Netphi

This article walks you through how to expose a class to COM from .NET Core (or .NET 5+) and how to create a COM client in Delphi that consumes the COM host.

Table of contents

• Introduction
• Table of contents
• Godzilla-type breaking change
  • What about .NET Framework?
• Expose .NET Core components to COM
  • Create the library
  • Generate the COM host
  • Register the COM host for COM
• Generate the type library

Godzilla-type breaking change

According to the official Microsoft documentation...

Unlike in .NET Framework, there is no support in .NET Core or .NET 5+ for generating a COM Type Library (TLB) from a .NET assembly. The guidance is to either manually write an IDL file or a C/C++ header for the native declarations of the COM interfaces. If you decide to write an IDL file, you can compile it with the Visual C++ SDK's MIDL compiler to produce a TLB.

... there is no support in .NET Core or .NET 5+ to automatically generate a .tlb file, which is later needed for the generation of the COM client in Delphi.

What about .NET Framework?

While many projects have already migrated to .NET Core or .NET 5+, there are still some applications running on the .NET Framework. If you're one of them and looking to use .NET in Delphi, you're in luck. Simply follow this YouTube tutorial.

Expose .NET Core components to COM

The official Microsoft documentation already provides a guidance on creating and registering the COM host for .NET components. However, there is are multiple potential pitfalls that aren't covered.

Create the library

The first step is to create the library.

[ComVisible(true)]
[Guid("A028C948-7924-45EB-B679-6484818A8585")]
public interface IServer
{
    double ComputePi();
}

and the implementation

[ComVisible(true)]
[Guid("D34D391D-5138-48E8-9D3C-6E10002A253E")]
[ComDefaultInterface(typeof(IServer))]
public class Server : IServer
{
    public double ComputePi()
    {
        return Math.PI;
    }
}

Generate the COM host

To enable COM hosting for your project, follow these steps:

1. Open the .csproj project file and insert <EnableComHosting>true</EnableComHosting> within a <PropertyGroup></PropertyGroup> tag.
2. Depending on the architecture of the COM client, execute one of the following commands to build your project:

• For x86 architecture:

dotnet build -r win-x86 --no-self-contained .\path\to\ProjectName.csproj

• For x64 architecture:

dotnet build -r win-x64 --no-self-contained .\path\to\ProjectName.csproj

Upon successful execution, you'll find the following files in the output:

• ProjectName.dll
• ProjectName.deps.json
• ProjectName.runtimeconfig.json
• ProjectName.comhost.dll

Register the COM host for COM

Open an elevated command prompt and run regsvr32 ProjectName.comhost.dll. That will register all of your exposed .NET objects with COM.

Generate the type library

While there are many ways to generate the type library, the offical one, due to the end of support in .NET Core and .NET 5+, is to manually create a .idl and use the MIDL compiler to generate the .tlb. Another way is to continue to use .NET Framework just for regasm's capability to create a .tlb from a .cs file. Another one is to use the third party tool dscom.

1. Download the dscom32.exe and the dscom.exe from the dscom releases.
2. Copy the dscom32.exe and the dscom.exe to a binaries/ folder on the .sln root.
3. Open your .csproj and add the following MSBuild command (For x86 only at the moment, there is a better way for x64)

<PropertyGroup>
  <WarningsAsErrors>$(WarningsAsErrors);DSCOM001</WarningsAsErrors>
</PropertyGroup>

<PropertyGroup>
  <SkipManualTlbExport>false</SkipManualTlbExport>
</PropertyGroup>

<Target Name="GenerateTypelib" AfterTargets="CoreBuild" Condition="'$(SkipManualTlbExport)' != 'true'">
  <Exec Command="$(SolutionDir)/binaries/dscom32.exe tlbexport $(TargetPath) /out:$(ProjectDir)$(TargetName).tlb" />
</Target>

4. Open an elevated command prompt and run dscom32.exe tlbregister ProjectName.tlb.