Flashes a screen-inverted rectangle around a window like in Microsoft Spy++.

InvertWindowBorder.cs

[DllImport("user32.dll", SetLastError = true)]
[return: MarshalAs(UnmanagedType.Bool)]
static extern bool GetWindowRect(IntPtr hwnd, out RECT lpRect);

[DllImport("user32.dll")]
static extern IntPtr GetWindowDC(IntPtr hWnd);

[DllImport("gdi32.dll")]
static extern int SaveDC(IntPtr hWnd);

[DllImport("gdi32.dll")]
static extern int SetROP2(IntPtr hdc, int fnDrawMode);

[DllImport("user32.dll")]
static extern int GetSystemMetrics(int smIndex);

[DllImport("gdi32.dll")]
static extern IntPtr CreatePen(int fnPenStyle, int nWidth, int crColor);

[DllImport("gdi32.dll")]
static extern IntPtr SelectObject(IntPtr hdc, IntPtr hgdiobj);

[DllImport("gdi32.dll")]
static extern IntPtr GetStockObject(int i);

[DllImport("gdi32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
static extern bool Rectangle(IntPtr hdc, int nLeftRect, int nTopRect,
                                         int nRightRect, int nBottomRect);

[DllImport("gdi32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
static extern bool DeleteObject(IntPtr hObject);

[DllImport("gdi32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
static extern bool RestoreDC(IntPtr hdc, int nSavedDC);

[DllImport("user32.dll")]
static extern IntPtr ReleaseDC(IntPtr hWnd, IntPtr hDC);

[StructLayout(LayoutKind.Explicit)]
struct RECT
{
    [FieldOffset(0)]
    int left;

    [FieldOffset(4)]
    int top;

    [FieldOffset(8)]
    int right;

    [FieldOffset(12)]
    int bottom;

    int Width
    {
        get { return right - left; }
        set
        {
            left += value;
            right -= value;
        }
    }

    int Height
    {
        get { return bottom - top; }
        set
        {
            top += value;
            bottom -= value;
        }
    }
}

const int R2_NOT         = 0x6,
          SM_CXBORDER    = 0x5,
          PS_INSIDEFRAME = 0x6,
          NULL_BRUSH     = 0x5;

static void InvertWindowBorder(IntPtr hwnd)
{
    if (hwnd == IntPtr.Zero)
        return;

    RECT rect;

    GetWindowRect(hwnd, out rect);

    if (rect.Width == 0 && rect.Height == 0)
        return;

    var hDc = GetWindowDC(hwnd);
    if (hDc == IntPtr.Zero)
        return;

    var savedDc = SaveDC(hDc);

    SetROP2(hDc, R2_NOT);

    var penWidth = GetSystemMetrics(SM_CXBORDER) * 3;
    var hPen = CreatePen(PS_INSIDEFRAME, penWidth, 0);
    SelectObject(hDc, hPen);

    var hBrush = GetStockObject(NULL_BRUSH);
    SelectObject(hDc, hBrush);

    Rectangle(hDc, 0, 0, rect.Width, rect.Height);

    DeleteObject(hPen);
    DeleteObject(hBrush);

    RestoreDC(hDc, savedDc);
    ReleaseDC(hwnd, hDc);
}