VSTO add-ins vs. Modern Outlook Add-ins

What is the main difference between a VSTO Add-in and a modern Office Add-in for Outlook?

What are VSTO add-ins?

Visual Studio Tools for Office (VSTO) was originally a set of development tools made available by Microsoft, allowing developers to extend Microsoft functionally or provide custom applications inside a Microsoft Office application.

Using VSTO, developers could provide a Visual Studio add-in, plugin, or small application, which allows Microsoft Office 2003 and later versions of Office applications to host a .NET Framework Common Language Runtime (CLR) to expose specific functionality via .NET. inside a Microsoft Office application like Word, Excel, or Outlook, on a Windows laptop. 

In common language, what it means is that VSTO add-ins were a way for developers to provide and extend custom functionality inside a Microsoft Application, delivered as a "plugin" or "add-in", installed on each individual PC (running Windows), or deployed using SCCM (Microsoft System Center Configuration Manager).

VSTO add-ins are therefore earlier Microsoft Office integration solutions that run only in Microsoft Office Application on Windows, by native code that runs on the user's PC.

VSTO add-ins therefore directly impacts performance of the Office Application, since code runs on the user's device inside the Microsoft application.

What are modern Outlook Add-ins?

Modern Add-ins for Microsoft Application like Outlook (Or Word, Excel, and others) are modern Microsoft integrations built by third parties, like AskCody, into Outlook using modern web-based technology. In today's modern world, modern Microsoft Add-ins therefore run across all platforms and not just Windows.

A modern Outlook Add-in is very much like a modern App known from Apple's App Store or Google Play. With modern Outlook Add-ins, the add-in, or App, is available everywhere Outlook is available. Modern Add-ins are therefore available in Outlook on Windows, on Mac, on iPad, on iPhone or Android, or online in Microsoft 365 - truly across Microsoft Outlook on all platforms.

This provides users with a consistent experience across all platforms, and allows developers to provide extended, custom functionality to all users, no matter where, or how, they want to use a Microsoft Application like Outlook. Modern Add-ins are therefore no longer limited or restricted to Windows, which provides users with a much better experience.

Unlike VSTO add-ins, Office Add-ins don't involve code that runs on the user's device or in the Office client, thereby not impacting any user experience or performance, but requires internet access, since these add-ins are very much like Apps on a smartphone.

How are modern Add-ins different from VSTO?

Modern Outlook add-ins are different from VSTO add-ins, which are the older integrations specific to Outlook running on Windows. Outlook add-ins don't have any code physically installed on the user's device or Outlook client. For an Outlook add-in, Outlook reads the manifest and hooks up the specified controls in the UI, and then loads JavaScript and HTML. The web components all run in the context of a browser in a sandbox.

Modern Outlook Add-ins can do anything a webpage can do inside a browser, but now inside Outlook. Developers, like AskCody, therefore use the Office Add-ins platform to add new functionality to Office clients like Outlook, and bring external data into Outlook, automate tasks or processes, or expose third-party functionality in Office clients, and more. 

Outlook add-ins have three key aspects:

  • The same add-in and business logic works across desktop (Outlook on Windows and Mac), web (Microsoft 365 and Outlook.com), and mobile.
  • Outlook add-ins consist of a manifest, which describes how the add-in integrates into Outlook (for example, a button or a task pane), and JavaScript/HTML code, which makes up the UI and business logic of the add-in.
  • Outlook add-ins can be acquired from AppSource or sideloaded by end-users or administrators.
  • They can be deployed using Centralized Deployment inside Microsoft 365.

Modern Office Add-ins vs. VSTO Add-ins?

Note: This table is based on an Outlook Add-in but applies to all Microsoft Applications

  Modern Office Add-in VSTO Add-in
Availability/Platform support

Available every Outlook is. Add-ins are available in Outlook on Windows, Mac, iPad, iPhone and Microsoft Online (Outlook Online / OWA).

This provides a consistent experience and allows users to use whatever version of Outlook they want to do their job.

Only runs on Outlook for Windows.
Impact on Microsoft Application (Outlook)

Doesn't impact Outlooks performance since no code runs natively on the PC.

Add-ins have no risk of being degraded or disabled since modern Add-ins doesn't impact Outlooks performance or experience.

Impacts Outlooks performance.

The VSTO Add-in risks being disabled if load time or performance impacts the overall experience of Outlook.

Runing multiple VSTO add-ins in Outlook might cause Outlook to crash. 

Deployment Deployed through Centralized Deployment in Microsoft 365, controlled by Active Directory Installed individually by each PC or through SCCM with installation packages and launch applications. 
Version Control Updated automatically when new versions are being made available. Requires manual updates through SCCM.

Advantages of using Modern Outlook Add-ins

Modern Outlook Add-ins provide the following advantages over add-ins built using VBA, COM, or VSTO.

  • Cross-platform support. Office Add-ins run in Office on the web, Windows, Mac, and iPad.

  • Centralized deployment and distribution. Admins can deploy Office Add-ins centrally across an organization.

  • Easy access via Microsoft AppSource. You can make your solution available to a broad audience by submitting it to AppSource.

  • Based on standard web technology. You can use any library you like to build Office Add-ins.

Why is this important?

Implementing third-party tools and integrations like AskCody into your Microsoft Applications such as Outlook impacts performance, experience, and productivity for your users and organization. As an IT Pro deciding which applications to implement and provide to your team and organization, you must ensure you provide the most user friendly and powerful solution, that gives your users the tools and features they need across all platforms, not just inside Outlook on Windows. Also, you cannot risk that Outlook (or other Microsoft Applications) crash due to poor performance or long load times, since this dramatically degrades productivity.