C# Dialog Box Window Rectangle

Question

I'm hooking dialog boxes (CommonDialog and MessageBox) so that I can center them into the parent Form once they are displayed. Everything works like a charm for what concerns MessageBox, but when the dialog inherits from CommonDialog (OpenFileDialog, SaveFileDialog, etc...) the rectangle I retrieve is always wrong and the centering gets messed up. This is the code I use to get the dialog box rectangle:

[DllImport("User32.dll", CallingConvention=CallingConvention.StdCall, CharSet=CharSet.Unicode, ExactSpelling=true, SetLastError=true)]
[return: MarshalAs(UnmanagedType.Bool)]
private static extern Boolean GetWindowRect([In] IntPtr handle, [Out] out RECT rectangle);

[DllImport("Dwmapi.dll", CallingConvention=CallingConvention.StdCall, CharSet=CharSet.Unicode, ExactSpelling=true, SetLastError=true)]
private static extern UInt32 DwmGetWindowAttribute([In] IntPtr handle, [In] DWMWINDOW_ATTRIBUTE attribute, [In, Out] IntPtr attributeValue, [In] UInt32 attributeSize);

[DllImport("Dwmapi.dll", CallingConvention=CallingConvention.StdCall, CharSet=CharSet.Unicode, ExactSpelling=true, SetLastError=true)]
private static extern UInt32 DwmIsCompositionEnabled([Out, MarshalAs(UnmanagedType.Bool)] out Boolean enabled);

internal static Boolean AeroThemeEnabled
{
    get
    {
        Boolean result = false;

        if (Environment.OSVersion.Version.Major >= 6)
            DwmIsCompositionEnabled(out result);

        return result;
    }
}

internal static Boolean GetRectangle(IntPtr windowHandle, out Rectangle rectangle)
{
    RECT nativeRectangle;

    if (AeroThemeEnabled)
    {
        IntPtr attributeValue = IntPtr.Zero;
        UInt32 size = GetSize(typeof(RECT));

        try
        {
            attributeValue = Marshal.AllocCoTaskMem((Int32)size);

            UInt32 result = DwmGetWindowAttribute(windowHandle, DWMWINDOW_ATTRIBUTE.DWMWA_EXTENDED_FRAME_BOUNDS, attributeValue, size);

            if (result == 0)
                nativeRectangle = Marshal.PtrToStructure<RECT>(attributeValue);
            else if (!GetWindowRect(windowHandle, out nativeRectangle))
            {
                rectangle = Rectangle.Empty;
                return false;
            }
        }
        finally
        {
            if (attributeValue != IntPtr.Zero)
                Marshal.FreeCoTaskMem(attributeValue);
        }
    }
    else if (!GetWindowRect(windowHandle, out nativeRectangle))
    {
        rectangle = Rectangle.Empty;
        return false;
    }

    Int32 x = nativeRectangle.Left;
    Int32 y = nativeRectangle.Top;
    Int32 width = nativeRectangle.Right - x;
    Int32 height = nativeRectangle.Bottom - y;

    rectangle = new Rectangle(x, y, width, height);

    return true;
}

In my working environment (Win7 with VS2013 and Framework 4.6.2), as I previously said, the method GetRectangle returns wrong rectangle Width and Height when the dialog box I have to center inherits from CommonDialog. During my tests, I found out that both GetWindowRect and DwmGetWindowAttribute return the same values, so the main workaround I implemented is totally useless. I also discovered that the retrieved rectangle is always 40 pixels or so wider than the real one (I measured it taking a screenshot).

Now, this is seriously driving me to madness. The only solutions I can figure out are:

1. Detect if the window handler belongs to a CommonDialog and, in that case, manually fix the rectangle size. I don't know how, since GetClassName returns #32770 for both MessageBox and CommonDialog. I also doubt that GetWindowLong can provide useful information that can let me recognize the dialog box type.

2. Manually resize the CommonDialog as soon as it is shown so I can know its real size.

Any other suggestion is more than welcome.