Show a c# Windows Forms on dual monitor

Actually this can be done very easily with the Screen class. 
Here is a small sample that display a second form, full screen, on your second monitor if there is one. 

Note: The code is to be put in your main Form class.

protected override void OnLoad(EventArgs e)
{
    base.OnLoad(e);

    if (Screen.AllScreens.Length > 1)
    {
        
        Form2 frm = new Form2();

        // Important !
        frm.StartPosition = FormStartPosition.Manual;

        // Get the second monitor screen
        Screen screen = GetSecondaryScreen();
        
        // set the location to the top left of the second screen
        frm.Location = screen.WorkingArea.Location;

        // set it fullscreen
        frm.Size = new Size(screen.WorkingArea.Width, screen.WorkingArea.Height);

        // Show the form
        frm.Show();

    }
}

public Screen GetSecondaryScreen()
{
    if (Screen.AllScreens.Length == 1)
    {
        return null;
    }

    foreach (Screen screen in Screen.AllScreens)
    {
        if (screen.Primary == false)
        {
            return screen;
        }
    }

    return null;
}