Vb6 Collection in C++
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.
So for the C portion, the tlh and tli files were generated by
#import "msvbvm60.dll" no_namespacewe 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)