Mine

// FIXME: This is also seriously LAME. The MinSliderSize property is "protected"
// according to gtkrange.h, and thus should be exposed and accessible through 
// this sub-class, but GAPI does not bind protected structure fields. LAME LAME.

[DllImport("libgobject-2.0-0.dll")]
private static extern void g_type_query(IntPtr type, IntPtr query);

private int min_slider_size_offset = -1;

public int MinSliderSize {
    get { 
        if(min_slider_size_offset < 0) {
            IntPtr query = Marshal.AllocHGlobal(5 * IntPtr.Size);
            g_type_query(Gtk.Widget.GType.Val, query);
            min_slider_size_offset = (int)Marshal.ReadIntPtr(query, 2 * IntPtr.Size + 4) + IntPtr.Size + 8;
            Marshal.FreeHGlobal(query);
        }

        int size = (int)Marshal.ReadIntPtr(Handle, min_slider_size_offset);
        Console.WriteLine(size);
        return size;
    }
}