Bluehill.NativeCom

Build native in-process COM servers (DLLs) on .NET with Native AOT. This library + source generator removes most of the boilerplate for COM activation, class factories, and DLL exports, making it practical to ship shell extensions and other in-proc COM components written in C#.

Windows-only. Requires .NET 8 or later and NativeAOT publishing.

What’s inside

• Bluehill.NativeCom.IClassFactory — COM IClassFactory ([GeneratedComInterface]) with CreateInstance and LockServer.
• Bluehill.NativeCom.ClassFactoryAttribute<T> — annotate a factory class to wire it to a target COM class.
• Bluehill.NativeCom.DllHelper — helper that implements CreateInstance for your target class using StrategyBasedComWrappers.
• Source generator (Bluehill.NativeCom.SourceGenerator) that:
  • Validates your setup and reports diagnostics.
  • Generates partial implementation of your factory (method bodies for CreateInstance and LockServer).
  • Emits UnmanagedCallersOnly exports: DllGetClassObject and DllCanUnloadNow.

Requirements

• Windows
• .NET 8+ (recommended TargetFramework: net8.0-windows)
• Native AOT publishing
• C# 12 (default in .NET 8)

Installation

Add the following packages to your class library project:

• Bluehill.NativeCom
• Bluehill.NativeCom.SourceGenerator

You can add them via NuGet Package Manager or dotnet CLI.

Quick start

1. Enable NativeAOT and unsafe code in your .csproj:

   <PropertyGroup>
     <TargetFramework>net8.0-windows</TargetFramework>
     <PublishAot>true</PublishAot>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>

2. Define COM interface(s) using the new source-generated COM attributes:

   using System;
   using System.Runtime.InteropServices;
   using System.Runtime.InteropServices.Marshalling;
   
   [GeneratedComInterface]
   [Guid("a08ce4d0-fa25-44ab-b57c-c7b1c323e0b9")]
   [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
   public partial interface IExplorerCommand {
   {
       ...
   }

3. Implement the COM class that provides the behavior. It must be annotated with [GeneratedComClass] and have a CLSID via [Guid]:

   using System;
   using System.Runtime.InteropServices; // for [Guid]
   using System.Runtime.InteropServices.Marshalling;
   
   [GeneratedComClass]
   [Guid("22222222-2222-2222-2222-222222222222")] // CLSID used to activate this class
   public partial class ExplorerCommand : IExplorerCommand
   {
       ...
   }

4. Declare a factory class and connect it to your target via [ClassFactory<TTarget>]. Do not implement the methods yourself — the generator emits the method bodies.

   using Bluehill.NativeCom;
   using System.Runtime.InteropServices.Marshalling;
   
   [GeneratedComClass]
   [ClassFactory<ExplorerCommand>]
   public partial class ExplorerCommandFactory : IClassFactory
   {
       // The generator provides CreateInstance(...) and LockServer(...).
   }

5. Publish with NativeAOT:

   # From your project directory
   dotnet publish -c Release

The native DLL (your COM server) will be in the publish\native folder.

How it works

• The source generator scans for classes annotated with [ClassFactory<TTarget>]. It validates that:
  • The factory class implements Bluehill.NativeCom.IClassFactory.
  • Both the factory and the target class have [GeneratedComClass].
  • The target class has a [Guid] attribute (its CLSID).
• For each valid pair, the generator produces:
  • A partial factory implementation that forwards CreateInstance to DllHelper.CreateInstanceHelper<TTarget>().
  • A partial LockServer method that do nothing.
• It also generates an internal static Dll class with UnmanagedCallersOnly exports:
  • DllGetClassObject: routes the requested CLSID to the correct factory CreateInstance.
  • DllCanUnloadNow: returns S_FALSE (1), since .NET Native AOT DLLs cannot be unloaded.

Opting out of DLL exports

If you need to provide your own exports implementation, define the symbol NO_GENERATE_DLLGCO in your project. Example:

<PropertyGroup>
  <DefineConstants>$(DefineConstants);NO_GENERATE_DLLGCO</DefineConstants>
</PropertyGroup>

With this symbol defined, the generator will NOT emit DllGetClassObject.

Diagnostics

The generator reports clear diagnostics to help you fix setup issues:

• NATIVECOM0001: Required type '{0}' couldn't be found.
• NATIVECOM0002: Factory class does not implement IClassFactory.
• NATIVECOM0003: Factory class does not have GeneratedComClassAttribute.
• NATIVECOM0004: Target class does not have GeneratedComClassAttribute.
• NATIVECOM0005: Target class does not have GuidAttribute.

COM registration (in-proc)

This package generates DllGetClassObject and DllCanUnloadNow, but it does NOT generate DllRegisterServer/DllUnregisterServer. You are responsible for registration (e.g., via an installer, script, or custom code). At minimum, in-proc registration requires keys like:

• HKCR\CLSID{Your-Target-Class-CLSID}\ (default: your class name or description)
• HKCR\CLSID{Your-Target-Class-CLSID}\InprocServer32\ (default: full path to your compiled DLL, ThreadingModel=Both) Ensure you register the DLL for the correct bitness (x64 vs x86) and that your process matches. Shell extensions typically require additional shell-specific registration.

Notes & limitations

• In-process servers only (DLL). No out-of-proc server support.
• Aggregation is not supported (CreateInstance rejects non-null pUnkOuter).
• NativeAOT is required to export UnmanagedCallersOnly entry points.
• This library relies on .NET source-generated COM (GeneratedComInterface/GeneratedComClass) available in .NET 8+.

License

MIT License — see LICENSE.