Visual Studio For Office Runtime Jun 2026

At its core, the VSTO Runtime is a set of in-process COM (Component Object Model) shims and loaders. When a developer writes an Excel Add-in using C# or VB.NET, that code runs inside the Common Language Runtime (CLR). However, Microsoft Office applications (Excel, Word, Outlook) are native C++ applications that understand COM, not .NET. The VSTO Runtime acts as a simultaneous translator and security guard.

Separate 32-bit and 64-bit versions exist to match your Office installation How to Download and Install visual studio for office runtime

When Office launches, it looks for registered add-ins. The VSTO Runtime intercepts this call, spins up the .NET runtime (if not already loaded), verifies the add-in's security certificates and manifests, and finally creates a managed AppDomain to host the developer's code. It ensures that a .NET exception in an add-in does not crash the entire Word document and that the add-in has the appropriate permissions to access the file system or database. At its core, the VSTO Runtime is a

Implementing the IDTExtensibility2 interface, which allows Office and your custom code to "talk" to each other. Why You Need It The VSTO Runtime acts as a simultaneous translator