VSTO Add-in Development

Our VSTO add-in development team builds and maintains the .NET Framework add-ins that load directly into Windows desktop Office, and we move them to the modern web model before that model runs out of road. VSTO and COM add-ins are compiled components that plug into the desktop Office process on Windows only, which is why they never appear in the new Outlook, on Mac, or in Office on the web. Microsoft has said it will not carry VSTO or COM past .NET Framework, so there is no .NET 5 or later path. Classic Outlook stays supported until at least 2029, which is the window most teams use to plan a calm move rather than a scramble. We keep your current Excel add-in and Outlook add-in work stable today while we design the web replacement.

A logistics firm came to us with a decade-old COM add-in that priced freight quotes inside Excel and Outlook. It still worked, but their IT team had just been told the new Outlook for Windows would become the default, and the add-in simply would not load there. We spent the first phase cataloguing every feature and ranking it by how often staff actually used it. The heavy calculation logic moved into a shared service, and the task pane was rebuilt in Office.js so it now opens on classic Outlook, new Outlook, and the web. The pricing team kept the exact workflow they knew, and the move shipped in stages so no quarter-end was ever put at risk.

This work fits teams running feature-rich VSTO or COM add-ins that must keep working on classic Office now and reach a supported model before the new Outlook default arrives. Microsoft now switches Microsoft 365 enterprise users to the new Outlook by default in March 2027, a year later than first planned, so there is time to plan but not to ignore it. If a feature has no direct web equivalent yet, we say so and design a supported workaround rather than promise parity the platform cannot deliver. And if your add-in is small and only ever runs on classic desktop Office, we will tell you plainly when staying put a little longer is the cheaper, sensible call. As Office.js experts, we would rather scope the honest path than sell you a rebuild you do not need.