Unable to install or run the application. The application requires that assembly Version 8.0.0.0 be installed in the GAC Assembly Cache first. Please contact your system administrator.

I ran into an interesting scenario today.

"Unable to install or run the application. The application requires that assembly <Assembly Name> Version 8.0.0.0 be installed in the GAC Assembly Cache first. Please contact your system administrator."

I went and checked in the application files that it has been included as Prerequisite auto and changed that to include started getting message on screen saying that "Strong name signature not valid for this assembly <Assembly Name>.dll"

Why is it being shown: The application was developed with references to an assembly that was registered in the GAC. At runtime the application will still expect the assembly to be available in the GAC and throw an error it it’s not found.

How to resolve: It’s important to go to the Publish tab before publishing a Clickonce application and examine the files under the Application Files section. <Attach Screenshot here)

Any external assembly that the project references will be by default included into the manifest as Pre-requisite-auto, this means that the assembly is presumed to exist in the GAC before the application is deployed. If you are seeing this error during deployment it means that a required assembly that was presumed to be installed as pre-requisite was not found on the target machine.

If you are sure that the assembly should exist on the Target machine, Pre-requisite Auto should be your option

If the assembly does not exist on the Target machine make sure you set it to exclude and install the required assemblies into the GAC using a bootstrapper setup.exe.