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;

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
    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

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