LNK2019 unresolved external symbol in unmanaged C++ code

Question

I would like to use external C++ libraries in a .Net standard project. I have read something about p/invoke and unmanaged code so I started from something really simple. I created a simple C++ project, the following are the header file and related implementation:

Sum.h

#ifndef SUM_H_INCLUDED
#define SUM_H_INCLUDED

#ifdef _WIN32
# define EXPORTIT __declspec( dllexport )
#else
# define EXPORTIT
#endif

namespace Something
{
  namespace Really
  {
    namespace Simple
    {

      class SimpleClass
      {

      public:
        SimpleClass();
        double SumValues( double x, double y);
      };

  #pragma region Object handlers

      // create instance
      extern "C"
      {
        EXPORTIT SimpleClass* CreateInstance();
      }

      //Invoke Sum Method
      extern "C"
      {
        EXPORTIT double InvokeSumValues( SimpleClass*instance, double x, double y);
      }

      // Delete instance
      extern "C"
      {
        EXPORTIT void InstanceDelete(SimpleClass*instance);
      }
  #pragma endregion

    }
  }
}
#endif

Sum.cpp

#include "Sum.h"

using namespace Something::Really::Simple;

SimpleClass::SimpleClass()
{
  // empty constructor
}

double SimpleClass::SumValues(double x, double y) {
  return x + y;
}

#pragma region Class handlers

SimpleClass* CreateInstance()
{
  return new SimpleClass;
}

double InvokeSumValues(SimpleClass*inst, double x, double y )
{
  return inst->SumValues( x, y);
}

void InstanceDelete(SimpleClass*inst)
{
  delete inst;
}

#pragma endregion

If I try to build the project I get some

LNK2019 unresolved external symbol

The only way I found to solve that, is to move the methods implementation from the .cpp to the .h file, but I would like to keep the method definitions and the related implementation separated.

Basically my question is, how I get it to work?

Just for the sake of completeness, this is how I would invoke the C++ from the C# implementation:

[DllImport( @"Simple.dll", EntryPoint = "CreateInstance", CallingConvention = CallingConvention.Cdecl )]

private static extern IntPtr CreateInstance();

[DllImport( @"Simple.dll", EntryPoint = "InvokeSumValues", CallingConvention = CallingConvention.Cdecl )]

private static extern IntPtr InvokeSumValues(IntPtr inst, double x, double y);

private static void SumValues(double x, double y )
    {
        IntPtr inst = CreateInstance();
        InvokeSumValues( inst, x, y );
    }