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Winforms: How to get Mouse Events for Transparent Control on a Transparent Form

I have a transparent form that overlays the desktop in a c# .NET winforms application. The transparency is done by setting the BackColor to bright orange and then setting the TransparencyKey to the same bright orange.
So far this works great and it creates a transparent form. I then want to create a control on the transparent form that draws a rectangle around items on the desktop. So essentially you can think of a transparent button with a rectangular border. To make this happen I currently extend Control and setup the control like this inside of the parent form which is also transparent:
public class CustomControl : Control
{
public CustomControl(Size size, Point point)
{
SetStyle(ControlStyles.SupportsTransparentBackColor, true);
this.BackColor = Color.Transparent;
this.Size = size;
this.Location = point;
}
protected override void OnPaint(PaintEventArgs e)
{
base.OnPaint(e);
Pen pen = new Pen(Color.Red, 2f);
e.Graphics.DrawRectangle(pen, this.ClientRectangle);
}
}
This creates the exact effect I'm looking for in that there is a control with a solid opaque rectangle border inside a transparent form and you can see the desktop through the control and form (see image below on left side).
The problem is that the transparent area of the control does not receive any mouse events. In fact, it basically doesn't exist. For example, when your mouse goes over the red rectangular border of the control that triggers the Mouse Hover, Mouse Enter, and Mouse Leave events and changes the cursor to whatever you set as this.Cursor for the control. Once, the mouse is in the transparent portion of the control none of these things trigger.
How can I keep things the way they are now in regards to the look of the control but still receive mouse events in the transparent area. In particular I want to receive Mouse Hover and have the controls Cursor value be honored. Note that if I make the BackColor of the control anything other than Color.Transparent then mouse events work fine.
Thank you!
Update
Based on Hans comment I would like to add that we have implemented the above and that it has usually worked (i.e., the transparent area does respond to mouse events). The reason this issue came up is because I recently rebuilt my machine with all the Windows 8.1 updates and latest ATI graphic drivers and after that the above setup no longer works ever (transparent areas on the control no longer receive any mouse events and for all intents and purposes are not part of the control) On my colleague's machine it almost always works although we've occasionally noticed that it won't work but we could never consistently reproduce the issue.
My assumption was that we did something wrong to cause the transparent areas to not respond to mouse events. However, based on Hans' comment, it seems as though the above code should never work and that the only reason it worked was because of a bug in Aero.
The exact color of our transparency key is rgb(255, 128, 0). In addition we did notice that any label controls placed on the transparent form look terrible (per Han's comment).
Update 2
Based on Han's additional comment regarding the Aero transparency bug I have the following updated questions.
Is there any information on this bug somewhere that explains what the bug is?
What is the intended behavior (assume there are no bugs) for transparent areas of a control? Should mouse events (e.g., Mouse Hover) work on the transparent area?
The Final Answer (that usually works)
The answered provided by Reza below did work for me on some of my computers. However, my primary desktop continued to stubbornly refuse to cooperate. Even when copying the exact project between computers using the same version of windows and .NET the problem existed. The issue when it arose was that transparent areas would not trigger mouse events and were not treated as part of the control.
In addition, I noticed the same thing Reza noticed which was that certain TransparencyKey colors flat out did not work. Although I don't know any details of the bug I would have to agree with Hans that there are bugs involved with transparency on WinForms and if anyone is starting from scratch I would go the WPF route and save yourself any possible future headaches.
In the end we ended up implementing a work around based on some of Hans' answers that required using a timer to check the mouse location and nesting two forms (one with opacity set) to be able to manage the mouse cursor. This solution has worked on all our systems and will hopefully hold up until we move over to WPF. I accepted Reza's answer because it seems to work in most places but be aware that it might not work you and there is no rhyme or reason as to why.
For detailed information on the two workaround we implemented please see the following questions and answers by Hans.
MouseHover and MouseLeave Events controlling
How can I add transparency to a c# form while keeping controls visible?

Important
Consider set BackgroundColor of form to Red and TransparencyKey to Red and BackGroundColor of transparent control to Transparent and it will work!
the strange thing that I see is, the approach doesn't work with for example Magenta but works with Red and Blue.
I think you should create your transparent control this way:
Transparent Control Code
public class TransparentControl : Control
{
public TransparentControl()
{
this.SetStyle(ControlStyles.SupportsTransparentBackColor, true);
}
private const int WS_EX_TRANSPARENT = 0x20;
protected override CreateParams CreateParams
{
get
{
CreateParams cp = base.CreateParams;
cp.ExStyle = cp.ExStyle | WS_EX_TRANSPARENT;
return cp;
}
}
}
And for the mouse events and rendering border when the mouse enters, here is the sample I did Using .Net 4.5 On Windows 8.1:
Create a Form and put a TransparentControl that we created using above code, on it, then handle MouseEnter and MouseLeave and Paint events and paint the border when mouse is in control bouds and handle Click event and show a message.
Form Code
private bool drawBorder;
private void transparentControl1_MouseLeave(object sender, EventArgs e)
{
drawBorder = false;
transparentControl1.Invalidate();
}
private void transparentControl1_MouseEnter(object sender, EventArgs e)
{
drawBorder = true;
transparentControl1.Invalidate();
}
private void transparentControl1_Paint(object sender, PaintEventArgs e)
{
if(drawBorder)
{
using (var pen = new Pen(this.ForeColor, 5))
{
e.Graphics.DrawRectangle(pen, 0, 0, this.transparentControl1.Width - 1, this.transparentControl1.Height - 1);
}
}
}
private void transparentControl1_Click(object sender, EventArgs e)
{
MessageBox.Show("Clicked");
}
Screenshot
The mouse cursor is in the control's area so the black border has been painted.
Important Note
If you draw border the same color as form's transparency key, the border will not be shown.

I think it's okay for the mouse cursor to go in the middle of that overlay box and move with the mouse cursor and the overlay box to make it look like it's a mouse cursor.
Like this picture

I have had similar problem when I use a picturebox control on a transparent form and use the mouseclick event over the control to trigger code. Sometimes the mouseclick is captured and sometimes it isn't. No pattern to it other than the color used as the form background. Red works fine but many other colors do not. Even black does not work consistently. I have found an almost black color that works fine when form.backcolor and form.transparencykey are set to this color. Based on this and others experiences it appears a bug in VB Studio and the way transparency is handled.
Form1.BackColor = Color.FromArgb(64, 0, 0) ' a color that works with transparency and allows picturebox to be clicked

Make a form's background transparent

How I make a background transparent on my form? Is it possible in C#?
Thanks in advance!

You can set the BackColor of your form to an uncommon color (say Color.Magenta) then set the form's TransparencyKey property to the same color. Then, set the FormBorderStyle to None.
Of course, that's just the quick and easy solution. The edges of controls are ugly, you have to keep changing the background color of new controls you add (if they're Buttons or something like that) and a whole host of other problems.
It really depends what you want to achieve. What is it? If you want to make a widget sort of thing, there are much better ways. If you need rounded corners or a custom background, there are much better ways. So please provide some more information if TransparencyKey isn't quite what you had in mind.

Put the following in the constructor of the form:
public Form1()
{
this.TransparencyKey = Color.Turquoise;
this.BackColor = Color.Turquoise;
}
Note: This method prevents you from clicking through the form.

Update:
How to: Give Your Control a Transparent Background
Deprecated:
How to: Create Transparent Windows Forms:
Note: As transparent forms are only supported in Windows 2000 or
later, Windows Forms will be
completely opaque when run on older
operating systems, such as Windows 98,
regardless of the value set for the
Opacity property.

A simple solution to get a transparent background in a winform is to overwrite the OnPaintBackground method like this:
protected override void OnPaintBackground(PaintEventArgs e)
{
//empty implementation
}
(Notice that the base.OnpaintBackground(e) is removed from the function)

Transparent User Control in .net

I'm trying to make a web style button with user control. I need to make User Control's background transparent. How to do this without making controls invisible. Also I'll need transparent Label and PictureBox.
Trying to make something like this:
this.SetStyle(ControlStyles.SupportsTransparentBackColor, true);
this.BackColor = Color.Transparent;

All three controls you list already have the ControlStyles.SupportsTransparentBackColor style turned on. Just set the BackColor property in the designer, Web tab. It is very unclear why that's not good enough for you.
It is otherwise an illusion, Windows Forms implements it by asking the Parent control to draw itself in the OnPaintBackground() method so it provides the background pixels. One notable thing that doesn't work is overlapping controls. You only see the Parent pixels, not the pixels of the overlapped control. That's fixable but the code is fugly.
The only other transparency option is Form.TransparencyKey. That's true transparency, implemented by using overlays in the video adapter. Problem is that this only works on toplevel windows. Forms, not controls.
These restrictions are inherent in the Windows Forms rendering model, using individual windows for the controls. A web browser doesn't have that same restriction, it emulates controls by drawing them. Layers of paint, that makes transparency trivial by just not painting. WPF uses that rendering model as well.

If you mean you want "partial transparency", see this for nice solution:
Opacity of Buttons/TextBoxes - VB.NET
(setting the background color using Alpha value)
I know the above sample is VB but it applies to C# as well. :)

You can use this code :
ImageButton() is my constructor.
public ImageButton()
{
InitializeComponent();
this.SetStyle(ControlStyles.Opaque, true);
this.SetStyle(ControlStyles.OptimizedDoubleBuffer, false);
}
protected override CreateParams CreateParams
{
get
{
CreateParams parms = base.CreateParams;
parms.ExStyle |= 0x20;
return parms;
}
}

How to clear graphics on a transparent control in windows forms

So, I am pretty unfamiliar with windows forms development.
I'm trying to create a "hey I'm busy doing stuff" component that just spins a shape around. I want this control to be transient and draw on top of any other controls. The class inherits directly from Control.
So I have this in the constructor:
SetStyle(ControlStyles.Opaque, true);
and this:
protected override CreateParams CreateParams
{
get
{
CreateParams parms = base.CreateParams;
parms.ExStyle |= 0x20;
return parms;
}
}
Which gets me a control that will draw on top of other controls.
Now my problem is this. I repaint the control a few times a second to give the appearane of a smooth animation. However, I can't figure out how to clear what was drawn in the previous frame. Using e.Graphics.Clear(Color.Transparent) in OnPaint turns the whole control black.
Is there a way to just clear the drawn contents of a control?
I've noticed that Resizing the the control will clear the background.
Things that Don't Work
Overriding OnPaintBackground to do nothing. Or just calling base.OnPaintBackground. Same results.

Okay, I found the solution here:
https://web.archive.org/web/20141227200000/http://bobpowell.net/transcontrols.aspx
The Parent controls actually must be invalidated in order to retain the transparent background.

You may have to override OnPaintBackground that this article presents: http://saftsack.fs.uni-bayreuth.de/~dun3/archives/creating-a-transparent-panel-in-net/108.html
You may also need to Invalidate the control when it needs to be cleared to force OnPaintBackground to be called.

Setting the parent of a usercontrol prevents it from being transparent

I've created a simple user control which is manually created with something like
MyUserControl ctrl = new MyUserControl();
The control have been designed to have BackColor = Color.Transparent and that works fine, until I set the Parent of the control to a form at which time it turns into the color of the form.
Might sound like its transparent but what it does is hide all the controls that exist on the form as well. I'm not 100% sure its the control that gets a solid background or something else thats happening when i hook it up, which prevents other controls from showing.
Basically if you do this
Create a form
Drop a button on it
In the click handler for the button you do the following
Example
MyUserControl ctrl = new MyUserControl();
ctrl.Parent = this;
ctrl.BackColor = Color.Transparent;
ctrl.Size = this.Parent.ClientRectangle.Size;
ctrl.Location = this.Parent.ClientRectangle.Location;
ctrl.BringToFront();
ctrl.Show();
Basically I want the usercontrol to overlay the entire form, while showing the underlaying controls on the form (hence the transparent background). I do not want to add it to the forms control collection because it doesn't really belong to the form, its just being shown ontop of everything else
I tried doing the same, but without setting the parent, but then the control didnt show at all.
Thanks!
EDIT: If I override the OnPaintBackground method in the usercontrol and prevent the background from being painted then it works, however that messes up with the transparent parts of a PNG image im painting in the control using DrawImage, which makes sense.

Windows Forms doesn't really support transparent controls.
You can work around this limitation by overriding the CreateParams property of the control and setting a custom style (look it up on google).
Further you have to override the painting of your control so that not only your control but also the parent control is redrawn. The reason is that the background must be painted before your control paints itself.
Finally you should override the OnPaintBackground method, as you have done, to make sure no background is painted.
Quite clumsy, and not perfect, but it should work.