[c#] Disabling Minimize & Maximize On WinForm?

Bind a handler to the FormClosing event, then set e.Cancel = true, and set the form this.WindowState = FormWindowState.Minimized.

If you want to ever actually close the form, make a class-wide boolean _close and, in your handler, set e.Cancel to !_close, so that whenever the user clicks the X on the window, it doesn't close, but you can still close it (without just killing it) with close = true; this.Close();

(And just to make my answer complete) set MaximizeBox and MinimizeBox form properties to False.