What VSTO Add-ins Are — and Why the Question Comes Up
VSTO (Visual Studio Tools for Office) add-ins are COM-based plugins written in C# or VB.NET that run inside Windows desktop Office. For years they were the default way to extend Excel, Word, and Outlook with serious logic, and many businesses still depend on them. The catch in 2026 is reach: VSTO add-ins run only on Windows desktop Office. They cannot run on Mac, Office on the web, or mobile, they install per machine, and Microsoft's investment has clearly shifted toward the web-based Office.js model. That does not make VSTO obsolete — but it does make 'should this be VSTO or Office.js?' a question worth answering deliberately.
Key Takeaways
COM-based .NET
Written in C#/VB.NET, with deep access to the desktop Office object model.
Windows desktop only
No Mac, no Office on the web, no mobile — a hard limit for hybrid and BYOD teams.
Per-machine deployment
Installed via ClickOnce or MSI on each device, which complicates rollout and updates.
Deep desktop power
Can reach Win32-level capabilities and advanced automation that Office.js does not expose.
Microsoft favors Office.js
New platform investment is going to web add-ins, so VSTO is effectively in maintenance mode.
VSTO vs Office.js: A Practical Comparison
VSTO still wins in a narrow set of cases: a Windows-only desktop user base, an existing VSTO codebase that works and rarely changes, offline desktop scenarios, or automation that genuinely needs Win32-level depth the web API cannot reach. For almost everything new, Office.js is the better default — one codebase across Windows, Mac, and web, central deployment without touching machines, and a future Microsoft is actively building on. The deployment difference alone is significant: pushing a VSTO update means redistributing installers, while an Office.js update is just a new web deploy your users pick up automatically. If you are starting fresh, or any of your users are on Mac or the web, that tilts the decision strongly toward Office.js.
Pro tip: let platform reach decide first
Before comparing features, ask one question: does every user sit on Windows desktop Office, today and for the foreseeable future? If the answer is anything but a firm yes, build Office.js. Reach is the constraint VSTO can never fix, and it is the one that hurts most later.
Keep VSTO where it already works and the requirements are stable. For anything new — or anywhere users are on Mac or the web — choose Office.js, and plan a migration for the legacy add-ins that still matter.
Conclusion
VSTO add-ins are not dead, but they are a specialist tool now rather than the default. Use them only where deep Windows-desktop access is non-negotiable, and move strategic add-ins to Office.js for reach and lower maintenance. Orfys advises on and executes both.