Multiple Classes in DLL file

what I've got here is pretty much a dumb and noobie question, but I just can`t find an answer on the web. When I want to create a DLL project (out of an existing c++ project), I read that I need to implement the following lines in the beginning of the .h files:

#ifdef _EXPORTING
#define CLASS_DECLSPEC    __declspec(dllexport)
#else
#define CLASS_DECLSPEC    __declspec(dllimport)
#endif

I looked at the example in the MSDN:

// MathFuncsDll.h
#ifdef MATHFUNCSDLL_EXPORTS
#define MATHFUNCSDLL_API __declspec(dllexport) 
#else
#define MATHFUNCSDLL_API __declspec(dllimport) 
#endif

Now I want to understand, do I need to change the "_EXPORTING" and the "CLASS_DECLSPEC" for every new class I make? For example if I'd create a class named "foo" within the same project as "MathFuncsDll.h" I'd need to put the following lines at the start of the .h file:

// FooDll.h
#ifdef FOO_EXPORTS
#define FOO_API __declspec(dllexport) 
#else
#define FOO_API __declspec(dllimport) 
#endif

Or is some line is the same in all .h files of the project?

And another thing, if I use a namespace to reference the whole dll as one and extract the classes from it, do I need to put the using namespace *NAME* in every .h file?

No, you need not to create new macros for each classes:

class MATHFUNCSDLL_API Foo {...};
class MATHFUNCSDLL_API Boo {...};
class MATHFUNCSDLL_API MyNewClass {...};

For the second q: don't use using namespace inside header file: "using namespace" in c++ headers

Your header can looks like the following:

#pragma once
namespace foo {
    class MATHFUNCSDLL_API Foo {...};
    class MATHFUNCSDLL_API Boo {...};
    class MATHFUNCSDLL_API MyNewClass {...};
}

EDITED

// mylibdef.h
#pragma once
#ifdef _EXPORTING
#define CLASS_DECLSPEC    __declspec(dllexport)
#else
#define CLASS_DECLSPEC    __declspec(dllimport)
#endif

// myclass1.h
#pragma once
#include "mylibdef.h"
namespace mylib {
class CLASS_DECLSPEC MyClass1 {...};   
}

// myclass2.h
#pragma once
#include "mylibdef.h"
namespace mylib {
class CLASS_DECLSPEC MyClass2 {...};   
}