lock application after time 'n' c#

Hello

Simple way is using ShowDialog()

1. Use Timer Control in MainForm Look at here

2. After "n" seconds or after clicking a Button,(in MianForm):

private void MyTimer_Tick(object sender, EventArgs e)
{
   MyTimer.Stop();
   AuthenticationForm.ShowDialog();
}

AuthenticationForm is a winFrom that you've created it for entering UserName and Password.

The application wont be accessible by user while the AuthenticationForm is opening.

3. If you want to pause another processes in the application, you can make it manually. (Use Event Handler)

4. If you use multiThreading then the solution may not work !

5. I prefer to set Enable of MainForm and other Forms to false instead of using ShowDialog(), then you need to store all opened form in a Collection for example List<Form>. You can overrid OnLoad(EventArgs e) method of Forms to store themselve into a static List. Form.OnLoad Method
It may be threadsafe but you must look at here first: Threading