Disabling the Close-Button temporarily

Question

I need to disable just the close button (minimize and maximize should be allowed) temporarily.

Every solution I\'ve tried disables all the buttons

Answer 1

Using Win32 API, you can do this the following way:

[DllImport("User32.dll")]
private static extern uint GetClassLong(IntPtr hwnd, int nIndex);

[DllImport("User32.dll")]
private static extern uint SetClassLong(IntPtr hwnd, int nIndex, uint dwNewLong);

private const int GCL_STYLE = -26;
private const uint CS_NOCLOSE = 0x0200;

private void Form1_Load(object sender, EventArgs e)
{
    var style = GetClassLong(Handle, GCL_STYLE);
    SetClassLong(Handle, GCL_STYLE, style | CS_NOCLOSE);
}

You will need to use GetClassLong / SetClassLong to enable the CS_NOCLOSE style. You then can remove it with the same operations, just use (style & ~CS_NOCLOSE) in SetClassLongPtr.

Actually, you can do this in WPF apps as well (yeah, I know, the question is about WinForms, but maybe someone will need this some day):

private void MainWindow_OnLoaded(object sender, RoutedEventArgs e)
{
    var hwnd = new WindowInteropHelper(this).Handle;
    var style = GetClassLong(hwnd, GCL_STYLE);
    SetClassLong(hwnd, GCL_STYLE, style | CS_NOCLOSE);
}

Still, you should consider what others have suggested: just show a MessageBox or another kind of message to indicate that user should not close the window right now.

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Edit: Since window class is just a UINT, you can use GetClassLong and SetClassLong functions instead of GetClassLongPtr and SetClassLongPtr (as says MSDN):

If you are retrieving a pointer or a handle, this function has been superseded by the GetClassLongPtr function. (Pointers and handles are 32 bits on 32-bit Windows and 64 bits on 64-bit Windows.)

This resolves the problem described by Cody Gray regarding the absense of *Ptr funcs in 32-bit OS.