandler ctrl.Invalidated, AddressOf
ChildControlInvalidated
Next
End If

Dim hScreenDC As IntPtr = GetDC(IntPtr.Zero)
Dim g As Graphics = Graphics.FromHdc(hScreenDC)
Using (g)
Dim screenPt As Point = Me.PointToScreen(pt)
g.DrawString(text, Me.Font, Brushes.Red, screenPt.X, screenPt.Y)
End Using

ReleaseDC(IntPtr.Zero, hScreenDC)
End Sub

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button2.Click
showText = ""
For Each ctrl As Control In invalidateChildControls
RemoveHandler ctrl.Invalidated, AddressOf
ChildControlInvalidated
Next
invalidateChildControls.Clear()
Me.Refresh()
End Sub

Private Sub ChildControlInvalidated(ByVal sender As Object, ByVal e As
System.Windows.Forms.InvalidateEventArgs)
If (showText <"") Then
DrawText(showText, showFnt, showPt)
End If
End Sub
End Class

Hope it is what you want.
Sincerely,
Linda Liu
Microsoft Online Community Support

[Phil]

>Also any control that redraws itself (e.g. a button when you hover over

with the mouse) will draw itself over the text.

We can solve this problem by handling the child control's Invalidated
event
and draw the text on the screen again.

I had thought of that, but didn't want to have to add a separate handler for
every child control on the form. I see your code identifies all the child
controls at runtime, so that gets around this problem.

>

>Another problem is if there is another window partly obscuring the form,

the text can still get written over the top of this other window.

I think we could redraw the text in the Activated event handler of the
form.

I had thought of that, but a form can become visible without becoming
activated. e.g. if another window is moved across it. This is why I put the
code into the Paint event, but then, it can still draw over the other
window, if the form is still partially covered.

I'll try your new code, and see how it works.

Thanks,
Phil.

[Linda Liu [MSFT]]

Hi Phil,

How about the problem now?

If there's any question, please feel free to let me know.

Thank you for using our MSDN Managed Newsgroup Support Service!

Sincerely,
Linda Liu
Microsoft Online Community Support

[Phil]

Hi Phil,

>
How about the problem now?

Hello,

Sorry for not getting back to you, been busy with other stuff.
I did try your updated version, and it is better, but as I suspected it does
still suffer from the problem with the text not showing when the form is
visible but inactive. I've got a couple of different uses for this, it'll
probably be OK for one of them, and there may be another way around it for
the other application.
I did think I might have a go at building this into a custom
TransparentLabel control which I could use for both but that may have to
wait until I have a bit more time.
I did have one idea: As you are now identifying all the controls that will
be overwritten, I wonder if it might be better to get a graphics object for
each of these and write to that rather than writing to the screen. This
could be done in the paint event, without the danger of overwriting other
windows. I haven't had chance to try this out yet though.

Cheers,
Phil.

[Linda Liu [MSFT]]

Hi Phil,

Thank you for your reply!

I wonder if it might be better to get a graphics object for each of these

and write to that rather than writing to the screen.

If the text is going to be painted on one child control fully, you can get
the graphics object for this child control and draw the text on this
control.

However, if the text couldn't be painted on one control fully, e.g. the
text is long and would be painted in an area that covers several child
controrls, it's difficult to draw the text using the graphics object of
each related child control, because each graphics object should only paint
a part of the text and the part is difficult to determine.
Sincerely,
Linda Liu
Microsoft Online Community Support

[Phil]

>I wonder if it might be better to get a graphics object for each of these

and write to that rather than writing to the screen.

If the text is going to be painted on one child control fully, you can get
the graphics object for this child control and draw the text on this
control.

Yes, that will be the case for one particular application I have in mind,
but in another case, my text will likely cover a number of other controls,

>
However, if the text couldn't be painted on one control fully, e.g. the
text is long and would be painted in an area that covers several child
controrls, it's difficult to draw the text using the graphics object of
each related child control, because each graphics object should only paint
a part of the text and the part is difficult to determine.

I haven't tried this yet, but presumably I could just paint the text to all
of them (and to the form). Presumably anything I try to paint outside of the
control's visible rectangle, would not be shown anyway, so this could work.

[Linda Liu [MSFT]]

Hi Phil,

I am reviewing this post and would like to know the status of this issue.

If you have any question, please feel free to let me know.

Thank you for using our MSDN Managed Newsgroup Support Service!

Sincerely,
Linda Liu
Microsoft Online Community Support