For the demonstration I consider a Visual Studio 2017 Solution containing two projects,

  • One project containing C++ Code
  • Other project contain C# driving code doing the demonstrating interop

C++ code looks like,

extern "C"             //No name mangling
__declspec(dllexport)  //Tells the compiler to export the function
int                    //Function return type     
__cdecl                //Specifies calling convention, cdelc is default, 
                       //so this can be omitted 
  test(int number) {
  return number + 1;

Please note the way return type specified compare to traditional function declaration.

C# driver code looks like following,

using System;
using System.Runtime.InteropServices;

namespace p1
  public static class NativeTest
    static void Main(string[] args)
      Console.WriteLine("Interop test: " + test(100));

    private const string DllFilePath = @"D:\Code\CSharp\InterOp\Debug\Cpp.dll";

    [DllImport(DllFilePath , CallingConvention = CallingConvention.Cdecl)]
    private extern static int test(int number);

    public static int Test(int number) {
        return test(number);

My projects dir structure looks like,

│   p1.sln
│   │   Cpp.vcxproj
│   │   Cpp.vcxproj.filters
│   │   Cpp.vcxproj.user
│   │   Main.cpp
│   │
│   └───Debug
│       │   Cpp.dll
│       │   Cpp.exp
│       │   Cpp.ilk
│       │   Cpp.lib
│       │   Cpp.log
│       │   Cpp.pdb
│       │   Main.obj
│       │   vc141.idb
│       │   vc141.pdb
    │   App.config
    │   p1.csproj
    │   Program.cs
    │   └───Debug
    │           p1.exe
    │   └───Debug
    │       │   p1.exe
    │       │   p1.pdb

For the project structure above, I can use a relative location of the file,

private const string DllFilePath = @"..\..\..\Cpp\Debug\Cpp.dll";

Additionally, I have changed following for C++ project,

Output Directory: \((SolutionDir)\)(ProjectName)$(Configuration)
Configuration Type: Dynamic Library (.dll)

When we are passing arrays and pointers we need data marshalling. ref, Using arrays and pointers in C# with C DLL

msdn documentation provides instructions for this for managed C++ to unmanaged C++. How to: Call Native DLLs from Managed Code Using PInvoke

So, for a new project it makes sense. Or if it is feasible to create managed C++ wrapper around the unmanaged code. It gives us an assembly that can be directly referenced from C#. Ref, Possible to call C++ code from C#?