Building a COM Proxy/Wrapper for Legacy VB6 Code

Proxy Patterns and Remote Calls with Visual Basic 6### Overview

Visual Basic 6 (VB6) remains in use in many legacy environments. When modernizing, integrating, or maintaining these applications, developers frequently need to interact with remote services, wrap legacy components, or insert intermediaries for cross-process communication. The Proxy design pattern — an object that controls access to another object — and related remote-call techniques can help VB6 applications achieve decoupling, security, caching, and network transparency.

This article covers:

What proxy patterns are and when to use them in VB6
Types of proxies relevant to VB6 (local/remote, virtual, protection, caching, and COM proxies/wrappers)
Techniques for making remote calls from VB6 (COM, DCOM, ActiveX EXE, SOAP/HTTP, sockets, and third‑party libraries)
Implementation patterns and practical examples with code snippets
Error handling, security, performance, and testing considerations
Migration and modernization tips

What is the Proxy Pattern?

A proxy is an object that acts as a surrogate or placeholder for another object to control access to it. In practice, the proxy implements the same interface as the real subject and forwards client requests, optionally adding behavior such as:

Lazy initialization (virtual proxy)
Access control (protection proxy)
Caching results (caching proxy)
Remote communication (remote proxy)
Logging, monitoring, or instrumentation

In VB6, proxies are commonly used when:

The real component lives in another process or on another machine.
Starting or initializing the real component is expensive.
You want to add authorization or validation before forwarding calls.
You need to adapt a legacy API to a newer one without changing callers.

Proxy Types and How They Map to VB6

Virtual Proxy
- Defers creation of the real object until needed.
- Useful for heavy objects (large data loads, complex initialization).
Protection Proxy
- Checks permissions before forwarding operations.
- Often used to prevent unauthorized changes or to gate expensive calls.
Caching Proxy
- Returns cached results for repeated requests.
- Useful when remote calls are slow or rate-limited.
Remote Proxy (the most relevant for VB6)
- Hides network communication details and marshals calls across process or machine boundaries.
- Implemented via COM/DCOM, ActiveX EXE, SOAP wrappers, or custom socket/RPC layers.
Adapter vs Proxy
- A proxy forwards calls preserving interface; an adapter changes the interface. In VB6 you might implement an adapter when wrapping COM objects with different interfaces.

Remote Call Options from VB6

VB6 is flexible but dated; choose an approach based on deployment, performance, security, and available infrastructure.

COM in-process (DLL) and out-of-process (EXE)
- ActiveX DLL: in-process; fast but requires same-machine and compatible runtime.
- ActiveX EXE: out-of-process; provides process isolation and marshaling via COM apartments.
DCOM
- Extends COM across machines. Requires network configuration, security setup, and can be brittle with firewalls and modern OS restrictions.
COM+ / MTS
- For transaction and role-based behavior; integrates with COM components.
SOAP / HTTP (Web Services)
- VB6 can consume SOAP services via MS SOAP Toolkit (deprecated) or by hand-crafting HTTP requests and parsing XML. Modern practice is to implement a small wrapper service (e.g., .NET, Node, or Java) that exposes a simple HTTP/JSON API and communicates with VB6 via HTTP.
Sockets / TCP
- Winsock control (MSWINSCK) for direct TCP/UDP communication. Useful for custom protocols, realtime, or lightweight remote messaging.
Named Pipes / Memory-Mapped Files / COM+ Queues
- IPC options for same-machine high-performance scenarios.
Third-party libraries
- Libraries offering modern protocols (gRPC, REST adapters) often need an intermediate wrapper due to VB6 limitations.

Implementing a Remote Proxy in VB6: Patterns & Examples

Below are practical patterns and code examples showing how to implement proxies in VB6.

Note: All code snippets are VB6-style. They omit full error handling for brevity — add robust error checking and logging in production.

1) Simple Local Proxy (interface forwarding)

Use when you want transparent substitution and to add light behavior (logging/protection/caching).

Example: wrap a legacy COM object with a proxy class in an ActiveX DLL.

' Class: CRealService (CoClass implemented elsewhere) ' Class: CServiceProxy (in your VB6 ActiveX DLL project) 'CServiceProxy - public methods that mirror CRealService Option Explicit Private m_real As CRealService Private Sub Class_Initialize()     ' don't create real object until first call (lazy init) End Sub Private Sub EnsureReal()     If m_real Is Nothing Then         Set m_real = New CRealService     End If End Sub Public Function GetData(id As Long) As String     EnsureReal     ' Example of additional behavior: caching or logging     Debug.Print "Proxy:GetData called for id=" & id     GetData = m_real.GetData(id) End Function

Deploy this ActiveX DLL; consumers reference the DLL and use CServiceProxy instead of CRealService.

2) Protection Proxy (permission check)

' In CServiceProxy Public Function DeleteRecord(id As Long, userRole As String) As Boolean     If userRole <> "admin" Then         Err.Raise vbObjectError + 1000, "CServiceProxy", "Insufficient permissions"         Exit Function     End If     EnsureReal     DeleteRecord = m_real.DeleteRecord(id) End Function

3) Remote Proxy using ActiveX EXE (out-of-process)

Create an ActiveX EXE exposing the real service. Clients create the EXE object; COM marshaling handles IPC.

Server (ActiveX EXE) class:

' Project type: ActiveX EXE ' Class: CRemoteServicePublic (Instancing: MultiUse or GlobalMultiUse) Public Function GetRemoteValue() As String     GetRemoteValue = "Hello from ActiveX EXE" End Function

Client:

Dim srv As CRemoteServicePublic Set srv = New CRemoteServicePublic Debug.Print srv.GetRemoteValue()

To add a proxy layer, implement a local DLL class that creates the ActiveX EXE object and forwards calls — allowing caching or retry logic locally.

4) Remote Proxy over SOAP/HTTP

If you have a web service, a VB6 proxy can call it using MSXML2.ServerXMLHTTP or WinHTTP, then parse XML/JSON.

Example using MSXML2.ServerXMLHTTP and JSON (simple approach requires a JSON parser or lightweight custom parsing):

Private Function HttpPost(url As String, body As String) As String     Dim http As Object     Set http = CreateObject("MSXML2.ServerXMLHTTP.6.0")     http.Open "POST", url, False     http.setRequestHeader "Content-Type", "application/json"     http.send body     HttpPost = http.responseText End Function Public Function GetUserName(userId As Long) As String     Dim url As String, payload As String, resp As String     url = "https://api.example.com/user"     payload = "{""id"":" & CStr(userId) & "}"     resp = HttpPost(url, payload)     ' parse resp JSON to extract name (use third-party JSON library or simple parsing)     GetUserName = ParseNameFromJson(resp) End Function

Practical tip: place HTTP logic in a proxy class so callers remain unaware of transport.

5) Sockets-based Remote Proxy

Using the Winsock control for custom TCP services:

Server side (any language) exposes a simple TCP API. VB6 client proxy:

' Form with a Winsock control named Winsock1 Private Sub ConnectAndRequest(host As String, port As Integer, request As String)     Winsock1.RemoteHost = host     Winsock1.RemotePort = port     Winsock1.Connect     ' wait for Connect event, then send request End Sub Private Sub Winsock1_Connect()     Winsock1.SendData requestString End Sub Private Sub Winsock1_DataArrival(ByVal bytesTotal As Long)     Dim s As String     Winsock1.GetData s     ' process response End Sub

Wrap Winsock usage in a COM class that exposes synchronous methods (using events internally to simulate synchronous behavior or implement callbacks).

Error Handling, Retry, and Timeouts

Always implement timeouts for network calls (ServerXMLHTTP has .setTimeouts).
Wrap remote calls in retry logic with exponential backoff for transient errors.
Surface meaningful error codes/messages to callers; don’t leak transport internals unless needed.
Use Err.Raise with custom vbObjectError codes for consistent error handling across VB6 projects.

Security Considerations

Avoid sending credentials in plain text; use HTTPS where possible.
For COM/DCOM, configure authentication levels and set appropriate ACLs. DCOM over the internet is discouraged.
Validate all inputs in the proxy before sending to the real service.
Sanitize and limit data returned from remote calls to prevent injection attacks in downstream components.

Performance and Resource Management

Use connection pooling where possible for HTTP connections (keep-alive) or reuse Winsock connections.
For in-process proxies, minimize object creation overhead by reusing instances when safe.
Cache frequently requested but rarely changed data in a caching proxy with TTL.
Measure marshaling overhead for COM and consider moving hot paths in-process if performance is critical.

Testing and Debugging Strategies

Create mock implementations of remote services and swap them via the proxy for unit testing.
Log requests/responses and timing information in the proxy to diagnose latency and errors.
Use network inspection tools (Fiddler, Wireshark) for HTTP/sockets traffic.
For COM/DCOM issues, use DCOMCNFG to inspect configuration and permissions.

Migration and Modernization Advice

Where feasible, implement a thin modern wrapper service (REST/JSON) around complex legacy logic. Let VB6 call that wrapper via HTTP — easier to secure and evolve.
Consider rewriting performance-critical remote code into a .NET COM-visible assembly or a small native DLL if rehosting in-process is acceptable.
Gradually replace VB6 proxies with modern client libraries when moving clients off legacy platforms.

Example: End-to-End Remote Proxy Scenario

Scenario: VB6 desktop app must call a remote inventory service. Direct DCOM is blocked by firewalls. Solution:

Implement an HTTP REST wrapper (small .NET service) over the inventory backend.
In VB6, create a CInventoryProxy class that uses MSXML2.ServerXMLHTTP to call REST endpoints, parse JSON responses, and expose simple VB6-friendly methods (GetStock, ReserveItem, etc.).
Add caching for GetStock with a TTL to reduce network load.
Add retry logic and clear error mapping for user-friendly messages.

This approach isolates network details, avoids DCOM complexity, and lets you modernize the backend independently.

Summary

A proxy in VB6 is an effective pattern to decouple clients from remote or expensive services, add security/caching, and provide a migration path to modern services.
Choose the right remote transport: COM/DCOM for rich Windows-only integrations; HTTP/REST for cross-platform, firewall-friendly access; sockets for custom low-latency protocols.
Implement robust error handling, security, caching, and testing strategies in the proxy layer.
For long-term sustainability, consider wrapping legacy services with modern RESTful facades and using lightweight VB6 proxies to bridge old clients to new servers.