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Visual Studio Code – How to correctly create COM classes with .NET8?

I need to create a COM class in .NET8 that needs to be accessible to Excel.

After watching this video, I implemented the following test bed class:

namespace COMTestBedCS
{
    [Guid("26a0aa6d-5aba-458f-92b4-b9a30ae0c65c")]
    [GeneratedComInterface]
    public partial interface ITestBed
    {
        int GetXPTO();
        void SetXPTO(int value);
    }

    [Guid("3e178f98-522e-4e95-8a9c-6d80dc48b7d5")]
    [GeneratedComClass]
    public partial class TestBed : ITestBed
    {
        private int _XPTO = 1024;

        public int GetXPTO() => _XPTO;
        public void SetXPTO(int value)=>_XPTO = value;
    }
}

The project compiles correctly, without errors. However, when I try to reference this test bed in Excel, I get the following error: Can't add a reference to the specified file.

If I try to use regsvr32, I get the following error:

regsvr32 error message

What am I doing wrong?

For completion sake, here’s the project file:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
  </PropertyGroup>

</Project>

Update

I was able to compile the code correctly and register it using regsvr32. However, I’m still unable to reference it in Excel.

First, I can’t find it on the reference list. If I try to browse for the dll (either the assembly or the comhost), it fails with the message: Can't add a reference to the specified file.

All that’s left now it to add it as a Reference in the Excel VBA.

New Code

Project

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <RootNamespace>COMTestBedCS</RootNamespace>
    <TargetFramework>net8.0-windows</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <AllowUnsafeBlocks>False</AllowUnsafeBlocks>
    <EnableComHosting>true</EnableComHosting>
    <RegisterForComInterop>True</RegisterForComInterop>
    <Platforms>x86</Platforms>
    <RegisterAssemblyMSBuildArchitecture>x86</RegisterAssemblyMSBuildArchitecture>
  </PropertyGroup>

</Project>

Code

using System.Runtime.InteropServices;
using System.Runtime.InteropServices.Marshalling;

namespace COMTestBedCS
{
    [Guid("26a0aa6d-5aba-458f-92b4-b9a30ae0c65c")]
    [ComVisible(true)]
    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    public interface ITestBed
    {
        int GetXPTO();
        void SetXPTO(int value);
    }

    [Guid("3e178f98-522e-4e95-8a9c-6d80dc48b7d5")]
    [ComVisible(true)]
    [ClassInterface(ClassInterfaceType.None)]
    internal class TestBed : ITestBed
    {
        private int _XPTO = 1024;

        public int GetXPTO() => _XPTO;
        public void SetXPTO(int value)=>_XPTO = value;
    }
}

OBS.: To compile successfully, I had to, for some reason, build the project twice. The first time Visual Studio is unable to successfully build, with the following error:

MSB3217 
Cannot register assembly "<assembly file>" Could not load file or 
assembly 'System.Runtime, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. 
The system cannot find the file specified.`

Then I have to rebuild it again without cleaning the project.
Only then the project compile successfully.

Update 2

If anyone wants to help, I put this test bed project on GitHub here.

Tags:

2

Answers


  1. 0
    1. Add the <EnableComHosting>true</EnableComHosting> property into the <PropertyGroup> in the csproj. This will tell dotnet to generate *.comhost.dll.
    2. Make the TestBed class internal instead of public. This will solve the build problem (I don’t know why, but from a general point of view, the implementation (the TestBed class) should not be accessible outside the assembly in any way other than the generated factory).
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  2. 0

    First, you must follow what’s detailed here Expose .NET Core components to COM, Generate the COM host:

    Open the .csproj project file and add
    <EnableComHosting>true</EnableComHosting> inside a
    <PropertyGroup></PropertyGroup> tag.

    But doing this will raise "The "GenerateClsidMap" task failed unexpectedly." error, as explained here:

    SYSLIB diagnostics for COM interop source generation

    SYSLIB1098 .NET COM hosting with EnableComHosting only supports
    built-in COM interop. It does not support source-generated COM interop
    with GeneratedComInterfaceAttribute.

    GeneratedComInterfaceAttribute is a new to .NET 8 feature, that you can’t use here in this scenario (exposing and hosting your own interfaces), so you’ll have to replace your code by something like this to revert back to "built-in" COM interop:

    [ComVisible(true)]
[Guid("26a0aa6d-5aba-458f-92b4-b9a30ae0c65c")]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public partial interface ITestBed
{
    int GetXPTO();
    void SetXPTO(int value);
}

[ComVisible(true)]
[Guid("3e178f98-522e-4e95-8a9c-6d80dc48b7d5")]
[ClassInterface(ClassInterfaceType.None)]
public partial class TestBed : ITestBed
{
    private int _XPTO = 1024;

    public int GetXPTO() => _XPTO;
    public void SetXPTO(int value) => _XPTO = value;
}

    Once the project is build, you can call regsvr32 <myfile>.comhost.dll (which has been generated by .NET Core) with sufficient rights.

    PS: You cannot use the RegisterForComInterop .csproj directive anymore as it’s reserved for .NET Framework (it internally uses the same code as Regasm which you cannot use either).

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