.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.
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.
VB6ComHost (class library, .NET Standard 2.0, Windows-only at runtime):
- Registers a COM-visible STA
IMessageFilterviaCoRegisterMessageFilter(see KB Q247791 context). - Exposes
IServiceProviderfrom that filter (via a CCW shim) soQueryServicecan answer the Office-styleIMsoComponentManager/SID_SMsoComponentManagerpair with a minimal managed stub. VB6ComHost.Open()/CreateInstance(progId)/Disposefor hosting on the current STA thread.VB6StaMessagePump— a smallPeekMessage/DispatchMessageloop for hosts that are not WinForms or WPF.
VB6ComHost.ConsoleSample (SDK-style .NET Framework 4.8 executable, x86): demonstrates [STAThread], Microsoft.CSharp for dynamic, multiple modeless windows (N), modal demo (M), 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.
| Requirement | Why |
|---|---|
| Windows | COM, ole32, user32, VB6 runtime are Win32-centric. |
| STA thread | VB6 COM objects and the message filter registration expect single-threaded apartment; use [STAThread] or equivalent before opening the host. |
| 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. |
lib/com/x86/ # Prebuilt sample ActiveXLibrary.dll (COM, 32-bit)
vb6/ActiveXLibrary/ # VB6 project sources for that DLL
src/
VB6ComHost.slnx # Visual Studio solution (.slnx)
VB6ComHost/ # Class library (netstandard2.0)
VB6ComHost.ConsoleSample/ # SDK-style console host (net48, x86)
Build from the repo root or src:
dotnet build src/VB6ComHost.slnxOpen src/VB6ComHost.slnx in Visual Studio (2022 with .slnx support, or newer).
When an ActiveX DLL shows Show vbModal and the caller is a .NET COM host, some configurations report that ShowModal returns to managed code before the VB6 form is actually dismissed, which can make it look as if several “modal” windows can be open at once or as if the console prints “returned” while a form is still visible. That behavior comes from the VB6 runtime + hosting boundary, not from the message pump being “disconnected.” The sample is primarily aimed at modeless hosting; treat modal as a demo only unless you validate it for your DLL and host.
See LICENSE (MIT).
