CLI/C++ link error 2028 and 2019 - templates

Question

I've got a problem with passing templates (I think that's the cause) from the CLI project.
I've got 3 layers of classes (for encapsulation reasons and technical reasons).
I'll show an example of the buggy part (please no comments about the encapsulation reasons, they're not demonstrated here)
Class A is in a C++ DLL :
class A {
public:
template<class T>
void foo(T blah) { //Do stuff }
}
Class B wraps class A (also regular non-ref class):
class B {
public:
template<class T>
void foo(T blah) { a->foo(blah); }
private:
A* a;
}
Class C is a ref class, which calls class B with an explicit type :
ref class C {
public:
void foo(int blah) { b->foo(blah); }
private:
B* b;
}
They compile alright (.obj is created), but the linker doesn't link the objects correctly.
I get 2 linker errors for the method:
error LNK2028: unresolved token
(0A000645) "public: void __cdecl
B::foo(class utils::CustomString const
&,int const &)"
(??$foo#_N#B#Namespace##$$FQEAAXAEBVCustomString#utils##AEB_N#Z)
referenced in function "private: void
__clrcall C::foo(int)" (??$foo#_N#Namespace##$$FAE$AAMXPE$AAVString#System##_N#Z)
error LNK2019: unresolved external
symbol error LNK2019: unresolved
external symbol "public: void __cdecl
B::foo(class int const &)"
??$foo#_N#B#Namespace##$$FQEAAXAEBVCustomString#utils##AEB_N#Z)
referenced in function "private: void
__clrcall C::foo(int)" (??$foo#_N#Namespace##$$FAE$AAMXPE$AAVString#System##_N#Z)
Edit
I don't have the lines with me now (not on same PC) but it says it couldn't link B.foo referenced in C.foo
I'm compiling the ref class with /clr in debug mode /MDd (yes it has to be in debug mode because of other dependencies that are all compiled the same way)
Anyone knows why this is happening ? And more important: how to solve this ?
Edit:
When setting class B (the wrapper) to be compiled with /GL (Whole Program Optimization) I get a different error:
LNK2001:
error LNK2001: unresolved
external symbol "public: bool __cdecl
Interface::B::foo(int &)const "
(??$foo#_J#B#Namespace##$$FQEBA_NAEBVCustomString#123#AEA_J#Z)

Answer 1

The problem is a bug with visual studio's linker.
First of all, you need to set the /GL (Whole Program Optimization) flag so it links the template after compile.
Then you need to use one of these work-arounds:
BUG: LNK2001 on Member Function When Use Nested Class Template

Answer 2

This is standard lossage with C++ templates, they don't have external linkage like .NET generics. You have to put the template function definition in a header file so you can #include it in the source code file that contains the ref class. Just like your code snippet does. It isn't otherwise specific to C++/CLI.
Be sure to wrap the #include with #pragma managed if you compile the rest of the native classes without the /clr option (like you should). Like this:
#pragma managed(push, off)
#include "FileWithTemplateDefinition.h"
#pragma managed(pop)

Answer 3

These linker errors are pointing out that you are missing a definition somewhere (as opposed to a declaration). Not having access to your code makes this difficult for us to determine, especially without having any verbose linker errors.
[EDIT]
This code compiles and links for me just fine in a managed C++ project:
#pragma managed
class A {
public:
template<class T>
void foo(T blah) { int i = 0; }
};
//Class B wraps class A (also regular non-ref class):
class B {
public:
template<class T>
void foo(T blah) { a.foo(blah); }
private:
A a;
};
// Class C is a ref class, which calls class B with an explicit type :
ref class C {
public:
C() { b = new B(); }
~C() { }
!C() { delete b; }
void foo(int blah) { b->foo(blah); }
private:
B* b;
};