Using Outlook Add-In with .NET 10 (C# Project)

Question

I am trying to use the MS examples for Outlook Add-In but the examples don't seem to work with ,NET 10 (just get timeout and exception loading the assembly), I tried using both the COM assembly reference directly, and also the Office interop NuGet package.

Answer 1

Dear @Paul Coulson,

Welcome to Microsoft Q&A Forum!

Thanks for sharing the details and based on my research, the behavior you’re seeing: "timeouts and exception loading the assembly” when using the Microsoft samples, may be the result of a platform mismatch. Here are some insights and the recommended paths forward:

1.VSTO/COM add‑ins are .NET Framework–only (not .NET 10). Outlook VSTO/COM add‑ins load in‑process with Office and rely on the .NET Framework 4.x runtime. Microsoft’s guidance is clear: Office add‑ins built with VSTO/COM are not supported on .NET Core/5+/9/10 and attempting to run them under modern .NET leads to load failures. Reference: Walkthrough: Office Programming in C#

2.The “new Outlook for Windows” doesn’t load COM/VSTO add‑ins. The new Outlook client (often called “Monarch”) only supports Office Web Add‑ins (Office.js). COM/VSTO add‑ins will not load there, which produces the same failure symptoms you’re seeing. References:

• Office Add-ins platform overview
• Develop Outlook add-ins for the new Outlook on Windows

3.Suggestion A (recommended if you’re on the new Outlook): migrate to an Office Web Add‑in. Rebuild the add‑in using Office.js (or Blazor WebAssembly for C# in a web add‑in). This works across Windows/Mac/Web/Mobile and is the only option in the new Outlook. You can consult in these links:

• Migrate from COM add-ins to web add-ins
• Office Add-in code samples

4.Suggestion B (if you need deep, in‑process Outlook automation and you’re on classic Outlook): stay on .NET Framework + VSTO. Target .NET Framework 4.8 and use the VSTO Outlook Add‑in template in Visual Studio. This runs in classic Outlook for Windows. Reference: Walkthrough: Create your first VSTO Add-in for Outlook

Also, I found this method If you converted to SDK‑style projects, ensure VSTO MSBuild targets are imported so the .vsto and manifests are generated. Example and rationale: “Migrating to SDK‑Style” notes

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I hope this information can help you to clarify it. Also, loading a VSTO/COM add‑in under .NET 10 or in the new Outlook causes timeouts and assembly load exceptions because the required VSTO runtime and COM add‑in host pipeline may be not available in that environment.

As a Microsoft Q&A moderator, my role is to guide discussions and connect users with helpful resources. While I don’t have the right to access to individual account or your environment in order to help you further. However, I still try my best to support you using the resources available. So, please kindly review my answers carefully and try them. 

Wish you a pleasant day! 

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