Vb6 Collection in C++


Author: Dave
Date: 03.21.16 - 12:02pm



Working between C/C++ and VB for simple types is fairly easy. VB6 was designed to interact with the WinAPI which is just a bunch of C functions in libraries.

For simple native types like numbers or strings its pretty easy. But what happens if your C function needs to return a variable sized list of strings?

Now you cant use simple C types anymore and have to start using COM types which is more complex and takes more background to deal with.

There are also a couple in between steps you can use. You C code can write the results to a file. VB6 then reads the results from the temp file when the C function returns.

You can also use callbacks. The vb app registers a callback procedure with the C function. Every time it generates a result entry it calls back the vb module function and feeds it data. The vb module pushs the string into an array or collection and then when the C function returns it reads this module level array/collection as the return result. Its ugly, but its not that hard.

What i really wanted was a way for the C code to dynamically add items to an array or collection. Using SafeArrays from C is fairly well documented, but they seem pretty bulky and can be locked etc. Adding items to a vb6 collection object was what I really wanted, but I was not able to find any working code online about it.

So here is one way to make it work. Its actually surprising how little it boils down to. Thankfully there is a lot of complexity being hidden and done automatically for us.

C++ code:
#include "msvbvm60.tlh" 

void addStr(_CollectionPtr p , char* str){
	_variant_t vv;
	vv.SetString(str);
	p->Add(&vv.GetVARIANT());
}

void __stdcall addItems(_CollectionPtr *pColl)
{
#pragma EXPORT
	
	addStr(*pColl, "this is my string1");
	addStr(*pColl, "this is my string2");
	addStr(*pColl, "this is my string3");
	addStr(*pColl, "this is my string4");
}




VB6 Code:
'we cant create a vba.collection in C, so we use a reference passed in from vb
Private Declare Sub addItems Lib "col_dll" (ByRef col As Collection)

Private Sub Command1_Click()

    Dim c As New Collection
    Dim x, tmp
    
    addItems c
    Me.Caption = c.Count & " items returned"
    
    For Each x In c
        tmp = tmp & x & vbCrLf
    Next
    
    Text1 = tmp
    
End Sub


So for the C portion, the tlh and tli files were generated by
#import "msvbvm60.dll" no_namespace
we then manually modified them to only include the collection object and we now import them manually. (they gave compile error as auto generated)

These files give us the smart pointer definition for the VB6 _CollectionPtr object. The VBA.Collection type can not be created by C++ code. So in order to use it we just pass in a live collection instance from our VB6 caller.

Its actually surprisingly easy to use this mechanism to transfer arbitrary amounts and types of data back from C++ to VB6. The demo app includes several methods designed to test for memory leaks, dangling pointers, corruption, and object ownership. This technique is passing on all fronts so looks good.

If you wanted to step through each element in a vb6 collection while on the C side, you could do something like the following (error handling omitted)

//remember vb collections start at 1
for(long i=1; i <= pColl->Count(); i++){
	_variant_t index = i;
	_variant_t vTmp = pColl->Item(&index);

	//automatically converts (some) types to strings for you
	//I think I read this can throw an exception in some cases..(VT_UNKNOWN?)
	_bstr_t b = vTmp; 

	printf("Item %d is type.vt=%d  CStr('%s')
",i, vTmp.vt, (char*)b);
}


Download Sample Code








Comments: (1)

On 11.19.22 - 4:36pm Dave wrote:
finally got around to figuring out how to create a new vb6 collection from C using vb runtime exports. Also available on git

 
Leave Comment:
Name:
Email: (not shown)
Message: (Required)
Math Question: 24 + 85 = ? followed by the letter: W 



About Me
More Blogs
Main Site
Posts: (year)
2024 (3)
     VB6 JSON
     ffmpeg voodoo
     RegJump Vb
2023 (9)
     VB6 Virtual Files
     File handles across dlls
     python abort / script timeout
     py4vb
     VB6 Python embed w/debugger
     python embedding
     VB6 IDE Enhancements
     No Sleep
     A2W no ATL
2022 (4)
     More VB6 - C data passing
     Vb6 Asm listing
     Byte Array C to VB6
     Planet Source Code DVDs
2021 (2)
     Obscure VB
     VB6 IDE SP6
2020 (4)
     NTFileSize
     BSTR from C Dll to VB
     Cpp Memory Manipulation
     ActiveX Binary Compatability
2019 (5)
     Console tricks
     FireFox temp dir
     OCX License
     Extract substring
     VB6 Console Apps
2018 (6)
     VB6 UDTs
     VB6 Debugger View As Hex tooltips
     VB6 - C Share registry data
     VB6 Addin Missing Menus
     VB6 Class Init Params
     VB6 isIn function
2017 (6)
     Python and VB6
     Python pros and cons
     download web Dir
     vc rand in python
     VB6 Language Enhancement
     Register .NET as COM
2016 (22)
     VB6 CDECL
     UDT Tricks pt2
     Remote Data Extraction
     Collection Extender
     VB6 FindResource
     CDO.Message
     DirList Single Click
     Reset CheckPoint VPN Policy
     VB6 BSTR Oddities Explained
     SafeArrays in C
     BSTR and Variant in C++
     Property let optional args
     Misc Libs
     Enum Named Pipes
     Vb6 Collection in C++
     VB6 Overloaded Methods
     EXPORT FUNCDNAME Warning
     VB6 Syncronous Socket
     Simple IPC
     VB6 Auto Resize Form Elements
     Mach3 Automation
     Exit For in While
2015 (15)
     C# self register ocx
     VB6 Class Method Pointers
     Duktape Debug Protocol
     QtScript 4 VB
     Vb6 Named Args
     vb6 Addin Part 2
     VB6 Addin vrs Toolbars
     OpenFile Dialog MultiSelect
     Duktape Example
     DukTape JS
     VB6 Unsigned
     .Net version
     TitleBar Height
     .NET again
     VB6 Self Register OCXs
2014 (25)
     Query Last 12 Mos
     Progid from Interface ID
     VB6 to C Array Examples
     Human Readable Variant Type
     ScriptBasic COM Integration
     CodeView Addin
     ScriptBasic - Part 2
     Script Env
     MSCOMCTL Win7 Error
     printf override
     History Combo
     Disable IE
     API Hooking in VB6
     Addin Hook Events
     FastBuild Addin
     VB6 MemoryWindow
     Link C Obj Files into VB6
     Vb6 Standard Dlls
     CStr for Pascal
     Lazarus Review
     asprintf for VS
     VB6 GlobalMultiUse
     Scintilla in VB6
     Dynamic Highlight
     WinVerifyTrust, CryptMsgGetParam VB6
2013 (4)
     MS GLEE Graphing
     printf for VB6
     C# App Config
     Tero DES C# Test
2012 (10)
     VC 2008 Bit Fields
     Speed trap
     C# Db Class Generator
     VB6 vrs .NET (again)
     FireFox Whois Extension
     git and vb6
     Code Additions
     Compiled date to string
     C# ListView Sorter
     VB6 Wish List
2011 (7)
     C# Process Injection
     CAPTCHA Bots
     C# PE Offset Calculator
     VB6 Async Download
     Show Desktop
     coding philosophy
     Code release
2010 (11)
     Dll Not Found in IDE
     Advanced MSScript Control
     random tip
     Clipart / Vector Art
     VB6 Callback from C#
     Binary data from VB6 to C#
     CSharp and MsScriptControl
     HexDumper functions
     Js Beautify From VB6 or C#
     vb6 FormPos
     Inline Asm w VB6
2009 (3)
     The .NET Fiasco
     One rub on computers
     Universal extractor