Interfaces (In the assembly named \"Interfaces\". In project :- Interfaces)
namespace Interfaces
{
public interface IDoSomeWork1
{
Please make sure, that you have added assembly references of the missing assembly in the project where your web.config file exists.
I was missing this. I already have added those assembly references in the project which was using Unity to resolve the class, but missed to add it in the parent project where configuration file was located. This has resolved my problem.