If this is your first time unit testing, then VSTO can be a steep learning curve. As suggested by @Sam Holder, you can put part of your logic in a separate assembly to encourage you to separate your logic from interaction with the office infrastructure.
However, while you cannot add a project using the usual approach of adding a link to a project, you can add a link to the output from this project using the option Browse:
- Right-click on the test project and select Add Link.
- In the window that appears, click the "Browse" button below.
- Go to the bin \ debug folder of the VSTO project and select the dll of the projects.
- Click OK.
Obviously, you had to create an add-on for the above.
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