dscom 0.14.0

dSPACE COM tools

This is an unstable prerelease. Anything may change at any time!

dscom generates a type library that describes the types defined in a common language runtime assembly and is a replacement for tlbexp.exe and TypeLibConverter.ConvertAssemblyToTypeLib.
The tool consists of a library and a command line tool. The library can be used in net5+ or in net48 projects.

Fortunately, .NET still supports COM, but there is no support for generating TLBs.
From the 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.

https://docs.microsoft.com/en-us/dotnet/core/native-interop/expose-components-to-com

One main goal is to make dscom behave like tlbexp.exe.

Happy IUnknowing and IDispatching 😉

• dSPACE COM tools
  • Command Line Client
    • Installation
    • 32Bit support
    • Usage
  • Library
  • Migration notes (mscorelib vs System.Private.CoreLib)
    • Why can I load a .NET Framework library into a .NET application?
  • Limitations

Command Line Client

The command-line interface (CLI) tool dscom is a replacement for tlbexp.exe and OleView (View TypeLib).

It supports the following features:

• Convert an assembly to a type library
• Convert a type library to YAML file
• Register a type library
• Unregister a type library

Installation

The installation is quite simple. You can use dotnet tool to install the dscom binary.

dotnet tool install --global dscom

Here you can find all available versions:
https://www.nuget.org/packages/dscom/

Alternatively you can download dscom.exe from the relase page.
https://github.com/dspace-group/dscom/releases

32Bit support

dscom installed by dotnet tool install can only handle AnyCPU or 64Bit assemblies. Depending on whether you want to process 32bit or 64bit assemblies, you need to download different executables from the release page.

• dscom.exe to create a 64Bit TLB from a AnyCPU or a 64Bit assembly
• dscom.exe to create a 32Bit TLB from a AnyCPU assembly (with the switch --win32)
• dscom32.exe to create a 32Bit TLB from a AnyCPU or a 32Bit assembly

https://github.com/dspace-group/dscom/releases

Usage

Use dscom --help to get further information.

c:\> dscom --help
Description:
  dSPACE COM tools

Usage:
  dscom [command] [options]

Options:
  --version       Show version information
  -?, -h, --help  Show help and usage information

Commands:
  tlbexport <Assembly>         Export the assembly to the specified type library
  tlbdump <TypeLibrary>        Dump a type library
  tlbregister <TypeLibrary>    Register a type library
  tlbunregister <TypeLibrary>  Unregister a type library

Library

If you miss the TypeLibConverter class and the ConvertAssemblyToTypeLib method in .NET, then the dSPACE.Runtime.InteropServices might help you. This method should behave compatible to the .NET Framework method.

public object? ConvertAssemblyToTypeLib(
  Assembly assembly,
  string tlbFilePath,
  ITypeLibExporterNotifySink? notifySink)

https://www.nuget.org/packages/dSPACE.Runtime.InteropServices/

Example:

using dSPACE.Runtime.InteropServices;

// The assembly to convert
var assembly = typeof(Program).Assembly;

// Convert to assembly
var typeLibConverter = new TypeLibConverter();
var callback = new TypeLibConverterCallback();
var result = typeLibConverter.ConvertAssemblyToTypeLib(assembly, "MyTypeLib.tlb", callback);

// Get the name of the type library
var typeLib2 = result as System.Runtime.InteropServices.ComTypes.ITypeLib2;
if (typeLib2 != null)
{
    typeLib2.GetDocumentation(-1, out string name, out _, out _, out _);
    Console.WriteLine($"TypeLib name: {name}");
}

// The callback to load additional type libraries, if necessary
public class TypeLibConverterCallback : ITypeLibExporterNotifySink
{
    public void ReportEvent(ExporterEventKind eventKind, int eventCode, string eventMsg)
    {
        Console.WriteLine($"{eventCode}: {eventMsg}");
    }

    public object? ResolveRef(System.Reflection.Assembly assembly)
    {
        // Returns additional type libraries
        return null;
    }
}

Migration notes (mscorelib vs System.Private.CoreLib)

Both assemblies are ComVisible=false but lot of .NET Framework types are ComVisible=true. But this is not the case for .NET (.NET Core and .NET >= 5).
Unlike mscorelib (the good old .NET Framework), no tlb is shipped for .NET.

As example the System.Exception class:

In case of mscorelib the System.Exception class is ComVisible=true:

[Serializable]
[ClassInterface(ClassInterfaceType.None)]
[ComDefaultInterface(typeof(_Exception))]
[ComVisible(true)]
public class Exception : ISerializable, _Exception

In case of System.Private.CoreLib (.NET Core and .NET >=5), the Exception class is ComVisible=false

[Serializable]
[TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
public class Exception : ISerializable
{

The _Exception class interface (default interface in this case) is not available in .NET (.NET Core and .NET >=5).

Why can I load a .NET Framework library into a .NET application?

The magic is in TypeForwardedFromAttribute.
If you try to load an .NET Framework assembly inside a .NET (.NET Core and .NET >=5) application, the runtime will forward the original type to a type defined in the System.Private.CoreLib assembly.

classextern forwarder System.Exception
{
    .assemblyextern System.Private.CoreLib
}

Therefore you should make sure that you do not use any types from the mscorelib typelib in your .NET Framework project if you plan to migrate to .NET 5+

Limitations

• No imports of the mscorelib typelib (all types are VT_UNKNOWN)
  • _Object not supported
  • _EventArgs not supported
  • _Delegate not supported
  • _Type not supported
  • ISerializable not supported
  • ICloneable not supported
  • IDisposable not supported
  • ...
• TypeLibExporterFlags is not supported
• ITypeLibExporterNotifySink is not COM visible
• TypeLibConverter is not COM visible
• AutoDual is not supported
  • see: https://docs.microsoft.com/en-us/previous-versions/visualstudio/visual-studio-2015/code-quality/ca1408-do-not-use-autodual-classinterfacetype?view=vs-2015&redirectedfrom=MSDN
• LCID only NEUTRAL is supported
• No GAC support
  • see: https://docs.microsoft.com/en-us/dotnet/core/compatibility/core-libraries/5.0/global-assembly-cache-apis-obsolete
• IEnumerator is converted to IEnumVARIANT (stdole)
• Guid is converted to GUID (stdole)
• Color is converted to OLE_COLOR(stdole)
• No support for UnmanagedType.CustomMarshaler
• No support for .NET Framework assemblies with AssemblyMetadataAttribute value ".NETFrameworkAssembly"