Rectangle Intersection Test (with C#)

For a software project I needed to check whether two rectangles intersect (or overlap). What made my problem complicated was that one of the rectangles could be rotated. While this problem seems to be trivial (to a human being), it’s not that simple to implement. It took me a while to find the right answer.

Intersecting Rectangles

Now, the solution to this problem is called a separating axis test. Basically this means: If I can find an axis (read: line) that separates both rectangles, then they don’t intersect/overlap. (Actually this works for any convex polygon; see below) Of course, if the two rectangles don’t intersect, there are undefinitely many possible separating axes. Fortunately we can use the edges of each rectangle as axes and testing all of the is sufficient.

An axis separating two rectangles

I won’t get into the details of this algorithm here – it’s sufficiently good described in the article mentioned above – but basically you check for each point on which side of the separating axis it is. If all points of the rectangle A are on one side and all of rectangle B are on the other side, then we’ve found a separating axis.

Here’s the implementation of the method testing where a single edge (represented by points x1 and x2) is a separating axis:

/// <summary>
/// Does axis separation test for a convex quadrilateral.
/// </summary>
/// <param name="x1">Defines together with x2 the edge of quad1 to be checked whether its a separating axis.</param>
/// <param name="x2">Defines together with x1 the edge of quad1 to be checked whether its a separating axis.</param>
/// <param name="x3">One of the remaining two points of quad1.</param>
/// <param name="otherQuadPoints">The four points of the other quad.</param>
/// <returns>Returns <c>true</c>, if the specified edge is a separating axis (and the quadrilaterals therefor don't 
/// intersect). Returns <c>false</c>, if it's not a separating axis.</returns>
bool DoAxisSeparationTest(Point x1, Point x2, Point x3, Point[] otherQuadPoints) {
  Vector vec = x2 - x1;
  Vector rotated = new Vector(-vec.Y, vec.X);

  bool refSide = (rotated.X * (x3.X - x1.X)
                + rotated.Y * (x3.Y - x1.Y)) >= 0;

  foreach (Point pt in otherQuadPoints) {
    bool side = (rotated.X * (pt.X - x1.X) 
               + rotated.Y * (pt.Y - x1.Y)) >= 0;
    if (side == refSide) {
      // At least one point of the other quad is one the same side as x3. Therefor the specified edge can't be a
      // separating axis anymore.
      return false;

  // All points of the other quad are on the other side of the edge. Therefor the edge is a separating axis and
  // the quads don't intersect.
  return true;

This method is then called for each edge of each rectangle. If the method returns true, the actual intersection test method can return “not intersecting”. If the method returns false for all edges, the rectangles intersect.


I’ve created an C#/WPF example implementation (under FreeBSD license) that you can download and experiment with.

Rectangle Intersection Test Project (for Visual Studio 2010)

Remarks: The algorithm above works for every convex polygon. Instead of four times two edges you then have n times m edges. For concave polygons, however, this algorithm doesn’t work because there may be no separating axis even though the polygons don’t intersect.

WPF crashes on exit

So, I’m working on my WPF application and everything runs fine, but when I close it, I get this error message (together with this doesn’t-say-me-anything stacktrace):

[System.ComponentModel.Win32Exception]	{"Invalid window handle"}
  WindowsBase.dll!MS.Win32.HwndWrapper.DestroyWindow(object args) + 0x11a bytes	
  WindowsBase.dll!MS.Win32.HwndWrapper.Dispose(bool disposing, bool isHwndBeingDestroyed) + 0x8c bytes	
  WindowsBase.dll!MS.Win32.HwndWrapper.Dispose() + 0x14 bytes	
  PresentationCore.dll!System.Windows.Interop.HwndSource.Dispose(bool disposing) + 0x1f6 bytes	
  PresentationCore.dll!System.Windows.Interop.HwndSource.WeakEventDispatcherShutdown.OnShutdownFinished(object sender, System.EventArgs e) + 0x33 bytes	
  WindowsBase.dll!System.Windows.Threading.Dispatcher.ShutdownImplInSecurityContext(object state) + 0x49 bytes	
  mscorlib.dll!System.Threading.ExecutionContext.runTryCode(object userData) + 0x51 bytes	
  mscorlib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) + 0x6a bytes	
  mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool ignoreSyncCtx) + 0x7e bytes	
  mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) + 0x2c bytes	
  WindowsBase.dll!System.Windows.Threading.Dispatcher.ShutdownImpl() + 0x72 bytes	
  WindowsBase.dll!System.Windows.Threading.Dispatcher.PushFrameImpl(System.Windows.Threading.DispatcherFrame frame) + 0xe1 bytes	
  WindowsBase.dll!System.Windows.Threading.Dispatcher.PushFrame(System.Windows.Threading.DispatcherFrame frame) + 0x49 bytes	
  WindowsBase.dll!System.Windows.Threading.Dispatcher.Run() + 0x4c bytes	
  PresentationFramework.dll!System.Windows.Application.RunDispatcher(object ignore) + 0x17 bytes	
  PresentationFramework.dll!System.Windows.Application.RunInternal(System.Windows.Window window) + 0x6f bytes	
  PresentationFramework.dll!System.Windows.Application.Run(System.Windows.Window window) + 0x26 bytes	
  PresentationFramework.dll!System.Windows.Application.Run() + 0x1b bytes	
  GuideDock.exe!GuideDock.App.Main() + 0x94 bytes
  mscoreei.dll!__CorExeMain@0()  + 0x38 bytes	
  [Frames below may be incorrect and/or missing, no symbols loaded for mscoree.dll]	
  kernel32.dll!@BaseThreadInitThunk@12()  + 0x12 bytes	
  ntdll.dll!___RtlUserThreadStart@8()  + 0x27 bytes	
  ntdll.dll!__RtlUserThreadStart@8()  + 0x1b bytes

This problem seemed to appear only randomly until I figured it out today. The problem can be reproduce by this XAML/C# code (together with a WPF window):

public partial class MainWindow : Window {
  public MainWindow() {
    this.m_touchCanvas.MouseLeave += (s, e) => CrashAppOnClose();
  private void CrashAppOnClose() {
    Window wnd = Window.GetWindow(this.m_touchCanvas);
    // This line throws a Win32Exception with "Ivalid window handle".
    wnd.PointToScreen(new Point());

Now, when the user closes the window (without using the mouse; eg. with Alt+F4) while the mouse is still in the window, the call to wnd.PointToScreen() (line 11) results in the Win32Exception above. Unfortunately, the doesn’t seem to be any way to check whether this exception will be thrown – I already tried Window.IsLoaded as suggested here with no luck.

What’s more annoying is that the call to PointToScreen() does not appear in the stacktrace. I can’t even imaging how this is possible. That’s why it took me ages to figure this one out.

Btw: I’d like to send a bug report to Microsoft but they haven’t got my account working in three months.

Download the example project


  • The problem only arises on 64-bit Windows system. A correct exception is thrown on 32-bit systems.
  • On 64-bit systems the Win32Exception can’t be caught in a try ... catch block. Both PointToScreen() and the point where the exception is thrown are on the same thread.
  • I’ve managed to create a bug report for this problem.

Given the 32-bit error message – which reads “This Visual is not connected to a PresentationSource.” – I found a way to circumvent this problem. You need to use PresentationSource.FromVisual like this:

private void CrashAppOnClose() {
  Window wnd = Window.GetWindow(this.m_touchCanvas);
  if (PresentationSource.FromVisual(wnd) != null) {
    wnd.PointToScreen(new Point());