🧩 Vb6.ActiveX.Hosting

.NET-side support for hosting Visual Basic 6 ActiveX DLLs that show modeless (and optionally modal) forms and expect a cooperating Win32 host. The focus is a pure, managed C# approach on Windows: register an STA OLE message filter and related COM plumbing so VB6’s runtime (MSVBVM60) accepts the host, without requiring a native helper DLL for the scenario this project targets.

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⚠️ VB6 runtime message this project addresses

Non-modal forms cannot be displayed in this host application from an ActiveX DLL, ActiveX Control, or Property Page.

That message appears when VB6 decides the host is not a suitable container for modeless UI from an ActiveX DLL (or related binaries). In code, the same gate is often reflected as App.NonModalAllowed being False when you expected modeless forms to work. The hosting pattern here is aimed at satisfying the checks behind that message—together with a proper STA thread and a Windows message pump—so modeless VB6 forms can run inside a .NET process.

Related search terms people use alongside that error: VB6 modeless ActiveX DLL, NonModalAllowed, CoRegisterMessageFilter, IMessageFilter, IServiceProvider, IMsoComponentManager, KB 247791, VB6 host application.

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🎯 What this repository provides

Vb6.ActiveX.Hosting (class library, .NET Standard 2.0, Windows-only at runtime):

1. Registers a COM-visible STA IMessageFilter via CoRegisterMessageFilter (see KB Q247791 context).
2. Exposes IServiceProvider from that filter (via a CCW shim) so QueryService can answer the Office-style IMsoComponentManager / SID_SMsoComponentManager pair. The manager keeps VB6’s IMsoComponent registration and implements FPushMessageLoop as a nested PeekMessage / GetMessage pump so Show vbModal blocks until the form is dismissed.
3. ComHost.Open() / Open(ComHostOptions) starts a dedicated STA thread and message pump (MTA callers are fine). CreateInstance / CreateInstance<T> / Invoke / InvokeAsync run on that thread. The host tracks each CreateInstance RCW and releases it on dispose (or earlier via Release). Window events are posted off the STA thread (SynchronizationContext or the thread pool). GetWindows() snapshots open forms. CloseWindows() (or HostWindowEventArgs.Close()) posts WM_CLOSE.
4. ComHostException for activation failures (ProgID, HRESULT, regsvr32 hint). A 64-bit process throws PlatformNotSupportedException unless ThrowIf64BitProcess is false.
5. VB6StaMessagePump and COM interfaces live for interop / advanced hosts; prefer ComHost.

Vb6.ActiveX.Hosting.ConsoleSample (SDK-style .NET Framework 4.8 executable, x86): demonstrates an MTA console calling ComHost, Microsoft.CSharp for dynamic, sync CreateInstance (N / M), async InvokeAsync (I / A), await using / DisposeAsync, and UTF-8 console output (emoji look best in Windows Terminal).

vb6/ActiveXLibrary/ — sample VB6 ActiveX DLL sources (WindowLauncher with ShowNonModal / ShowModal).

lib/com/x86/ActiveXLibrary.dll — checked-in 32-bit build for convenience; the console project copies it to output on build. You still need regsvr32 (or registration-free COM) for ProgID activation.

Typical caller code (sync CreateInstance + using / Dispose):

var options = new ComHostOptions
{
  EventContext = SynchronizationContext.Current, // WinForms/WPF UI thread; omit for thread-pool events
  ShutdownTimeout = TimeSpan.FromSeconds(2),
};

using (ComHost host = ComHost.Open(options))
{
  host.ModelessWindowOpened += (_, e) => { /* not on the host STA thread */ };
  dynamic launcher = host.CreateInstance("ActiveXLibrary.WindowLauncher");
  launcher.ShowNonModal(1); // late-bound calls run on the host STA thread
  foreach (HostWindowInfo window in host.GetWindows())
  {
    _ = window.Title;
  }
} // Dispose closes remaining forms and releases tracked RCWs on the STA thread

Same host, async (InvokeAsync + await using / DisposeAsync). ShowModal does not block the caller:

await using (ComHost host = ComHost.Open())
{
  await host.InvokeAsync(() =>
  {
    dynamic launcher = host.CreateInstance("ActiveXLibrary.WindowLauncher");
    launcher.ShowModal(1);
  });
}

The NuGet listing uses src/Vb6.ActiveX.Hosting/README.md. Pack from that project (GitHub repo is currently jeromydean/VB6ComHost):

dotnet pack src/Vb6.ActiveX.Hosting/Vb6.ActiveX.Hosting.csproj -c Release

The package targets netstandard2.0 and net8.0-windows, is Windows-only, and must be consumed from a 32-bit (x86) process when loading classic VB6 DLLs.

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📋 Requirements

Requirement	Why
Windows	COM, ole32, user32, VB6 runtime are Win32-centric.
STA thread	VB6 COM and the message filter still require STA; ComHost.Open() creates that thread and pump so callers do not have to.
32-bit (x86) process when loading 32-bit VB6 DLLs in-process	VB6 ActiveX DLLs are normally 32-bit; a 64-bit host cannot load them in-process.
.NET	Library targets .NET Standard 2.0; the sample targets net48 and references Microsoft.CSharp for dynamic.

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📁 Repository layout

lib/com/x86/              # Prebuilt sample ActiveXLibrary.dll (COM, 32-bit)
vb6/ActiveXLibrary/       # VB6 project sources for that DLL
src/
  Vb6.ActiveX.Hosting.slnx              # Visual Studio solution (.slnx)
  Vb6.ActiveX.Hosting/                  # Class library (netstandard2.0)
  Vb6.ActiveX.Hosting.ConsoleSample/    # SDK-style console host (net48, x86)
  Vb6.ActiveX.Hosting.Tests/            # xUnit (net48, x86)

🔨 Build

Build from the repo root or src:

dotnet build src/Vb6.ActiveX.Hosting.slnx

Open src/Vb6.ActiveX.Hosting.slnx in Visual Studio (2022 with .slnx support, or newer).

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🔒 Modal forms and the nested message pump

When an ActiveX DLL calls Show vbModal, VB6 does not always run its own GetMessage loop. If the host advertises IMsoComponentManager (required for modeless hosting), VB6 registers an IMsoComponent and calls FPushMessageLoop. That call must pump messages until IMsoComponent.FContinueMessageLoop returns false (form closed). A stub that returns immediately makes ShowModal return to managed code while the form is still visible, so another “modal” can open. ComHost runs that nested pump on its dedicated STA thread.

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📄 License

See LICENSE (MIT).