Is defined in an assembly that is not referenced. You must add a reference to assembly

. . . is defined in an assembly that is not referenced. You must add a reference to assembly

There could be a number of reason you are receiving the error above in C#, but perhaps the first thing you should check is the are references within your projects.

For example if you created classOne and classTwo and classTwo inherits classOne, you need to be sure to not only qualify the use of classOne in classTwo with 'using namespace classOne', but you also have to add a reference to classOne's dll in classTwo if you built classOne's dll.

In addition to inheriting classes, like with classTwo above, you can also inherit interfaces.  When doing so, be sure to reference the interface with the using directive and adding a reference to the interfaces dll if it exists.  That is, if you built the interface (Command Build Class/Interface).

Also not only do you have to type the using directive and add references where necessary in classTwo, but you will also have to do it in classOne or any class that will make use of classTwo.  Otherwise you will get the error . . . is defined in an assembly that is not referenced. You must add a reference to assembly.