I am trying to call .NET Core 8 (C#) code from C++ dll in Excel VBA. The architecture I’m targeting is: Excel (VBA) → C++ native DLL → .NET Core 8 C# DLL The idea is that the VBA code calls the native DLL, and that DLL hosts the .NET Core runtime and forwards the call to a [UnmanagedCallersOnly] static method inside my C# assembly.

In my real project, VBA is not the final target — I only use Excel/VBA here as a simple test harness for my proof of concept. However, in the final architecture, other constraints force me to go through a native C++ DLL as the entry point, which then has to call into the C# .NET Core code.

My POC Project : (many paths are hard coded case is only a POC) https://drive.google.com/file/d/1hE2NVuktjwj2rp-7oRcvEcpqiMYAw9b0/view?usp=sharing

If you have a simple POC project for load C# Core (8.0 if posible) with an C++ (20 if posible) dll in 64 t'is perfect ;)

Details :

C# code (cs_core.dll) :

using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

namespace MyNamespace;

public static class ManagedExports
{
    [UnmanagedCallersOnly(CallConvs = new[] { typeof(CallConvStdcall) })]
    public static int Add(int a, int b) => a + b;
}

Project file:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
  </PropertyGroup>
</Project>

So this assembly is built against .NET 8, not .NET Framework.

C++ code (cpp_project.dll) :

extern "C" __declspec(dllexport) int __stdcall ManagedAdd(int a, int b)
{
    // Path to runtimeconfig.json
    fs::path runtimeconfig = L"C:\\POC\\cs_core.runtimeconfig.json";

    if (!init_dotnet(runtimeconfig.c_str()))
        return -1;

    if (!get_managed_add_fn(L"C:\\POC\\cs_core.dll",
        L"MyNamespace.ManagedExports, cs_core",
        L"Add"))
        return -2;

    return g_add(a, b);
}

VBA code (Excel) :

Private Declare PtrSafe Function ManagedAdd Lib "C:\POC\cpp_project.dll" _
    (ByVal a As Long, ByVal b As Long) As Long

Sub TestManagedAdd()
    MsgBox "Result = " & ManagedAdd(2, 3)
End Sub

The issue

When called from Excel, the DLL loads and unloads immediately. In Visual Studio debug output, I see:

'EXCEL.EXE' (Win32): Loaded 'C:\POC\cpp_project.dll'. Symbols loaded.
'EXCEL.EXE' (Win32): Unloaded 'C:\POC\cpp_project.dll'

But the expected C# code never runs, and no MessageBox/trace from DllMain shows up.

Also, Dependency Walker shows strange dependencies:

• cs_core.dll (net8.0) still appears to reference mscoree.dll and System.Runtime.dll from .NET Framework v4.0.30319, which is confusing.

Running corflags on cs_core.dll reports:

'EXCEL.EXE' (Win32): Loaded 'C:\POC\cpp_project.dll'. Symbols loaded.
'EXCEL.EXE' (Win32): Unloaded 'C:\POC\cpp_project.dll'

But the expected C# code never runs, and no MessageBox/trace from DllMain shows up.

Also, Dependency Walker shows strange dependencies:

cs_core.dll (net8.0) still appears to reference mscoree.dll and System.Runtime.dll from .NET Framework v4.0.30319, which is confusing.

Running corflags on cs_core.dll reports:

Version   : v4.0.30319
CLR Header: 2.5
PE        : PE32
ILONLY    : 1

ven though it was compiled with net8.0.

What I tried

Using dotnet exec cs_core.dll → fails with MissingMethodException: Entry point not found, which makes sense since it’s not an EXE.

Ensured cs_core.runtimeconfig.json is present next to the DLL.

Tried different .NET SDK versions (8.0.414, 9.0.304).

Added logging in DllMain with OutputDebugString → never triggered from Excel.

Checked with Dependency Walker (x64) → shows .NET Framework dependencies, even for net8.0 build.

Question

What is the correct way to wire Excel VBA → native C++ DLL → .NET Core 8 DLL ?

Environment

• Windows 10 (x64)
• Excel 2016 (x64)
• Visual Studio 2022 (17.14)
• .NET SDK 8.0.414 and 9.0.304 installed
• .NET runtimes up to 9.0.8 installed

Would appreciate any help — we spent several days on this, and we are stuck at this “DLL loads/unloads immediately” stage.

load C# core (8.0) dll in C++ (20) dll