ClickOnce deployment error saying Actipro assemblies must be in the GAC

Root

If a message similar to the following appears when you attempt to run a ClickOnce-deployed application, you have not configured your project references correctly.

Unable to install or run the application. The application requires that assembly
ActiproSoftware.???.Wpf30 Version ?.?.????.0 be installed in the Global Assembly Cache (GAC) first.

You must ensure that all Actipro assemblies that are needed by your project have their references set so Copy Local is true. That will allow them to be deployed and copied to the client machine properly, making this error message disappear.

If you don't do this, the end user machine is unable to locate the Actipro assemblies and therefore throws this exception.