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How can I call a managed CLR/C++ routine from unmanaged C++?

I'm having a linking problem that looks just like this one: http://www.experts-exchange.com/Programming/Languages/.NET/Q_23332235.html that unfortunately didn't end up getting resolved.

I have code in an unmanaged C++ project that needs to call code in a C# project (i.e, managed code).  To that end I have created a 'bridge' project in CLR/C++ which will wrap the C# code and give the unmanaged C++ code something to call into.  There are several descriptions of how to do this on the web and I've been working through this one stackoverflow.com/questions/1827102/managed-c-to-form-a-bridge-between-c-sharp-and-c.

Bridge Project (C++/CLR)
// This is the main DLL file and is named MixedModeDLL.cpp
#pragma once
#include "stdafx.h"
#include "MixedModeDLL.h"
#include "CppInterface.h"

double CalculateProduct(double x, double y)
{
	ManagedClass^ t = gcnew ManagedClass();
	double z = t->multiplyTwoValues(x, y);

	return z;
}

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That's it.  The header file CppInterface.h is pretty simple.  It just exports the routine CalculateProduct which calls the C# code (via machinations in MixedModeDLL.h which I'm happy to share but seems a distraction to the problem at hand) like this:
#pragma once

extern "C" __declspec(dllexport) double CalculateProduct(double x, double y);

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It should be noted that I have successfully called the CalculateProduct(...) routine from a C# driver program which implies that CalculateProduct(...) can be accessed by outside code (i.e., it looks like it is being exported).  I did this using this code:
private void button4_Click(object sender, EventArgs e)
{
   // Test of calling into the managed C++ (CLR) MixedModeDLL DLL.
   double h = 2.5;
   double k = 6.0;
   double p = NR.CalculateProduct(h, k);
}

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where NR is a C# class in the driver code defined like this:
class NR
{
   [DllImport("MixedModeDLL.dll", EntryPoint = "CalculateProduct")]
   public static extern double CalculateProduct(double x, double y);
}

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C++ Project:
In the C++ code I have a line that says the following:

extern "C" __declspec(dllexport) void testCPPcallingCS()
{
   double z = CalculateProduct(4.9, 3.2);
}

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(Note: (the export here is so that my C++ driver program can get access to testCPPcallingCS())

testCPPcallingCS() is defined in a module that includes a #include statement for  header file that mirrors CppInterface.h described above; where that one says export, this one says import.  That is:

#pragma once

 extern "C" __declspec(dllimport) double CalculateProduct(double x, double y);

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The compiler has no problem with this, but the linker gets fussy.  It reports two errors:
error LNK 2019: unresolved external symbol __imp_CalculateProduct referenced in function testCPPcallingCS
error LNK1120: 1 unresolved externals

It almost looks like the name of the CalculateProduct routine has been decorated with a prefix of __imp_.   I have checked that all of the expected DLLs are in the output directory (...\Driver_CPP\x64\Debug) and that the *.lib of the 'Bridge' CLR/C++ code (i.e., MixedModeDLL.lib) is located in the same directory.  In the aforementioned link the author says to insure that the linker is told to 'use the generated .lib file when linking'.  I have checked the Properties of MixedModeDLL and the native C++ project and see nothing that implies to me that the .lib file is not being used.

I'm developing using Microsoft Visual Studio 2010 (my C# driver routine is written in MSVS 2012) on a Windows 7 computer.  MixedModeDLL and the unmanaged C++ DLL are targeted to an x64 platform.
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e_livesay
Asked:
e_livesay
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1 Solution
 
jkrCommented:
COM/Interop is usually the way to go if you need to call .NET code from unmanaged code. See http://www.codeproject.com/Articles/12673/Calling-Managed-NET-C-COM-Objects-from-Unmanaged-C ("Calling Managed .NET C# COM Objects from Unmanaged C++ Code")
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e_livesayAuthor Commented:
I've seen references to COM but was hoping to avoid all of the overhead associated with it.  Also, I'm now very interested to figure out what I'm doing wrong.  The method that I'm trying to implement has been reported to work in various locations ( http://stackoverflow.com/questions/1827102/managed-c-to-form-a-bridge-between-c-sharp-and-c as well as here http://www.codeproject.com/Tips/695387/Calling-Csharp-NET-methods-from-unmanaged-C-Cplusp).

It seems that the only missing component is the importation of the function definition into my unmanaged C++ DLL.
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jkrCommented:
>>but was hoping to avoid all of the overhead associated with it.

Um, sorry to say so, but at the moment the overhead is at your side ;o)

It (at least to me) seems a lot easier to just use a COM IF comprised of a single method, especially since the price for that on the C# side is minimal at best and the C++ side is sort of straightfoward for the same reason, espcially if you can make use of ATL helpers in that context.
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sarabandeCommented:
It almost looks like the name of the CalculateProduct routine has been decorated with a prefix of __imp_.
it is more that CalculateProduct is managed code which could be called from C# or managed c++ or mixed-code c++ dlls but not from (pure) unmanaged code.

though the export definitions are valid, the linker was unable to provide a bridge from unmanaged to managed which would be required when calling the CalculateProduct from unmanaged.

as I don't have done much with mixed-code, I am not quite sure whether the problem could be solved with your approach. but you could try to add an exported (unmanaged)  wrapper function to the mixed-code dll which calls the CalculateProduct rather than to exporting the CalculateProduct itself.

that way the mixed-code dll should provide the bridge functionality needed.

Sara
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e_livesayAuthor Commented:
Turns out I hadn't told the the linker to look in the *.lib file.  I changed the import code from this:
#pragma once
extern "C" __declspec(dllimport) double CalculateProduct(double x, double y);

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to this:
#pragma once
#pragma comment (lib, "C:\\FullPath\\Driver_CPP\\x64\\Debug\\MixedModeDLL.lib")
extern "C" __declspec(dllimport) double CalculateProduct(double x, double y);

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and the code now works.
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e_livesayAuthor Commented:
I found this useful when figuring out the linker issue:
stackoverflow.com/questions/7611198/microsoft-visual-studio-2010-adding-lib-files-in-this-version
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e_livesayAuthor Commented:
The other supplied answers suggested that I use COM or that I add a wrapper function.  While the use of COM was a reasonable way forward I did not end up doing it.  Nor did I add a wrapper function.  After looking at my own code again I found out what was wrong with it and fixed it myself based on a comment that I'd made earlier in the conversation surrounding this Question (i.e., related to the __imp_ prefix reported by the linker).
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