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Exit application by clicking button on mainform when already pop up form is running

Form2 is user confirmation window,i want to close the application by stop button on mainform,but currently form2 is active and there is no dependency between form2 and mainform.How to close application ?

You must be opening Form2 using ShowDialog() method. Instead of that use Show() method and on MainForm button's click event call Application.Exit() method

You can use...
// for WinForms application
if (System.Windows.Forms.Application.MessageLoop)
{
System.Windows.Forms.Application.Exit();
}
// for Console application
System.Environment.Exit(1);

So your problem is the parent window will not have focus/input until the modal window (the confirmation dialog) has finished.
So, to avoid this, just make the confirmation dialog using a custom window (instead of a common dialog / msgbox) and show it. This way if the user wants to click your main form they can (the confirmation will be on background, covered by the main window because of this).

it looks like you're showing the dialog as ShowDialog() or showing a built in MessageBox which is also a ShowDialog that needs to be closed in order for focus to return back to main window. So what you should do is create a custom Dialog and use dialog.Show() method. This will allow you to interact with your main window while the dialog is open and you can click on a button to close your application regardless of the dialog being open or not. Following code should do the closing work for you
Application.Current.ShutDown();

C# How to go back from a window from ,where 2 different window forms lead to

I hope the title is not very confusing. I'll try to explain it as good as i can.
I am building a virtual museum with c# at visual studio. At form1 the user can choose if he will continue unregisterd ( leads to form2) or if he will continue as registered user( leads to form3). Some options are the same for the registered and the unregisterd user and some are not. So lets say that at both form2 and form3 i have a linklabel that leads to form4 with the code
this.close();
Form4 myForm4 = new Form4;
myForm4.ShowDialog();
So far so good. At form4 i put a "previous" button.
How can i make it to go back to the form that led me to form4?
if i click the linklabel from form3 and push the previous button , i want to go to form3 and if i click from form2 i want to return to form2.
One solution is to create similar form and go back without a problem. But i have 6 same options which means 12 new forms.
Can anyone help me how to do it?

Just don't close the form; hide it instead. Execution will return to it after ShowDialog(), where you can unhide it again:
this.Hide();
Form4 myForm4 = new Form4;
myForm4.ShowDialog();
this.Show();
You could actually trap the result of ShowDialog() and do something different besides just unhiding the form.
A more robust approach would be to use one form and change what is currently displayed in it by using either UserControls or borderless forms. Then you could have a List to track the order and allow easy traversal for back/forward buttons.

C# Windows Form - Window Always Focused

I have a form from which I can open another form. I would like to prevent the user from switching to other forms when form 2 is opened. Thus, the user can only switch to other forms when this windows is closed.
Any help to do so?

Use ShowDialog method which open a new form in Modal state
Form2 frmNew = new Form2();
frmNew.ShowDialog()

You should use a (Modal) Dialog instead of a Window.
A dialog can only be exited by pressing a Cancel/OK button (or Escape key), while windows have normally a Close button.
Example to show a dialog:
// C#
//Display frmAbout as a modal dialog
Form frmAbout = new Form();
frmAbout.ShowDialog();

form.ShowDialog() instead of form.Show()

Place form on top and disable others behind it

I have a Main Form and a Other Form.
The Main Form is always open, and at some time it will launch Other Form.
I tried:
form.TopMost = true;
But this only places the form on top. The Form behind (Main Form) still can be accessible.
How can I get the same behavior like when I do OpenFileDialog, and disable the main form behind it?
(Thanks in advance)

You need to make your form modal. To do this, use ShowDialog instead of Show to display it (the same way you do with a dialog).
Also, note that forms shown with ShowDialog are not actually closed and disposed when you click their Close button, so you should dispose them manually. The usual way to handle their lifetime is to use a using construct:
using (var form = new SomeForm())
{
form.ShowDialog();
// do stuff after the dialog is closed
}

form.showdialog(); where form is the the top form to be launched.so while lauching the top form just add form.showdialog()

.NET Compact Framework - modal Form without ShowDialog

Is there a way to show a modal form without ShowDialog metod calling? By showing a modal form I mean that the Form overlays the current Form and prevent user input to the bacground Form. The Form that is to be shown is a MessageBox style form that is not fullscreen.
Thanks
Dominik

I would assume that you could set the "dialog" form to stay on top (TopMost property) and then disable the main form in order to prevent input.
This is only a partial solution as the main form will still be able to be focused, closed etc. You will need to handle all these cases yourself in order to prevent them.
Is there any particular reason you don't want to use ShowDialog?

You could try to do this:
MyForm frm = new MyForm(); // this would be your modeless dialog
frm->show(this); // "this" being the instance that invokes it.