How do I create an "unfocusable" form in C#?

To disable activation by mouse:

class NonFocusableForm : Form
{
    protected override void DefWndProc(ref Message m)
    {
        const int WM_MOUSEACTIVATE = 0x21;
        const int MA_NOACTIVATE = 0x0003;

        switch(m.Msg)
        {
            case WM_MOUSEACTIVATE:
                m.Result = (IntPtr)MA_NOACTIVATE;
                return;
        }
        base.DefWndProc(ref m);
    }
}

To show form without activation (the only one way that worked for me in case of borderless form):

    [DllImport("user32.dll")]
    public static extern bool ShowWindow(IntPtr handle, int flags);

    NativeMethods.ShowWindow(form.Handle, 8);

Standard way to do this (seems like it doesn't work for all form styles):

    protected override bool ShowWithoutActivation
    {
        get { return true; }
    }

If there are other ways of activating the form, they can be suppressed in similar manner.