Andrei Smolin

How Add-in Express supports Office 365 / Office 2019

This blog is about how these products support Office 2019 and Office 365:

  • Add-in Express for Office and .NET
  • Add-in Express for Office and VCL
  • Add-in Express Outlook Regions for VSTO
  • Security Manager for Outlook

You might already know that Office 365 and Office 2019 are, in fact, subsequent builds of Office 2016: in any Office 365 and Office 2019 application, the Application.Version property returns a string that starts with “16”.

This means Add-in Express products supporting Office 2016 also work in Office 365 and Office 2019. Though, there are several details to take notice of.

Office 2019 and Office 365

Microsoft uses “Office 2019” for Office products that you buy as a one-time purchase, which means you pay a single, up-front cost to get Office apps for one computer. One-time purchases are available for both PCs and Macs. However, there are no upgrade options which means if you plan to upgrade to the next major release, you’ll have to buy it at full price (citation from As Office developers, you may run into a retail version of Office 2019 installed on the end-user’s machine and left non-updated. Be aware that that version may cause problems, so request the customer to update their Office version. The same applies to the initial retail version of Office 2016.

“Office 365” represents the “software as service” approach: Office 365 applications are delivered and updated online via Click-to-Run. On this page at they say that Office 365 is a subscription service.

Office 365 Insider (or Beta) channel

There are a few Office 365 update channels, each providing new builds at different update intervals. Different stability levels are assumed.

Insider is one of such update channels: it updates as often as 1-2 times a week. Every update is in fact a beta. To get a hand on the pulse, our testing team regularly run tests on the Beta channel builds.

Still, reacting to a change (once it is detected) may require some efforts from us and this will take some time. Also, Microsoft may introduce bugs with each update. And they did introduce bugs, quite rarely though. It also takes time for them to recognize the issue and fix it.

This means that Office 365 builds obtained through the Insider/Beta channel may not be supported immediately upon release. Using a non-tested beta build may break the add-in or even the add-in’s host application itself. We don’t recommend that your customers use the Insider channel.

Office from Microsoft Store

This is another flavor of Office. Because it works in a way that conflicts with per-user Office add-ins, only per-machine add-ins are supported. Find our research result in Office from Microsoft Store: a spreading source of issues. It looks like Microsoft has recognized the problem and we meet Office from Store on more and more rare occasions.

No support for Office Online, Office for Mac, Office for iOS

COM add-ins can only work on Windows Desktop. No other platform supports COM which is the base for Office on Windows. That is why, Add-in Express products do not support web versions of Office applications, also called Office Online. Nor do they support Office for Mac, iOS, etc.

For the avoidance of doubt, supported are only Office applications installed on Windows Desktop.

Security Manager for Outlook

Because Office 365 now updates regularly, you should update your Security Manager regularly, too. To get informed about new updates, subscribe to the Add-in Express RSS News Feed – please see this icon at the bottom of any page: Add-in Express RSS

Good luck!


  • Xavier N says:

    Hi Andrei,

    Read this blog post last month.

    With the introduction of .NET 5.0 and consolidation efforts of the different frameworks, porting to .NET Core (now just .NET 5.0) is not a question of when – it is just a question of how. I hope this post shed some light on the things you need to consider when choosing to port your Windows desktop app and make them cross platform.

    Not sure to understand all the in and out of the post but do you think it means com addins can be written in .net core and so be ported to macOs (and Linux) ? If so, will ADX think about a Addin Express version for .Net Core ?
    Regards. Xavier

  • Andrei Smolin (Add-in Express Team) says:

    Hello Xavier,

    In a .NET Core Class Library project, Visual Studio disables the “Register for COM Interop” flag. This means such a library cannot be used as a COM add-in. Besides, COM add-ins are only available on Windows Desktop. So, it doesn’t look like there ever be a .NET Core version of Add-in Express add-ins.

  • Alex says:

    Hi Andrei,

    regarding these ongoing modenization efforts by Microsoft, are you planning to add support for the new WebView2 control ( in the WebViewPane some time? Because currently the WebViewPane uses the old WebBrowser control, based on the Internet Explorer engine, if I looked correctly. Having an up-to-date browser engine in add ons would be really nice.


  • Dmitry Kostochko (Add-in Express Team) says:

    Hello Alex,

    I am afraid I do not quite understand your question. Microsoft is going to replace the web view control for Office Add-ins, i.e. for JScript add-ins. Add-in Express is a tool for creating COM Add-ins. We don’t use the System.Windows.Forms.WebBrowser control in our code. As for the WebViewPane advanced region – we use the WebViewOn and WebViewURL properties just for hiding the default content of an Outlook Folder. Then, we place your .NET form over that area.

    Is it what you were asking about? If not, please clarify your question.

  • Alex says:

    Hi Andrei,

    thanks for your clarification on this. I guess I’ve been too long out of outlook add in development and mixed up the WebViewPane with what’s rendered in our Form. I thought the webBrowser control was mandatory, but you’re right, its just a part of our form. I guess I took it from a random example back then.
    So thanks for bringing me on the right track again and sorry for taking your time ;-)

    Regards, Alex

  • Andrei Smolin (Add-in Express Team) says:

    No problem at all!

  • Jan van Veldhuizen says:

    A related question is: wehn I use the webviewurl for an Outlook folder, then it uses the IE browser via outlwvw.dll.
    As IE is being replaced by Edge, what will happen to this feature?

  • Andrei Smolin (Add-in Express Team) says:

    Hello Jan,

    The property doesn’t depend on the browser used. Add-in Express panes do not depend on the browser used, too.

    As to the WebViewURL property itself, they’ve restricted its use: check our blog at; look into the comments as well.

  • Jan van Veldhuizen says:

    The things, I have published an app that sits as an html application in Outlook Today, using the HomePage property.
    This app only runs within Outlook or in IE (because it supports ActiveX).
    I use this function to check in which context the app runs.

    function checkBrowser() {
    var isBrowserSupported
    if (window.external !== undefined && window.external.OutlookApplication !== undefined) {
    isBrowserSupported = true;
    outlookApp = window.external.OutlookApplication;
    outlookNS = outlookApp.GetNameSpace(“MAPI”);
    } else {
    try {
    isBrowserSupported = true;
    outlookApp = new ActiveXObject(“Outlook.Application”);
    outlookNS = outlookApp.GetNameSpace(“MAPI”);
    catch (e) {
    isBrowserSupported = false;
    return isBrowserSupported;

    So, I am quite sure that Outlook Today has kind of an IE control. So the question remains: what happens when Microsoft drops it?

  • Andrei Smolin (Add-in Express Team) says:

    Hello Jan,

    I see. Nobody in here is able to answer this question. You should address it to Microsoft. I believe they know about such an issue.

  • Jiri Mleziva says:

    I use Add-in Express version 9.2.4635 . Is it possible to use the custom add-in built with this version with Office 2019?
    I got the error

    Exception Source: Microsoft Word
    Exception Type: System.Runtime.InteropServices.COMException
    Exception Message: This command is not available.
    Exception Target Site: Open

    Could it be caused by the not up-to-date Add-in Express?

  • Andrei Smolin (Add-in Express Team) says:

    Hello Jiri,

    It shouldn’t be caused by Add-in Express. Instead, this should be caused by calling Open in some specific scenario. What’s your scenario? Do you reproduce the issue on other machines?

    Send answers to these questions and your call stack to the support email address; find it in {Add-in Express installation folder}\readme.txt. Make sure your email contains a link to this page.

  • David says:

    Hi, I am using addin-express to show website using WebBrowser, based on this tutorial:

    However i noticed that in newer outlook (2016 etc) WebBrowser is still using IE11 engine, I would really like it to use Edge/Chromium engine (it is available for normal addins installed via XML manifest). Is there any way to force WebBrowser to use newer browser or any other control that would give me a newer browser than IE?

  • Andrei Smolin (Add-in Express Team) says:

    Hello David,

    It seems you look for WebView2; see

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