Error LNK2028: unresolved token (0A00000D) "extern "C" void __cdecl

Question

I have two projects inside one solution (using VS 2013). First project - mtriangle. Contains C code.
Second project - GeoGui. Contains Windows Forms project (used C++).

I try to use functions from mtriangle in GeoGui.

I have the following code:

Project mtriangle

triangle.h

#ifdef __cplusplus 
extern "C" {
#endif
    __declspec(dllexport) void trimain(char *, struct triangulateio *, struct triangulateio *, struct triangulateio *);
#ifdef __cplusplus 
}
#endif 
#ifdef __cplusplus 
extern "C" {
#endif
    void trifree(VOID *memptr);
#ifdef __cplusplus 
}
#endif 

triangle.c

void trimain(char *triswitches, struct triangulateio *in, 
struct triangulateio *out, struct triangulateio *vorout)
{
 // Implementation here, will be provided if need 
}

Project GeoGui

mainForm.h

#pragma once
extern "C" 
{
   #include "triangle.h"
}

... 

private: System::Void dlgFileOpen_FileOk(System::Object^  sender, System::ComponentModel::CancelEventArgs^  e) {


                 struct triangulateio *in;

                 struct triangulateio *out;

                 struct triangulateio *voronoi;


                 trimain("p", in,out,voronoi);
}

When I try to build solution, error message appears:

Error   13  error LNK2028: unresolved token (0A00000D) "extern "C" void __cdecl trimain(char *,struct triangulateio *,struct triangulateio *,struct triangulateio *)" (?trimain@@$$J0YAXPADPAUtriangulateio@@11@Z) referenced in function "private: void __clrcall GeoGUI::mainForm::dlgFileOpen_FileOk(class System::Object ^,class System::ComponentModel::CancelEventArgs ^)" (?dlgFileOpen_FileOk@mainForm@GeoGUI@@$$FA$AAMXP$AAVObject@System@@P$AAVCancelEventArgs@ComponentModel@4@@Z)   C:\Users\Administrator\Documents\Visual Studio 2013\Projects\mtriangle\GeoGUI\GeoGUI.obj    GeoGUI

Error   14  error LNK2019: unresolved external symbol "extern "C" void __cdecl trimain(char *,struct triangulateio *,struct triangulateio *,struct triangulateio *)" (?trimain@@$$J0YAXPADPAUtriangulateio@@11@Z) referenced in function "private: void __clrcall GeoGUI::mainForm::dlgFileOpen_FileOk(class System::Object ^,class System::ComponentModel::CancelEventArgs ^)" (?dlgFileOpen_FileOk@mainForm@GeoGUI@@$$FA$AAMXP$AAVObject@System@@P$AAVCancelEventArgs@ComponentModel@4@@Z)    C:\Users\Administrator\Documents\Visual Studio 2013\Projects\mtriangle\GeoGUI\GeoGUI.obj    GeoGUI

Could you, please, advise - what I do incorrectly?

Thank you!

I read this article What is an undefined reference/unresolved external symbol error and how do I fix it?

and, especially, this section: "Symbols were defined in a C program and used in C++ code."

it recommends do the following:

If an entire library is included in a header file (and was compiled as C code); the include will need to be as follows;

> extern "C" {
>     #include "cheader.h" }

But as I pointed above in sample, I've already done it:

> extern "C" {
>        #include "triangle.h"
>     }

This is a difference.