Creating C++ DLL and using it in .NET

When using C++ code in C#, we often need to wrap the C++ code into a DLL (Dynamic Link Library) for several important reasons:

• Interoperability

C# and C++ are fundamentally different languages with distinct runtime environments. C# runs on the .NET Common Language Runtime (CLR), while C++ typically runs as native code

• Compilation and Linking

C++ code needs to be compiled into machine code that can be executed directly by the processor. C#, on the other hand, is compiled into intermediate language (IL) code that runs on the CLR

• Reusability and Maintenance

Wrapping C++ code in a DLL promotes code reuse and easier maintenance

Outline

1. How to create a C++ DLL
2. Calling DLL function in .NET

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How to create a cpp DLL

Source code: https://github.com/tuttelikz/notes/tree/main/cpp-dll-dotnet/CustomMath

Create a new DLL project

Open Visual Studio > Create a new project > Set "C++", "Windows" and "Library" filters to find the template > click Next

Give a project name

Set any name to the project > click Create

Create operators.cpp

Project > Add New Item > C++ File (.cpp) > Set name to .cpp file > Add

Add code to operators.cpp

We defined two simple functions as part of the library

#include "pch.h"

int multiply3(int n)
{
    return n * 3;
}

int add5(int n)
{
    return n + 5;
}

Create operators.h

Project > Add New Item > Header File (.h) > Set name to .h file > Add

Add code to operators.h

This code is responsible to export previously defined functions from the DLL

#pragma once
extern "C" __declspec(dllexport) int add5(int n);
extern "C" __declspec(dllexport) int multiply3(int n);

• extern means that the entity has external linkage, i.e. is visible outside its translation unit (C or CPP file).
• __declspec(dllexport) means that the symbol should be exported from a DLL. It is used when compiling the code that goes into the DLL.

Sources:

• https://stackoverflow.com/a/2288355/5151687
• https://learn.microsoft.com/en-us/cpp/build/exporting-cpp-functions-for-use-in-c-language-executables?view=msvc-170

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Calling DLL function in dotnet

Source code: https://github.com/tuttelikz/notes/tree/main/cpp-dll-dotnet/ClientApp

Create a console application

Open Visual Studio > Create a new project > Set "C#", "Windows" and "Console" filters to find the template > click Next

Give a project name

Set any name to the project > click Create

Create CustomMathApi.cs

Project > Add Class > Class > Set name to .cs file > Add

Add code to CustomMathApi.cs

A separate wrapper C# class is created to import DLL functions and interface calls from C#, like an API.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;

namespace CustomMathAPI
{
    public static class CustomMath
    {
        private const string _dllImportPath = @"C:\Users\saska\source\repos\notes\cpp-dll-dotnet\CustomMath\x64\Debug\CustomMath.dll";
        [DllImport(_dllImportPath, CallingConvention = CallingConvention.Cdecl)]
        public static extern int add5(int n);
        [DllImport(_dllImportPath, CallingConvention = CallingConvention.Cdecl)]
        public static extern int multiply3(int n);
    }
}

• We should use System.Runtime.InteropServices
• _dllImportPath is used to define a DLL function in managed code

Sources:

• https://learn.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.dllimportattribute.callingconvention?view=net-8.0#system-runtime-interopservices-dllimportattribute-callingconvention

Add code to Program.cs

This code calls the imported DLL functions and prints the result

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using CustomMathAPI;

namespace ClientApp
{
    internal class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine($"Calling dll CustomMath.add5(4): {CustomMath.add5(4)}");
            Console.WriteLine($"Calling dll CustomMath.multiply3(4): {CustomMath.multiply3(4)}");
            Console.ReadLine();
        }
    }
}

Check the output

Run the code to see the result: Debug > Start Debugging

Note: Solution configuration should be set same as DLL (Configuration > "Debug", Platform > "x64")

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Thanks for reading, hope this post was useful. Stay tuned, more to come 🙂

Alternate URL: https://github.com/tuttelikz/notes/tree/main/cpp-dll-dotnet