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Office Plug-Ins Don’t Have To Be Ugly, Slow, And Clunky

Updated: May 10, 2023

By John Kwan, Benjamin Woods, Jennet Hydyrova, and Tural Hajiyev

The Microsoft Office suite remains by far the most popular set of business applications for the legal community. [1] Plug-ins extend the functions of these applications and tailor the edited them to legal professionals’ needs. One major advantage for users is that they do not need to leave the familiar Office environment to take advantage of these enhancements.

As Shreya Vajpei, Assistant Manager at Khaitan & Co commented, “Given the amount of time that lawyers spend working in Microsoft Office 365, we would like to see more of them take advantage of the rich features and capabilities the suite has to offer. This includes third-party plug-ins that can add a lot of extra functionality to MS Word and MS Outlook and tailored to our sector’s needs.”

A common frustration, however, is that plug-ins tend to slow down Office applications. In some cases, they could even lead to crashes and valuable work being lost in the process. Many also observe that a lot of plug-ins have old-school interfaces (especially when compared with modern consumer apps) that detract from the user experience.

It is now possible to have your cake and eat it too. Plug-ins built using Microsoft’s “Office Add-in” framework can be beautiful, fast, and a breeze to deploy. In this article, we draw upon our recent experience developing SendCheck [2], a plug-in for Office Outlook.

Two types of plug-ins

Until around 2011, Office plug-ins were VSTO, or COM, add-ins (VSTO Add-ins). The standard mode of installing a VSTO Add-in is through the use of an MSI, and a VSTO Add-in runs only in Office on Windows. VSTO Add-ins are built using Microsoft’s proprietary .NET Framework. Significantly, Microsoft has announced that they will not be bringing VSTO support to .NET Core or the latest versions of .NET. [3]

The launch of Office 365 in 2011 heralded the introduction of a new kind of plug-ins, somewhat confusingly named “Office Add-ins”. Office Add-ins can be built using standard web technologies such as HTML, CSS, Sass, LESS, and JavaScript that enable one to create richer, more modern, and more streamlined interfaces. Office Add-ins can run across multiple platforms, including Windows, Mac, Outlook mobile, and in a browser.

For illustration, below are screenshots of SendCheck that we developed initially as a VSTO Add-in, and then as an Office Add-in. Although the core functions are materially identical across the two versions, we prefer the latter’s modern interfaces which integrate seamlessly with Outlook in a side pane (as opposed to a pop-up window) and provide a more pleasing user experience.

Whilst VSTO Add-ins, installed locally, tend to run upon starting up the relevant Office application, the code of an Office Add-in is generally only loaded when a user uses it.

Further, if the Office Add-in crashes, this generally will not crash the Office application itself.

In this connection, Microsoft itself noted that “Office Add-ins have a small footprint compared to VSTO Add-ins and solutions”. [6]

Office Add-ins are more secure

Further, Office Add-ins, being based on modern web technologies, have additional advantages from a development and security perspective:

  • One could set up automated testing and build pipelines for Office Add-ins on more operating systems than for VSTO Add-ins. Most CI/CD frameworks use Linux by default and therefore it can be easier to set up CI/CD for Office Add-ins out of the box. In turn, the development team could find it easier to run security tests on a timely basis.

  • Office Add-ins can be built on modern web technologies that are regularly updated, patched, and maintained. By contrast, as noted above, Microsoft has announced that they will not be bringing VSTO support to .NET Core or the latest versions of .NET. Because one is not able to use the latest versions of .NET, one is more likely to use third-party libraries with potential dependency vulnerabilities that might not be patched. This could present security risks, especially with the passage of time.

Requirements for using Office Add-ins

The full requirements can be found in Microsoft’s documentation. It is not sufficient, however, to check only the Office versions supported by the relevant Office Add-in. There are, in fact, three intertwined aspects that should be considered together:

  • Office version (e.g., Office 365 on Windows, Office 2016 on Windows). [7]

  • Availability of centralized deployment. [8] This requires Microsoft 365, Exchange Online, or active Exchange Online mailboxes.

  • Authentication and licensing. Microsoft recommends authentication via Azure Active Directory. [9] If Office 365 users are not connected to the organization’s Azure Active Directory, however, this would require an alternative method of authentication.

Concluding remarks

Legal professionals spend the bulk of their time using Office applications, and they call upon Office plug-ins to augment the functions of those applications without navigating to a different environment.

The proliferation of plug-ins, however, has led to significant concerns about performance and crashes. Indeed, many law firm CIOs and IT personnel we spoke to reported that they were actively looking to cull and remove plug-ins for this very reason. As Yuriy Fishman, Director of Enterprise Applications at Cleary Gottlieb Steen & Hamilton LLP, commented,

“Our lawyers expect a smooth and seamless experience when using the Office suite. We are always on the lookout for options to facilitate that.”

Further, accustomed to slick consumer apps outside of work, legal professionals are increasingly demanding intuitive, attractive interfaces that are easy to write with modern web technologies.

For the reasons set out above, modern Office Add-ins could avoid many of these issues and enable law firms and legal departments to make use of plug-ins without sacrificing performance, administrative ease, or user experience.

As Kristian Weberyd, Digital and Innovation Manager at Advokatfirman Vinge, commented, “Office plug-ins are a vital part of our attorneys’ toolkit. The enhancements made possible by Microsoft’s Office Add-in framework are a welcome development and could significantly improve the user experience.”



[1] E.g., “ABA Survey: Legal Software Use In Law Firms In 2021”, available at and “Microsoft 365’s Growing Dominance in Law Firms & Legal Departments”, available at

[2] Learn more through this link:

[3] “Create VSTO Add-ins for Office by using

[4] “Office Add-ins platform overview”, available at

[5] “Determine if Centralized Deployment of add-ins works for your organization”, available at

[6] “Create VSTO Add-ins for Office by using Visual Studio”, available at

[7] “Office client application and platform availability for Office Add-ins”, available at

[8] “Determine if Centralized Deployment of add-ins works for your organization”, available at

[9] “Authentication options in Outlook add-ins”,


About the Authors

John Kwan (TL) is the co-founder of 10BE5. He was an associate in the London and Brussels offices of Cleary Gottlieb Steen & Hamilton LLP between 2013 and 2019. He was born and raised in Hong Kong and holds a law degree from the University of Cambridge.

Dr. Benjamin Woods (TR) is 10BE5’s Head of Engineering. He holds a doctorate in Plasma Physics from the University of York.

Jennet Hydyrova (BL) is a Backend Engineer at 10BE5 based in Istanbul, Turkey.

Tural Hajiyev (BR) is a Frontend Engineer at 10BE5 based in Wroclaw, Poland.

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