How to send list through COM

Question

Can I send a list of objects through COM? I have my class and there is also a list of this class. And I need to send this through a COM function. Can I do that? If yes, then how? Do I need serialization marshaling?

Answer 1

If one side of the interface will be written in a language with a "simpler" type system, such as VB6 or script, then the SAFEARRAY is the way to go. COM's universal marshaller can take care of it.

If you have C++ on both sides of the interface, and you're happy to define the interface in IDL, and generate custom marshalling code, then IDL includes a "size_is" attribute that can be used. See here for details.

Answer 2

To any reasonable degree, there's no way to do so with std::list. I'm sure there is some maniac who could manage it but from my experience, it's not worth it. If doesn't have to be an std::list and just some linked list, then you'll have to either define a new COM interface and provide it's implementation (which can just be a wrapper around std::list) or find some thirdparty resource that has an adequate implementation for your purposes. If you don't have much experience with implementing COM interfaces, then you'll need to do some good amount of reading.

If it can be an array, then you can make your life a lot simpler by using a SAFEARRAY. And SAFEARRAYs can be easier to use with ATL's CComSafeArray, which is pretty much a wrapper. It tries to help/save you from issues, such as reference counting, when dealing with COM objects. You can then just pass the array between your COM objects. Just going to remind you that you still need to pay attention to who is responsible for de-allocating the memory.

Answer 3

COM does not pin down any particular collection type, it certainly doesn't have anything standard that models a list. By convention, you can model any collection with an interface. So say you have a collection of Foo objects that each implement IFoo. Then declare an IEnumFoo interface similar to:

interface IEnumFoo : IUnknown
{
    HRESULT Next( [in] ULONG celt,
                  [out, size_is(celt), length_is(*pceltFetched)] IFoo **rgelt,
                  [in,out,unique] ULONG *pceltFetched );
    HRESULT Skip( [in] ULONG celt);
    HRESULT Reset();
    HRESULT Clone( [out] IEnumFoo **ppenum );
}

And then simply return an interface pointer to an implementation of this interface to allow the client code to enumerate the list. Check the MSDN docs for IEnumVARIANT, a very common enumeration interface type for variants. Which also explains what the methods do.