If you try to install the x86 VSTO runtime on a machine with 64-bit Office, your add-ins will fail to load, and you may see "is not a valid Office Add-in" errors.

A frequent cause of mysterious add-in failures is a hybrid environment:

The 2010 runtime relies heavily on deployment technology and Code Access Security (CAS). CAS has been deprecated in newer .NET Frameworks (4.0+), though it is still supported for legacy scenarios. Organizations relying strictly on the 2010 runtime may face security audit challenges as they migrate to .NET Core or .NET 5/6/7+, which do not support the legacy VSTO architecture.

The is a critical engine that allows Microsoft Office applications (like Excel, Word, and Outlook) to run custom add-ins and solutions built with .NET. While "2010" is in the name, this runtime is essential for running Office-based solutions created in Visual Studio 2010, 2012, 2013, and 2015. What is the x64 Version?

: The runtime is often installed automatically when you install Office. For instance, installing 64-bit Office on 64-bit Windows typically installs the x64 runtime.