C# multiple class library with common base class/namespace problem

By default, use namespaced functions.

Classes are to build objects, not to replace namespaces.

In Object Oriented code

Scott Meyers wrote a whole Item for his Effective C++ book on this topic, "Prefer non-member non-friend functions to member functions". I found an online reference to this principle in an article from Herb Sutter: http://www.gotw.ca/gotw/084.htm

The important thing to know is that: In C++, functions that are in the same namespace as a class is, and that have that class as a parameter, belong to that class' interface (because ADL will search those functions when resolving function calls).

For example:

• let's say you have a namespace N
• let's say you have a class C, declared in namespace N (in other words, its full name is N::C)
• let's say you have a function F, declared in namespace N (in other words, its full name is N::F)
• let's say that function F has, among its parameters, a parameter of type C

... Then N::F is part of N::C's public interface.

Namespaced functions, unless declared "friend," have no access to the class' internals, whereas static methods have.

This means, for example, that when maintaining your class, if you need to change your class' internals, you will need to search for side effects in all its methods, including the static ones.

Extension I

Adding code to a class' interface.

In C#, you can add methods to a class even if you have no access to it. But in C++, this is impossible.

But, still in C++, you can still add a namespaced function, even to a class someone wrote for you.

See from the other side, this is important when designing your code, because by putting your functions in a namespace, you will authorize your users to increase/complete the class' interface.

Extension II

A side-effect of the previous point, it is impossible to declare static methods in multiple headers. Every method must be declared in the same class.

For namespaces, functions from the same namespace can be declared in multiple headers (the almost-standard swap function is the best example of that).

Extension III

The basic coolness of a namespace is that in some code, you can avoid mentioning it, if you use the keyword using:

#include <string>
#include <vector>

// Etc.
{
   using namespace std ;
   // Now, everything from std is accessible without qualification
   string s ; // Ok
   vector v ; // Ok
}

string ss ; // COMPILATION ERROR
vector vv ; // COMPILATION ERROR

And you can even limit the "pollution" to one class:

#include <string>
#include <vector>

{
   using std::string ;
   string s ; // Ok
   vector v ; // COMPILATION ERROR
}

string ss ; // COMPILATION ERROR
vector vv ; // COMPILATION ERROR

This "pattern" is mandatory for the proper use of the almost-standard swap idiom.

And this is impossible to do with static methods in classes.

So, C++ namespaces have their own semantics.

But it goes further, as you can combine namespaces in a way similar to inheritance.

For example, if you have a namespace A with a function AAA, a namespace B with a function BBB, you can declare a namespace C, and bring AAA and BBB in this namespace with the keyword using.

You can even bring the full content of a namespace inside another, with using namespace, as shown with namespace D!

namespace A
{
   void AAA();
   void AAA2();
}

namespace B
{
   void BBB();
}

namespace C
{
   using A::AAA;
   using B::BBB;
}

namespace D
{
   using namespace A;
   using namespace B;
}

void foo()
{
   C::AAA();
   // C::AAA2(); // ERROR, won't compile
   C::BBB();
}

void bar()
{
   D::AAA();
   D::AAA2();
   D::BBB();
}

Conclusion

Namespaces are for namespaces. Classes are for classes.

C++ was designed so each concept is different, and is used differently, in different cases, as a solution to different problems.

Don't use classes when you need namespaces.

And in your case, you need namespaces.

The ref keyword causes Pass by Reference semantics - that is, if the variable is re-assigned in the called function, it will re-assign the variable in the caller as well.

Obviously, this only works if a variable² (which can be re-assigne to) is directly passed as the argument and will not work if an arbitrary expression is passed. In this case, this is not a variable, rather a special expression which cannot be re-assigned, and so cannot be used.

As such, this would work: (But please see other answers and keep reading as to why this is likely not required and/or just silly.)

var me = this;
var slave = new ClassB(ref me);

However, Pass by reference should not be confused with Pass by Object [Sharing]¹ semantics. Pass by Object means that if an object is passed, that object is passed: the object is not copied/cloned/duplicated. If the object is mutated then the object is mutated. All Reference Types have Pass by Object semantics in C# unless either ref or out are used. (The class keyword declares a new reference type, as in the case in the post).

On the other hand, Value Types (e.g. struct), including int and Guid and KeyValuePair<K,V>, have Pass by Value semantics - in this case a copy is made and thus, if the value is modified, only the value struct (which is a copy) changes.

Happy coding

¹ Underneath C#/.NET achieves Pass by Object by passing a reference to an object by Value. However, the rules above correctly describe the observable semantics and effects.

² Unlike C#, which only allows variables to be used with ref, VB.NET allows Properties to be used. The VB.NET compiler automatically creates a temporary variable and implicit reading/writing instructions of the property during compilation.