Why am I getting 'Assembly '*.dll' must be strong signed in order to be marked as a prerequisite.'?
When I had this problem I fixed it by turning off the 'Enable ClickOnce security settings'.
Menu: Project | 'Project name' Properties... | Security tab | 'Enable ClickOnce security settings' check box.
My guess is that you're not working with strongly named assemblies. I've had this error when two projects reference slightly different versions of the same assembly and a more dependent project references these projects. The resolution in my case was to remove the key and version information from the assembly name in the .csproj files (it didn't matter anyway), and then do a clean build.
Changes between the different assembly versions were compatible with the parts of the solution referring to them. If this is not the case with you, you might have to do some more work to resolve the issue.
NuGet
With NuGet it's easy to get into this situation if:
- You install a package to one project in your solution.
- A new version of that package is deployed to the package source.
- You install it to another project in the same solution.
This results in two projects in your solution referencing different versions of that package's assemblies. If one of them references the other and is a ClickOnce app, you'll see this problem.
To fix this, issue the update-package [package name] command at the Nuget Package Manager Console to bring everything up to a level playing field, at which point the problem goes away.
You should manage NuGet packages at the solution level rather than at the project level unless there is a compelling reason not to. Solution level package management avoids the potential of multiple versions of dependencies. When using the management UI, if the Consolidated tab shows 1 or more packages have multiple versions, consider consolidating them to one.
See this answer.
Go to the publish page and click on "Application Files". From there you should see a list of your DLL's. Ensure that the ones that are giving you trouble have their Publish Status marked as "Include" rather than "Prerequisite".