C# ハンドルを元に対象プロセスのWindowサイズを取得

[DllImport("user32.dll")]
private static extern bool GetWindowRect(IntPtr hwnd, out RECT lpRect);

[StructLayout(LayoutKind.Sequential)]
private struct RECT
{
    public int left;
    public int top;
    public int right;
    public int bottom;
}

[StructLayout(LayoutKind.Sequential)]
private struct SIZE
{
    public int width;
    public int height;
}

private static SIZE GetWinSize(IntPtr _IntPtr)
{
    IntPtr handle = _IntPtr; //取得したいWindowのハンドル

    RECT rect;
    bool flag = GetWindowRect(handle, out rect);

    SIZE size;
    size.width = rect.right - rect.left;
    size.height = rect.bottom - rect.top;

    return size;
}