Public class is inaccessible due to its protection level

This error is a result of the protection level of ClassB‘s constructor, not ClassB itself. Since the name of the constructor is the same as the name of the class^* , the error may be interpreted incorrectly. Since you did not specify the protection level of your constructor, it is assumed to be internal by default. Declaring the constructor public will fix this problem:

public ClassB() { } 

^* One could also say that constructors have no name, only a type; this does not change the essence of the problem.