VSTO is a set of development tools that allows programmers to create custom applications that run inside Microsoft Office. Unlike VBA (Visual Basic for Applications), which lives inside the document itself, VSTO allows you to write code in C# or VB.NET using the full power of the .NET Framework.
VSTO 4.0 remains widely used in enterprise environments, but for newer Office add-in development, Microsoft recommends transitioning to (cross-platform, web-based) unless deep OS-level or high-trust COM integration is required. vsto 4.0
Here’s a concise, informative text on suitable for a developer overview, documentation snippet, or training material: VSTO is a set of development tools that
The "Dinosaur" That Still Roars: Why VSTO 4.0 Still Matters in 2026 which lives inside the document itself