# How to add Overlay Objects to display from a worker thread in C#

**URL:** <https://forum.commonvisionblox.com/t/how-to-add-overlay-objects-to-display-from-a-worker-thread-in-c/484>\
**Category:** .Net API\
**Tags:** display, csharp\
**Created:** [June 28, 2018, 4:50pm UTC](https://forum.commonvisionblox.com/t/how-to-add-overlay-objects-to-display-from-a-worker-thread-in-c/484 "2018-06-28T16:50:04Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Greg](https://avatars.discourse-cdn.com/v4/letter/g/71c47a/32.png) [@Greg](https://forum.commonvisionblox.com/u/Greg)\
**Post date:** [June 28, 2018, 4:50pm UTC](https://forum.commonvisionblox.com/t/how-to-add-overlay-objects-to-display-from-a-worker-thread-in-c/484/1 "2018-06-28T16:50:04Z")

</div>

Hello,

I’m fighting with trying to add Overlay Objects to Display, but from a worker thread. `AddOverlayObjectNET()` returns always with a true, but nothing is in shown in the display. Here my code:

```auto
Cvb.Plugin.TStaticTextOutPlugInData textData = new Cvb.Plugin.TStaticTextOutPlugInData("dummy text", false);
Cvb.SharedPixelList pixelList;
Cvb.Image.CreatePixelList(3, out pixelList);
double[] position = new double[2];
position[0] = 100;
position[1] = 100;
Cvb.Image.AddPixel(pixelList, position);

if (cvDisp1.InvokeRequired)
{
  cvDisp1.Invoke((MethodInvoker)delegate () { cvDisp1.RemoveAllOverlayObjects(); });
  cvDisp1.Invoke((MethodInvoker)delegate () { cvDisp1.AddOverlayObjectNET("StaticTextOut", "bla", false, false, 0xFF, 0xFF00, true, 1, pixelList, textData.ToIntPtr()); });
  cvDisp1.Invoke((MethodInvoker)delegate () { cvDisp1.Refresh(); });
}

```

This code works perfect as long I have it implemented in the main thread (of course then without the invokes).

Any idea what is wrong there?

cheers  
greg

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<div class="post-metadata">

**Author:** ![illusive](https://yyz1.discourse-cdn.com/flex027/user_avatar/forum.commonvisionblox.com/illusive/32/61_2.png) [@illusive](https://forum.commonvisionblox.com/u/illusive)\
**Post date:** [June 29, 2018, 12:47pm UTC](https://forum.commonvisionblox.com/t/how-to-add-overlay-objects-to-display-from-a-worker-thread-in-c/484/2 "2018-06-29T12:47:31Z")

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Hi @Greg,

we have investigated this and it turns out that this is in fact _not_ a multithreading issue but a pointer aliasing problem. In the method `AddOverlayObjectNET` the last parameter (the pointer to the plugin data) has been marked as `void*` in the display control’s MIDL description (in the .Net wrappers generated by _tlbimp_ and _aximp_ this parameter is exposed as an `IntPtr`). We fully expected this to result in 64 bit being passed to the control via the `IDispatch` interface, but as it turns out, this is marshaled as a variant of type `VT_HANDLE` which is internally treated as an integer (long story short: you pass in a pointer which is converted to an integer by the CLR and then passed to the control which converts it back to a pointer from which the upper 32 bits are garbage).

The fact that you are now seeing this in you non-UI threads is coincidence more than anything else - it just happens that the overlay plugin data structure seems to usually end up inside the 32 bit address range in your UI thread and - maybe only slighty - above that in your non-UI threads.

Unfortunately this is impossible to fix without adding a new function (or modifying the existing one, which is something we decided against so that we don’t break existing applications - after all the issue is non-existent in a 32 bit process), which is what we did. The fixed ActiveX control will ship with the new version (13.1) of :cvb: which is due to be available soon (currently we’re on what will hopefully be the final release candidate). The new display control should also run fine on a 13.0 installation (if you need the OCX before the 13.1 release please PM me). Sorry for the inconvenience!

By the way: Any call that does **not** pass a plugin data structure will **not** suffer from the flaw in `AddOverlayObjectNET`, nor will for example `AddLabel` be affected. So one possibility would be to work around the issue with a label instead of the StaticTextout plugin until the fix is available.

On a different matter: I’d advise to rearrange your lambdas so that you don’t waste performance on superfluous thread transitions and synchronizations (you can do all three operations in one go):

```csharp
if (cvDisp1.InvokeRequired)
{
  cvDisp1.BeginInvoke((MethodInvoker)delegate () 
  {
    cvDisp1.RemoveAllOverlayObjects(); 
    cvDisp1.AddOverlayObjectNET("StaticTextOut", "bla", false, false, 0xFF, 0xFF00, true, 1, pixelList, textData.ToIntPtr());
    cvDisp1.Refresh();
  });
}

```

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<div class="post-metadata">

**Author:** ![illusive](https://yyz1.discourse-cdn.com/flex027/user_avatar/forum.commonvisionblox.com/illusive/32/61_2.png) [@illusive](https://forum.commonvisionblox.com/u/illusive)\
**Post date:** [July 12, 2018, 9:23pm UTC](https://forum.commonvisionblox.com/t/how-to-add-overlay-objects-to-display-from-a-worker-thread-in-c/484/3 "2018-07-12T21:23:30Z")

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As :cvb: 13.01.000 is out now, this can finally be addressed properly: The Display control has a new method, `AddOverlayObjectNET2` which is used just like `AddOverlayObjectNET` except for the last parameter that caused the issue (the last parameter no longer expects an `IntPtr` that it later on mutilated but uses `long` (the C# 64 bit sized type `long`…) across the board.

Note that starting with :cvb: 13.01.000 `AddOverlayObjectNET` in 64 bit applications will **always** return `false` (at first sight this might look fairly inconvenient, but I think it’s better to make the problem appear quickly than lull the user in a false sense of security).

If `AddOverlayObjectNET2` doesn’t show up immediately in your programming environment, please close Visual Studio, then delete the _bin_ and _obj_ folder of your project then open your project again.