Variante 1:
private void Form1_SizeChanged(object sender, EventArgs e) { if (this.WindowState == FormWindowState.Maximized) { ... } }
Variante 2:
protected override void WndProc(ref Message m) { if ((UInt32)m.Msg == Constant.WM_SYSCOMMAND) { switch ((UInt32)m.WParam) { case Constant.SC_MAXIMIZE: break; case Constant.SC_RESTORE: break; default: break; } } base.WndProc(ref m); }