Error: Reference to type claims it is defined, but it could not be found

This error seems to cover a variety of scenarios. In my case, closing and re-opening Visual Studio was the trick. After restarting Visual Studio, I was able to build the project as expected.

Another way this could happen is if you're using several NuGet packages where one, probably central, package has been updated but some additional functionality packages haven't been.

To work through my recent example - the error was "Reference to type 'ConsumerSubscriptionConfigurator<>' claims it is defined in 'MassTransit', but it could not be found". This is because we had updated MassTransit from 2 to 3, but we had not updated some of the other optional packages (MassTransit.log4net and MassTransit.Autofac) to the same version number. It appears as if assembly redirection had kept everything working until we tried to use one more additional feature.

I had a similar problem. The site was running a cached version of the dll and not the one I had deployed to the bin directory. I cleared the temporary asp.net folder contents and this solved the issue.