Installing VSTO 4.0 causes VSTO 3.0 Addin to stop working

Question

I just installed Visual Studio 2010 yesterday. As part of this, I installed VSTO 4.0. Now, when I launch any Office application, my VSTO 3.0 add-ons do not load. Error in the event log

User URI: file: /// H: /PathToMyAddin/MyAddin.vsto Exception: in the configuration there are no permissions necessary to create the application domain.

Microsoft.VisualStudio.Tools.Applications.Runtime.CannotCreateCustomizationDomainException: In the configuration, there is no permission required to create the application domain. ---> System.Security.SecurityException: Individual functions in this application will not work, because the administrator listed File: /// H: /PathToMyAddin/MyAddin.vsto as untrustworthy. Contact your administrator for further assistance. at Microsoft.VisualStudio.Tools.Office.Runtime.RuntimeUtilities.VerifySolutionUri (Uri uri) at Microsoft.VisualStudio.Tools.Office.Runtime.DomainCreator.CreateCustomizationDomainInternal (String solutionLocation, String manifestName, String documentNamerpurmenterpumentleurpumentleurpumentleurpumentleurpumentleurpluentrlementpluentrturpu Contractor) The build zone that failed was: MyComputer

This seems to be due to the fact that trying to download another version of .NET is the same process / AppDomain. However, an error indicates that this is a permission issue.

Answer 1

It turns out that VSTO 4 does not by default trust add-ins in network locations. However, adding the following registry key

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\vsto runtime Setup\v4] "EnableVSTOLocalUNC"=dword:00000001 

and turning on my add-in again, it seems to work again as expected.

Another option for some is to simply install the add-in on the user's local computer, rather than in a network location