Exported Fortran DLL fails when including C++.lib file in build

I recently added a call to a C++ function embedded in a library (.lib) file to a fortran project that is then compiled into a DLL for use by C#.  This had the unfortunate effect of breaking my solution (.sln), which was previously comprised of only C# and Fortran languages.  My question is this: do conflicts arise if a fortran dll has to both 'export' functions for C# while simultaneously importing C++ functions contained in a .lib file?  To help illustrate this question, I've laid out below the working code followed by the amended code that broke.  I'll reiterate that when the .sln was working, C# was calling functions in a Fortran DLL that did not also include a C++ .lib compiled into it.  For brevity, I've only shown one call from C# to fortran ("MFNWT_INIT"):

C# code:

using System;
public static class CustomMODSIM
{
    public static Model myModel = new Model();

    //Fortran DLL interface
    [DllImport("MF_DLL.dll", CallingConvention = CallingConvention.StdCall)]
    public static extern void MFNWT_INIT();

    //...other imported functions cut out for brevity...

    private static void OnInitialize()
    {
        MFNWT_INIT();
        //Initialize other C# variables
        ...
    }
...
} 

Working Fortran DLL code:

MODULE MF_DLL
C
    CONTAINS
C
    SUBROUTINE MFNWT_INIT() BIND(C,NAME="MFNWT_INIT")
C 
    !DEC$ ATTRIBUTES DLLEXPORT :: MFNWT_INIT
C
    USE GLOBAL
    USE GWFBASMODULE
    INCLUDE 'openspec.inc'
C
    ...

Now, here are the steps I performed that broke it. As stated, I added a C++.lib file to the fortran project and compiled without any errors.  The following fortran module serves as the interface between the C++.lib file and the fortran code, it was added to the Fortran project:

Module ApplicationsInterface

    implicit none

    interface
        ! =============================================================!!
        ! Two tasks are performed in this subroutine:
        ! 1) Create DataTable for holding "Ideal Applications"        ! 2) Fill this DataTable with values stored in a CSV file generated by a VB code.
        ! =============================================================!! 
        subroutine IdlAppInit() bind(C, name="idappinit")
            import 
        end subroutine
    end interface

End Module ApplicationsInterface

And finally, I amended the first bit of fortran I showed above with lines 12 & 17 below, and it broke:

MODULE MF_DLL
C
    CONTAINS
C
    SUBROUTINE MFNWT_INIT() BIND(C,NAME="MFNWT_INIT")
C 
    !DEC$ ATTRIBUTES DLLEXPORT :: MFNWT_INIT
C
    USE GLOBAL
    USE GWFBASMODULE
C
    USE ApplicationsInterface  
C
    INCLUDE 'openspec.inc'
C
    ...
    CALL IdlAppInit()
    ...

The funny thing was, if I add only the USE ApplicationsInterface line, no problems, I can still step through the code in debug mode.  It is only when I add line 17 ("CALL IdlAppInit()") and try to "step into" it (debug mode) from C# that I get the error message:

"An unhandled exception of type 'System.DllNotFoundException' occurred in MODSIMStreamAquifer.exe" 

followed by

"Additional information: Unable to load DLL 'MF_DLL.dll': The specified module could not be found. (Exception from HRESULT: 0x8007007E)"

Why doesthe MF_DLL.dll suddenly not load?  Literally, it goes from working to not working if line 17 in the previous bit of fortran is commented/uncommented.  I also added a screen shot of the error message to this post.