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In C# WPF window, adding button for topmost beside standard window buttons

I'd like add a button for changing window state to topmost to the built-in window buttons such as maximize, minimize, and close [please refer to this pic.]
However, I'm having hard time finding the way out as WPF seems not to provide such an API. I even thought using the icon next to the window title functioning as the button for topmost, but looks not feasible.
Is there anyway like using .dll or could I inherit the window class and add the button and corresponding event handler anyhow?
Thanks.

It will be very difficult as you can see below link:
How to add an extra button to the window's title bar?
To make things easy, you should consider implementing a custom window for that. This window will have custom buttons including Close,Minimize,Maximize along with any other buttons as well.

Mono Gtk Window require focus for input

I haven't been able to find an example containing this functionality, and either i missed it in the documentation or it's not there.
I have a fullscreen GUI program, and when a user is required to type in a number, a calculator window popup has to appear in the center of the screen. The user types a number and clicks on enter, or hits cancel to continue past the window.
The problem is clicking on the fullscreen window behind the calculator brings that window to the front and hides the calculator without the intended entry being completed, which could get annoying for the user.
I guess the functionality I'm trying to create is what happens in most text editors/IDE's when you press the Open File button. Let me know if you want to see code, it's just two separate Window classes at the moment.

The Present() function of the Window object brings that particular window to the front. So adding a FocusOutEvent listener on the window you wish to keep in front like:
windowObj.FocusOutEvent += (obj, args) => windowObj.Present();
will work.
The alternative to this (and the better way) is to set the Modal property of the window to true.
If you've stumbled across this and you were looking for a popup window that suspends whatever called it to wait for input, see this question:
gtk# thread for window
You basically use the Dialog class instead of Window and add your elements to the ActionArea of the Dialog.
Hope this helps.

Get all window handles and respective label handles given process name

is it possible to get the handle of a specific component in an unknown number of window opened by the same program?
The program foo.exe contains a button that, when clicked, opens a form containing a Label and an Image ( with no upper bound on the number of forms opened). Is possible to get all the handle of the Label component of every window openend, given the process name "foo" ?

with EnumWindows or FindWindow you can find the application window depending if you already know the title or the process name or so on, you can even use FindWindowLike if you really do not much of it.
Once in one way or another you know the windows handle of your window, with GetDlgItem you can get info about your child controls.

C#/WPF, how to make a window (created with Window.ShowDialog()) title bar blink when clicking its parent window (like MessageBox does)?

I'm trying to create a custom MessageBox by using a WPF Window that is called with ShowDialog().
So far, I've managed to implement everything, except for one thing.
As you know, when you use MessageBox.Show("text"); you cannot set the focus or click the parent window (the one that called the MessageBox). If you do try to click the parent window, the MessageBox will blink briefly in order to alert you that you must close if first.
Windows created with Window.ShowDialog();, however, do not show that behavior. In fact, while you cannot set the focus to the parent window, the child (called with ShowDialog()) will never blink briefly.
My question is, is there any way to implement that in WPF? I've been searching for an answer but I must admit, I am stumped.
Thanks everyone!

You need to set the Owner of the modal window correctly, e.g. using the following code from within the owning window:
Window win = new SomeModalWindow();
win.Owner = this;
win.ShowDialog();

You would have to set Owner property of the child Window to the parent Window. See the MSDN Documentation here.

How do you send information from one window to another in the same program?

In the program I am trying to build, I have a menu button that opens a second window. The user puts information into the second window, presses the "Done" button, and the information is transfered into the main window. The problem I am having is opening the second window. I have both windows build in xaml files in Visual Studio but I can't find a way to show the second window. Using "Window window = new Window" does not fit my needs because 1) I already have the second window built and 2) I have tried this and I cannot figure out how to add children to the window; there is no window.children nor any grid to put the children into. Thank you in advance!
Moments after I pressed post, I thought of something I hadnt tried:
"WindowAdd add = new WindowAdd; //WindowAdd being the second window
add.Show();"
This does exactly what I want it to do. The next problem I have is sending the information the TextBoxes into the MainWindow. I am thinking cookies might work but am unsure. Anyone have any thoughts? Thanks in advance!

You need to create the Window in code, but instead of doing:
Window window = new Window();
You should use:
Window2 window = new Window2(); // Assuming the window's class name is Window2
This will construct and initialize an instance of your new window class, defined in XAML. Once you've done this, you can open the window and you'll see all of your controls.