Disable close button

@tayspen’s demo shows the common Win32 route (EnableMenuItem); @v_sirigiri expanded that with a toggle and @alegn asked the right question about MF_ENABLED. A few important clarifications that aren’t always obvious: graying the Close menu item alters the system menu but does not by itself cover every close path (Alt+F4, programmatic Close, or OS shutdown). Two more robust approaches are usually preferable depending on the goal — prevent closing, or remove the X visually.

To reliably block user-initiated closes, handle the FormClosing event and cancel when appropriate. This covers the title‑bar X and Alt+F4 in a simple, managed way:

private bool allowClose = false;

private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
    if (!allowClose && e.CloseReason == CloseReason.UserClosing)
    {
        e.Cancel = true;
    }
}

Set allowClose = true before calling Close() when an actual programmatic close is desired. Checking e.CloseReason avoids cancelling system-initiated shutdowns.

To hide the close button at creation time without P/Invoke, override CreateParams and clear the WS_SYSMENU bit so the X never appears:

protected override CreateParams CreateParams
{
    get
    {
        CreateParams cp = base.CreateParams;
        const int WS_SYSMENU = 0x00080000;
        cp.Style &= ~WS_SYSMENU;
        return cp;
    }
}

To toggle the X at runtime, the usual path is SetWindowLong/SetWindowLongPtr to add/remove WS_SYSMENU and then force a redraw (SetWindowPos with SWP_FRAMECHANGED or call DrawMenuBar). Beware 64‑bit: use the Ptr variants on x64. Finally, no client-side trick will stop a forced process kill or some OS shutdowns — combine menu/style changes with FormClosing checks for the most reliable behavior.