in window 7 (or possibly higher) the use FormWindowState.Maximizedis wrong, since the maximum size will be FormWindowState.Maximizedout of the FormWindowState.Maximizedtasks, but you can do it
this.WindowState = FormWindowState.Normal; // or default
this.FormBorderStyle = FormBorderStyle.None;
this.TopMost = true;
// do it here
this.Location = new Point(0,0);
var fullscreenSize = new Size(Screen.PrimaryScreen.Bounds.Width, Screen.PrimaryScreen.Bounds.Height);
this.Size = fullscreenSize;
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