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Posted 7 May 2006


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COM in plain C, Part 3

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7 May 2006CPOL30 min read
COM collections in C



Sometimes, we may need to maintain a list of items. For example, let's say that our COM component is designed to operate some hardware PCI card we've designed. And, let's say that a user can install several of these cards into one computer, and we want our component to control all of the available cards, letting an application retrieve information about each card, and access each card individually.

In other words, when our component runs, it needs to query what cards are in the system, and make available to an application, a list of information about all of them. For the sake of argument, let's assume that the only information we need to make available is the "name" of each card, where the first card in the system will be named "Port 1", the second card will be named "Port 2", etc.

Since we don't know ahead of time how many cards will be in a system, the best approach would be to create some struct that can be linked to others of its kind, in a linked list. For example, maybe we'll define an IENUMITEM struct to hold information about a card:

typedef struct _IENUMITEM {
   // To link several IENUMITEMs into a list.
   struct _IENUMITEM  *next;
   // This item's value (ie, its port name).
   char               *value;

And if we have three ports, our linked list of IENUMITEMs would look something like this (where we normally would GlobalAlloc the IENUMITEMs, but I'll just statically declare/initialize them below for quick edification):

IENUMITEM Port1 = {&Port2, "Port 1"};
IENUMITEM Port2 = {&Port3, "Port 2"};
IENUMITEM Port3 = {0, "Port 3"};

In COM lingo, we refer to a group of related items as a "collection". So, the linked list of three items above would be our collection.

But there's one problem with our IENUMITEM above. It has a char * member, which is not automation compatible. We could change this to a BSTR, which is automation compatible. Better yet, let's put a VARIANT right in the IENUMITEM. This has the advantage of making an IENUMITEM generic (i.e., it can store any kind of automation data type). And as we'll see later, we have to return an item's value to an application, using a VARIANT. So, since we need to deal with a VARIANT anyway, let's store it in our IENUMITEM that way. Here's the new definition of an IENUMITEM:

typedef struct _IENUMITEM {
   struct _IENUMITEM  *next;
   VARIANT            *value;

And, here is how we may allocate one, and set its value to "Port 1" (error checking omitted):

IENUMITEM  *enumItem;

enumItem = (IENUMITEM *)GlobalAlloc(GMEM_FIXED, sizeof(IENUMITEM));
enumItem->next = 0;
enumItem->value.vt = VT_BSTR;
enumItem->value.bstrVal = SysAllocString(L"Port 1");

Defining a collection object

Remember that some script languages do not have the concept of a pointer. Such a language can't possibly walk the above list on its own, because the first member of each IENUMITEM is a pointer to the next IENUMITEM. So, we need to provide an object that helps the application (using our COM object) walk this list, fetching each item's value.

Because Microsoft left it up to the Visual Basic programmers (ugh) to come up with this object, they chose to base it upon an IDispatch. In other words, the VTable for our object must start with the three IUnknown functions as all COM objects do (QueryInterface, AddRef, and Release), and then must be followed immediately with the four standard IDispatch functions (GetTypeInfoCount, GetTypeInfo, GetIDsOfNames, and Invoke). Our object must then have three more functions. In our IDL file, when we define the VTable (i.e., the interface) for this object we'll create, we must name these three extra functions Count, Item, and _NewEnum. In our actual VTable for the object, we can use any name we want for the pointers (although I'll stick to those names). Why? Because no one is ever going to directly call them. No one will ever even know that we're going to add these three functions to our VTable. These functions must be called only indirectly via the Invoke function of our object -- even if you're using a language like C that could have called them directly if only Microsoft's Visual Basic programmers had designed this to accommodate more powerful languages. That's the terrible price we all pay for letting Visual Basic programmers design this. Furthermore, in our IDL file, we must assign the ID DISPID_VALUE to the Item function, and DISPID_NEWENUM to the _NewEnum function. We must. The Count function can be assigned any positive number of our choosing for its ID. Does there appear to be any sense of consistency or logic to this whatsoever? No? Remember -- Visual Basic programmers.

We'll also need to generate a new GUID for this object's VTable, for use in our IDL file (i.e., the type library).

Let's look at the definition of this object. We can call it anything we like. I'll choose to call it an ICollection:

// Our ICollection VTable's GUID
// {F69902B1-20A0-4e99-97ED-CD671AA87B5C}
DEFINE_GUID(IID_ICollection, 0xf69902b1, 0x20a0, 0x4e99, 0x97, 
            0xed, 0xcd, 0x67, 0x1a, 0xa8, 0x7b, 0x5c);

// Our ICollection's VTable
#define INTERFACE ICollection
   // IUnknown functions
   STDMETHOD  (QueryInterface) (THIS_ REFIID, void **) PURE;
   // IDispatch functions
   STDMETHOD_ (ULONG, GetTypeInfoCount) (THIS_ UINT *) PURE;
                                    ITypeInfo **) PURE;
               UINT, LCID, DISPID *) PURE;
               REFIID, LCID, WORD, DISPPARAMS *,
               VARIANT *, EXCEPINFO *, UINT *) PURE;
   // Extra functions
   STDMETHOD (Count) (THIS_ long *);
   STDMETHOD (Item) (THIS_ long, VARIANT *);
   STDMETHOD (_NewEnum) (THIS_ IUnknown **);  

This shouldn't look totally unfamiliar. First, I ran GUIDGEN.EXE to create another GUID. I pasted it above, and gave it the variable name IID_ICollection. That's the GUID for our ICollection object's VTable.

Next, we define our ICollection object's VTable. We're using the same macro we used when we defined our IExample2 VTable in the last article. And as mentioned earlier, it starts with the three IUnknown functions, followed by the four IDispatch functions, just like IExample2 did when we added support for scripting languages. These functions are going to perform the same duties as the respective functions did in IExample2.

Finally, we add the three extra functions Count, Item, and _NewEnum. Later, we'll examine what they do.

Remember that the above macro automatically defines our ICollection object as so:

typedef struct {
   ICollectionVtbl  *lpVtbl;
} ICollection;

In other words, it is defined to have only one data member -- a pointer to our ICollection's VTable. This member is named lpVtbl, of course. But, we need to add an extra DWORD member to keep a reference count (just like we did to IExample2, and consequently defined a MyRealIExample2 to accommodate any extra data members). So, we'll define a MyRealICollection as so:

typedef struct {
   ICollectionVtbl  *lpVtbl;
   DWORD            count;
} MyRealICollection;

Now, let's take a look at how we'll define the VTable (interface) in our IDL file:

[uuid(F69902B1-20A0-4e99-97ED-CD671AA87B5C), oleautomation, object]
interface ICollection : IDispatch
   [propget, id(1)] 
   HRESULT Count([out, retval] long *);
   [propget, id(DISPID_VALUE)] 
   HRESULT Item([in] long, [out, retval] VARIANT *);
   [propget, id(DISPID_NEWENUM), restricted]
   HRESULT _NewEnum([out, retval] IUnknown **);

Note that we've used the new GUID created for our ICollection VTable.

We use the oleautomation keyword because our functions will accept only automation compatible data types, and return such.

Finally, we use the object keyword to indicate that this VTable belongs to some object. It's not going to be an object that an application gets hold of via our IClassFactory. (Later, we'll see how an application gets hold of one of our ICollection objects.) In fact, an application is never going to be told that our ICollection object is anything except a standard IDispatch object (i.e., our ICollection object is going to masquerade as an ordinary IDispatch as far as an application or script engine is concerned). Since our object is going to masquerade as an ordinary IDispatch, there's no need to specifically define our ICollection object itself inside of our IDL file (unlike we had to do with IExample2 when we listed what interfaces were in it, and which interface was the default). All scripting languages and applications already know all about an ordinary IDispatch object, so there's no need to put any information about this IDispatch-impersonator (ICollection) object itself in our IDL file. We need to define only its VTable as above, and mark that VTable as belonging to some object.

Note that, on the interface line, we do specify that this VTable contains IDispatch functions in the expected place and order. So, it certainly can masquerade as an IDispatch.

We, then, list the three extra functions. Notice that they are all defined as propget. Also notice that the Item function has an ID (DISPID) of DISPID_VALUE, and the _NewEnum function has an ID of DISPID_NEWENUM. (We also use the restricted keyword on that latter one, because an object browser is not supposed to show the _NewEnum function. It is supposed to be a function that only a script engine would call internally, but an actual script would never call.) For the Count function, we can choose any positive ID number, so I arbitrarily chose 1. (It doesn't matter that IExample2's Buffer function also has an ID of 1. These VTables are used in two different objects, so their IDs do not need to be unique between the two VTables).

Later, we'll actually get into writing these functions.

Before we change our IExample2 sources, let's just copy the entire IExample2 directory to a new directory named IExample3. We'll also rename all of the files to reflect this new directory (i.e., IExample2.h becomes IExample3.h, and IExample2.c becomes IExample3.c, etc.). And while we're at it, let's edit the files in this new directory, renaming our IExample2 object to an IExample3, to distinguish it from our previous sources. All we do is search and replace every instance of "IExample2" with "IExample3". And, don't forget to run GUIDGEN.EXE to make new GUIDs for IExample3, its VTable, and the type library. Replace the old GUIDs in IExample3.h, and update the UUIDs in IExample3.idl. After all, we don't want our new DLL (which we'll name IExample3.dll) to conflict with our previous IExample2.dll. I've done all this for you, and put the resulting files in an IExample3 directory.

Some helper functions

We need to construct our list of port names. Let's write a helper function to do that. We'll arbitrarily assume we have three ports, and so create three IENUMITEMs. We'll store the head of this list in a global variable PortsList. And, we also need a helper function to free the list when we're done with it.

IENUMITEM  *PortsList;

// This is just a helper function to free
// up our PortsList. Called when our DLL unloads.
void freePortsCollection(void)
   IENUMITEM *item;

   item = PortsList;

   // Is there another item in the list?
   while ((item = PortsList))
      // Get the next item *before* we delete this one
      PortsList = item->next;

      // If the item's value is an object, we need to Release()
      // it. If it's a BSTR, we need to SysFreeString() it.
      // VariantClear does this for us.

      // Free the IENUMITEM.

// This is just a helper function to initialize our Ports list.
// Called when our DLL first loads.
HRESULT initPortsCollection(void)
   IENUMITEM *item;

   // Add a "Port 1" IENUMITEM to our list.
   if ((PortsList = item = 
       (IENUMITEM *)GlobalAlloc(GMEM_FIXED, 
      item->next = 0;
      item->value.vt = VT_BSTR;
      if ((item->value.bstrVal = SysAllocString(L"Port 1")))
         // Add a "Port 2" IENUMITEM to our list.
         if ((item->next = (IENUMITEM *)GlobalAlloc(
                               GMEM_FIXED, sizeof(IENUMITEM))))
            item = item->next;
            item->value.vt = VT_BSTR;
            if ((item->value.bstrVal = SysAllocString(L"Port 2")))
               // Add a "Port 3" IENUMITEM to our list.
               if ((item->next = (IENUMITEM *)GlobalAlloc(
                                     GMEM_FIXED, sizeof(IENUMITEM))))
                  item = item->next;
                  item->next = 0;
                  item->value.vt = VT_BSTR;
                  if ((item->value.bstrVal = 
                       SysAllocString(L"Port 3")))

   // Error

We're also going to add a second global variable named CollectionTypeInfo to hold a ITypeInfo for our ICollection... uh, the IDispatch object. (We'll talk about why we need this, later). So, we need to add that global variable, and then let's write two helper functions -- one to initialize the variable to zero, and one to Release the ITypeInfo:

// Our ICollection's ITypeInfo. We need only one of these so
// we can make it global
ITypeInfo    *CollectionTypeInfo;

// This helper function initializes our ICollection TypeInfo.
// It's called when our DLL is loading.
void initCollectionTypeInfo(void)
   // We haven't yet created the ITypeInfo for our ICollection
   CollectionTypeInfo = 0;

// This helper function just Release()'s our ICollection TypeInfo.
// It's called when our DLL is unloading.
void freeCollectionTypeInfo(void)
   if (CollectionTypeInfo)

Now, we need to modify our DllMain to call these helper functions:

BOOL WINAPI DllMain(HINSTANCE instance, DWORD fdwReason, LPVOID lpvReserved)
   switch (fdwReason)
         MyTypeInfo = 0;

         // Initialize our ICollection stuff

         // Initialize our Ports list
         if (initPortsCollection())
            MessageBox(0, "Can't allocate the PortsList", "ERROR", MB_OK);

         OutstandingObjects = LockCount = 0;

         MyIClassFactoryObj.lpVtbl = (IClassFactoryVtbl *)&IClassFactory_Vtbl;


         // Free our Ports list

         // Free our ICollection ITypeInfo

         if (MyTypeInfo) MyTypeInfo->lpVtbl->Release(MyTypeInfo);


Our collection object's functions

Now, let's write the actual functions for our ICollection (actually, our MyRealICollection). Rather than put this code in IExample3.c, let's create a separate source file for it, called PortNames.c. And, we'll put the definition of our ICollection VTable and its GUID in a separate PortNames.h file. (We can also put our above helper functions in PortNames.c.)

The IUnknown functions (QueryInterface, AddRef, and Release) and IDispatch functions (GetTypeInfoCount, GetTypeInfo, GetIDsOfNames, and Invoke) are almost the same as the respective functions for our IExample3 object. So rather than reproduce the code here, I'll refer you to the file PortNames.c (in the directory IExample3).

One difference is that our ICollection functions are passed an ICollection object pointer (instead of an IExample3 object pointer), of course. And, ICollection's Release function is slightly different (since unlike IExample3's Release, there is no buffer member to free).

The other key difference is the call to GetTypeInfoOfGuid. Note that we're passing the GUID for our ICollection VTable (instead of the GUID for our IExample3 VTable, like we do in IExample3.c). Here's the deal. When we get an ITypeInfo for IExample3 (by calling loadMyTypeInfo in IExample3.c), we pass the GUID of IExample3's VTable to the OLE function GetTypeInfoOfGuid. The implication of this is that the default ITypeInfo Microsoft creates for us is good for getting information only about the functions in our IExample3 VTable. It cannot be used to get information about the functions in another object's VTable. But, we do need an ITypeInfo that will provide us with information about our ICollection's functions. So now, we have to call GetTypeInfoOfGuid again, but this time we pass the GUID for our ICollection object's VTable (i.e., the new GUID I created). This will return a second ITypeInfo (which we store in that global variable we added, named CollectionTypeInfo). This second ITypeInfo can be used with ICollection's IDispatch functions to get information about ICollection's functions. And it can be used with DispInvoke and DispGetIDsOfNames in ICollection's Invoke and GetIDsOfNames functions to do almost all of the work for us -- just like we did with IExample3's ITypeInfo.

Notice that ICollection's IDispatch functions use this new ITypeInfo, whereas IExample3's IDispatch functions use IExample3's ITypeInfo. They are not the same ITypeInfo, and cannot be used interchangeably.

All that's left is to write the three extra functions, Count, Item, and _NewEnum.

The Count function is simple. It is passed a pointer to a long. Count fills in that pointer with the total number of items in our list. For example, above we have three ports (IENUMITEM structs) in our list, so we would return a 3.

Here's our Count function:

STDMETHODIMP Count(ICollection *this, long *total)
   DWORD     count;
   IENUMITEM *item;

   // Count how many items in the list by just walking it all
   // the way to the last IENUMITEM, incrementing a count for
   // each item.
   count = 0;
   item = (IENUMITEM *)&PortsList;
   while ((item = item->next)) ++count;

   // Return the total.
   *total = count;


The Item function is also simple. It is passed a long that tells us which item is being queried (where 0 is the first item, 1 is the second item, etc.). It is also passed a VARIANT into which we copy the value of that item.

Here's our Item function:

STDMETHODIMP Item(ICollection *this, long index, VARIANT *ret)
   IENUMITEM  *item;

   // Assume we have nothing to return.
   ret->vt = VT_EMPTY;

   // Locate to the item that the caller wants.
   item = (IENUMITEM *)PortsList;
   while (item && index--) item = item->next;

   // Any more items left?
   if (item)
      // Copy the item's value to the VARIANT that the caller supplied.
      // If what we're returning to the caller is an object, we must AddRef()
      // it on the caller's behalf. The caller is expected to Release() it
      // when done with it. If what we're returning is a BSTR, then we must
      // SysAllocString a copy of it. Caller is expected to SysFreeString it.
      // Other datatypes are simply copied to the caller's VARIANT as is.
      // VariantCopy() does all this for us. It also returns S_OK if all
      // went well.
      return(VariantCopy(ret, &item->value));

   // If no more items, return S_FALSE.

As you'll note from the comment above, the OLE function VariantCopy does half the work for us.

For now, we'll gloss over the _NewEnum function. We'll stick in a dummy one that returns E_NOTIMPL.

Once we have all the ICollection functions written, we can statically declare its VTable:

static const ICollectionVtbl ICollectionVTable = 

How an application obtains our collection object

Let's consider how an application gets hold of one of our MyRealICollection objects. The easiest thing to do is to add another (extra) function to our IExample3 object. The application will call this new function to allocate and receive one of our MyRealICollection objects. (But we're going to lie to the application and tell it that it's just an ordinary IDispatch.)

We need to change the definition of IExample3's VTable (in IExample3.h), to add this new function, which I'll arbitrarily call GetPorts. I'll define it as being passed a handle to an IDispatch, which is where we'll return the pointer to our newly allocated MyRealICollection... er, IDispatch. Yeah, that's it. It's just an IDispatch. Wink, wink. Here's our updated IExample3 VTable:

// IExample3's VTable
#define INTERFACE IExample3
    // IUnknown functions
    STDMETHOD  (QueryInterface) (THIS_ REFIID, void **) PURE;
    // IDispatch functions
    STDMETHOD_ (ULONG, GetTypeInfoCount)(THIS_ UINT *) PURE;
    STDMETHOD_ (ULONG, GetTypeInfo) (THIS_ UINT, LCID, ITypeInfo **) PURE;
                UINT, LCID, DISPID *) PURE;
    // Extra functions
    STDMETHOD  (GetString) (THIS_ BSTR *) PURE;
    STDMETHOD  (GetPorts) (THIS_ IDispatch **) PURE;  // <--- Added GetPorts here

Notice that I added GetPorts to the end of the VTable. Also notice, I'm specifying that GetPorts will fill in an IDispatch pointer (even though it will really be our MyRealICollection). It's an IDispatch. Honest. Would I lie?

And I have to make the same change in the IDL file for our IExample3 VTable:

[uuid(CFADB388-9563-4591-AABB-BE7794AEC17C), dual, 
             oleautomation, hidden, nonextensible]
interface IExample3VTbl : IDispatch
   [helpstring("Sets the test string.")]
   [id(1), propput] HRESULT Buffer([in] BSTR);
   [helpstring("Gets the test string.")]
   [id(1), propget] HRESULT Buffer([out, retval] BSTR *);
   [helpstring("Gets the enumeration for our hardware ports.")]
   [id(2), propget] HRESULT Ports([out, 
           retval] IDispatch **);  // <--- Added GetPorts here

Notice, I made this newly added function a propget, just like Buffer. This is so that a script can use an ordinary assignment instruction to get our MyRealICollection... duh!... IDispatch object. The member is called "Ports" as far as the script is concerned. Never mind that we don't actually have a Ports data member in our IExample3 object. This is sort of a phony member. But the script doesn't need to know that.

And, I arbitrarily gave it a DISPID of 2.

Don't forget that we need to add this function to our statically declared IExample3Vtbl in IExample3.c:

static const IExample3Vtbl IExample3_Vtbl = {QueryInterface,
GetPorts}; // <--- Added GetPorts here

So, we need to write the GetPorts function:

static HRESULT STDMETHODCALLTYPE GetPorts(IExample3 *this, IDispatch **portsObj)
   // Create an IDispatch to enumerate our port names.
   // Caller is responsible for Release()'ing
   // it. NOTE: We're really returning a MyRealICollection,
   // but the caller doesn't know that.
   // He thinks we're returning an IDispatch,
   // which is ok because a MyRealICollection's
   // VTable starts with the 3 IUnknown functions
   // followed by the 4 IDispatch functions, just
   // like a real IDispatch object's VTable
   if (!(*portsObj = allocPortsCollection()))

The above simply calls another helper function (named allocPortsCollection) we'll put in PortNames.c. This helper function does the work of GlobalAlloc'ing a MyRealICollection and initializing it. It's fairly similar to how our IClassFactory's CreateInstance GlobalAllocs an IExample3 and initializes it. We even bump up the count of outstanding objects because our MyRealICollection... er, IDispatch object is going to be given to some application (which is expected to later Release it).

IDispatch * allocPortsCollection(void)
   MyRealICollection *collection;

   // Allocate a MyRealICollection
   if ((collection = (MyRealICollection *)GlobalAlloc(
                      GMEM_FIXED, sizeof(MyRealICollection))))
      // Store its VTable
      collection->lpVtbl = (ICollectionVtbl *)&ICollectionVTable;

      // AddRef it
      collection->count = 1;

      // Indicate another outstanding object since we'll be returning it
      // to an application which is expected to Release() it

   // Return it as if it were an IDispatch (which it can be used as)
   return((IDispatch *)collection);

We're done. You can compile the IExample3.dll. To register it, just take IExample2's register utility (RegIExample2) and replace every "IExample2" with "IExample3". After all, there is nothing about IExample3 which requires us to register it any differently than IExample2. Likewise, to un-register it, modify IExample2's un-register utility (UnregIExample2).

A VBScript example

Let's write a VBScript example that uses our collection to display the port names. I've written such an example (IExample3.vbs), and placed it in the IExample3 directory.

Of course, the VBScript needs to first call CreateObject to get one of our IExample3 objects. If you installed it properly, it should have a ProdID of "IExample3.object". Now that the script has our IExample3, it can simply access that phony "Ports" member to get one of our MyRealICollection... damn!... IDispatch objects to use. Here, we assign it to a variable named "coll".

Set coll = myObj.Ports

Next, we can call the Count function to determine how many port names there are. Actually, because our type library defined this function as propget, the script can use an assignment.

count = coll.Count

Now, we loop around, calling the Item function to get each port name, and display it:

For i = 0 To count - 1
   MsgBox coll.Item(i)

And, that's it.

A C example

For a C/C++ application to use our collection object, it needs to call our Count and Item functions indirectly, through the Invoke function. Hold on to your seats because this is going to be a bumpy ride. The MS Visual Basic programmers designed the IDispatch functions to be passed arguments, and return values, such that those folks could easily add COM support to VB and get it out the door quickly. But they didn't give any concern to how easy it would be to utilize the IDispatch functions from any other language. It's a major pain in the butt from C/C++.

In the directory IExample3App, is an example C application that does just what the VBScript above does. It obtains our Ports collection object, and uses it to display the names of all ports. I'm not going to discuss how the C app gets our IExample3 object. That is exactly the same as how it gets an IExample2 object (except we #include IExample3.h, and use the IExample3 object's GUID).

The main point of interest begins where we call our IExample3's GetPorts function to get our MyRealICollection... *cough*... IDispatch object. That starts where I put the following comment (in IExample3App.c):


Peruse all of that code and read the comments. They detail the steps you need to jump though to use IDispatch functions from C/C++. Then, take an MS Visual Basic programmer out to lunch to "thank" him, and slip lots of hot pepper into his food when he isn't looking.

An IEnumVARIANT object

If we look at the following lines in our Item function, something may alarm us:

// Locate to the item that the caller wants.
item = (IENUMITEM *)PortsList;
while (item && index--) item = item->next;

Every time someone calls our Item function, we have to start at the head of the list and search to the desired item. Assume we have 30,000 items in the list. Say, an application asks us to fetch item 28,000. We have to walk through 27,000 items before we get to the desired item. Now, let's say the application calls Item again to request item 29,000. Even though it's only one more link away, we start all over from the head of the list and walk though the 28,000 items. Obviously, this isn't very efficient.

We could perhaps add another data member to our MyRealICollection. This member would store the "position" where we last left off in the list. Microsoft thought about this, and then decided, instead of monkey-ing around with the collection object (which really isn't designed to be efficiently called from C/C++ anyway, thanks to VB developers), a second standard COM object, called an IEnumVARIANT, would be defined. The main purpose of an IEnumVARIANT is to store the current position that an application has "read from" within the list. But given how inefficient and troublesome it is for a C/C++ app to call our collection's Item function indirectly via Invoke, MS decided to have some functions in the IEnumVARIANT that an app can directly call to do what we previously did with the collection's Item function... and more. Specifically, an IEnumVARIANT has four functions in it (plus the three IUnknown functions, of course) named Next, Skip, Reset, and Clone.

  • The IEnumVARIANT's Next function makes our collection object's Item function superfluous. With a single call to Next, an app can return the values of several items at once (by supplying a count of how many items to return, and an array of VARIANTs to store all the values). So, an app could call Next to read four items. On the next call to Next, our IEnumVARIANT will automatically start reading at the fifth item in the list, and return however many values the app requests.
  • The IEnumVARIANT's Reset simply resets the position back to the start of the list.
  • The IEnumVARIANT's Skip sets the position to a particular point (i.e., it's analogous to seeking within a disk file, except, here it sets the position within our list).
  • The IEnumVARIANT's Clone allocates (and returns to the app) another IEnumVARIANT object whose position is the same as the IEnumVARIANT being cloned. This is used in case an app wants to nest loops where it's necessary to remember more than one position in a particular list.

Microsoft has already defined (in an include file that ships with your compiler) an IEnumVARIANT object and its VTable (i.e., Microsoft has already decided what functions are in the VTable, what those functions are passed, and what they return). So, we don't need to do that. But like with all the objects we've created so far, we need to add a couple extra data members to our IEnumVARIANT. So once again, we'll define a MyRealIEnumVariant that has these extra members.

But before we modify our IExample3 sources, let's again make a new directory named IExample4. We'll do that thing where we copy the sources to this new directory, and rename/edit them. Search and replace "IExample3" with "IExample4". Run GUIDGEN.EXE to create new GUIDs, and put them in IExample4.h, PortNames.h, and IExample4.idl. Once again, I've done this for you, and created an IExample4 directory with the new files.

In PortNames.c, we'll add the definition of our MyRealIEnumVariant object, write all its functions, and statically declare its VTable. This all done starting at the comment:

//=================== IEnumVARIANT functions ===================

In fact, you should now be quite familiar with what its QueryInterface, AddRef, and Release functions do. And the other four functions are rather trivial, so you can peruse the source code comments to get details about those functions.

The real question is "How does an app get hold of one of my IEnumVARIANT objects?". Remember before, how we ignored our collection object's _NewEnum function, and just had it return E_NOTIMPL? Well, guess what. That's the function an app calls to get one of our IEnumVARIANT objects, so now we have to write some real code for it.

In other words, to get one of our IEnumVARIANTs, the app must first get our IExample4 object, call our IExample4's GetPorts function to get our collection object, and then call our collection's _NewEnum function to get an IEnumVARIANT. It's not the short way home, but that's how it works. Some bad news: a C/C++ app can't directly call our collection's _NewEnum function. Just like with our collection's Count and Item functions, a C/C++ app has to indirectly call _NewEnum through Invoke. Blech. The good news is that, once our C/C++ app has the IEnumVARIANT, the collection object can be Release()d and we're done with that latter aberration.

So, let's examine our collection's _NewEnum function:

STDMETHODIMP _NewEnum(ICollection *this, IUnknown **enumObj)
   IEnumVARIANT *enumVariant;

   if (!(enumVariant = allocIEnumVARIANT())) return(E_OUTOFMEMORY);
   *enumObj = (IUnknown *)enumVariant;

This just calls our helper function named allocIEnumVARIANT which allocates and initializes our IEnumVARIANT (actually, a MyRealIEnumVariant) much like how our IClassFactory's CreateInstance allocates our IExample4 object, or our IExample4's GetPorts function allocates our collection object. There's really nothing new here.

But notice that _NewEnum asks the app to pass a handle where we return an IUnknown object -- not an IEnumVARIANT. Yes, it's true that we're really returning our IEnumVARIANT, but it is masquerading as an IUnknown object, and it can do that because its VTable starts with the three IUnknown functions.

"But why masquerade? Didn't you just say that _NewEnum is used to get hold of our IEnumVARIANT?"

Yes... in a roundabout manner. [Cue scary monster movie soundtrack.]

In a previous article, I alluded to the fact that a COM object could actually have many VTables inside of it. We say that such an object has "multiple interfaces". Microsoft decided that _NewEnum should be able to pass back an object that could have multiple interfaces, and the IEnumVARIANT may be just one of many VTables in it (and not even the first VTable in the object). So, what an app is expected to do is take this IUnknown object we give it, and call that object's QueryInterface, passing the GUID for an IEnumVARIANT (IID_IEnumVARIANT, which is defined in Microsoft's include files for us). And then, QueryInterface will return a pointer to the IEnumVARIANT VTable (i.e., essentially, the IEnumVARIANT embedded inside of whatever object that IUnknown object really is -- because you just know it's something else masquerading as an IUnknown).

In our case, the app is going to call our IEnumVARIANT's QueryInterface, asking us for an IEnumVARIANT. And, we're just going to return the same pointer again. Totally unnecessary. Inefficient. Illogical. But, that's sometimes how COM is when you've got parts of it that are designed by MS VB developers (who had a vested interest in making the whole IDispatch thing a lot like how VB internally calls its own built-in functions, thus minimizing their work, and foisting this design on everyone else regardless of how inconvenient and unwieldy that can be), and other programmers who wanted to use things like multiple interfaces (which a VBScript can't even directly use -- talk about a comedy of errors).

Anyway, we've got our IEnumVARIANT support done, so we can compile IExample4.dll. To register it, once again, you can modify RegIExample2.c, searching and replacing "IExample2" with "IExample4".

Another VBScript example

A VBScript can absolutely not call our collection's _NewEnum function (because this may return an object with multiple interfaces that need to be QueryInterfaced -- and a VBScript can't deal with such an object). So, a VBScript cannot obtain our IEnumVARIANT and call its functions.

Does this mean an IEnumVARIANT is useless to a VBScript? Nope. The VBScript engine can itself use our IEnumVARIANT. When does the engine do that? When the script uses a For Each loop to enumerate the items in our collection. In the directory IExample4 is a VBScript named IExample4.vbs which uses such a For Each loop. This is a slightly different way for the script to do the same thing that IExample3.vbs did, but it's more efficient under-the-hood (because the engine uses our IEnumVARIANT's Next function, instead of the VBScript using our collection object's Item function). And, it's slightly less coding on the part of the script because the engine retrieves the item's value on behalf of the script. Here's how it's done:

set myObj = CreateObject("IExample4.object")
Set coll = myObj.Ports
For Each elem In coll
   MsgBox elem

The first two lines are the same as IExample3.vbs (except that we use the ProdID for IExample4.dll now).

But the loop is different. When the VBScript engine executes that For Each line, it gets hold of our IEnumVARIANT (by calling our collection's _NewEnum, and doing a QueryInterface for IID_IEnumVARIANT). It stores away this IEnumVARIANT internally so it can be used on subsequent iterations of the loop. Then, it calls our IEnumVARIANT's Next function, asking for an item's value to be returned. Of course, the first time Next is called, we return the port name for the first item (i.e., the string "Port 1"). The VB engine stuffs this string into the variable "elem". This is all done in that one VBScript instruction. Now, the script simply displays the value of "elem" (which is the string "Port 1"). On the next iteration of the loop, the VB engine once again calls our IEnumVARIANT's Next function, asking for one more item's value. This is the second time Next has been called, so of course, we return the second item's value, which is the string "Port 2". The VB engine now updates the elem variable to this new value. And the script displays "Port 2". This continues on until the VB engine calls our IEnumVARIANT's Next, and we have no more items to return. At that point, Next returns S_FALSE (instead of S_OK) to the engine. And, the engine drops out of the loop (and Releases our IEnumVARIANT).

Another C example

In the directory IExample4App, there is a C example that demonstrates how to get and use our IEnumVARIANT. We still have to mess with the collection object and its Invoke. But at least, the loop is more efficient. And the whole thing is less involved than using the collection object's Item function.

A more generic approach

If you look at the IEnumVARIANT and the collection functions in PortName.c, you'll see that these are hard-wired to work upon only our list of port names (i.e., PortsList). But with a small bit of retooling, we can rewrite those functions to work upon any linked list of IENUMITEMs we throw at them. In other words, we can make these functions generic, so if our component needs to maintain several different types of lists, we can more easily provide further collection and IEnumVARIANT objects that reuse these same functions without any further modification. So, let's separate just the IEnumVARIANT and the collection functions into a new, separate source file named IEnumVariant.c. The only stuff we'll leave in PortNames.c is the code that specifically accesses our PortsList.

But first, we'll do that thing where we create a new IExample5 directory, copy the files there, and rename/edit them. You should know the routine by now. I've done the work, and created an IExample5 directory.

Instead of simply declaring a global variable that is the list itself, we'll wrap the list in another struct we'll call a IENUMLIST, as so:

typedef struct {
   struct _IENUMITEM *head;
   DWORD             count;

The head member is where the list is stored. And we've added a count field that will be incremented each time we create another collection or IEnumVARIANT object that uses this particular list.

Now, we change our PortsList global to this new struct:

IENUMLIST    PortsList;

The key to making our collection and IEnumVARIANT functions more generic is adding an extra data member to our MyRealIEnumVariant and MyRealICollection objects. We'll add a member to our MyRealICollection that holds a pointer to the IENUMLIST it should operate upon. And, we'll write a new helper function that will allocate a MyRealICollection object. This helper function will be passed that IENUMLIST pointer, and will store it in this new data member added to our MyRealICollection. I've written such a function (allocICollection) and put it in IEnumVariant.c. It's passed a pointer to whatever IENUMLIST we want our collection object to operate upon.

We'll also add a data member to our MyRealIEnumVariant. It will do the same thing as the new data member added to our MyRealICollection (i.e., hold a pointer to the IENUMLIST that our IEnumVARIANT operates upon).

Other changes are trivial, but what we're left with in PortNames.c is just a little bit of code to create and delete our PortsList, and to create a collection object specifically to wrap PortsList.

To create another list, and add support for a collection and IEnumVARIANT objects, all we need do is create another source file, much like what is in PortNames.c. In fact, let's do this.

Let's assume that we want to create a list of network cards in a system. And for each network card, we want to provide two pieces of information: the name of the network card, and its MAC address. We'll put the code in NetCards.c and NetCards.h.

We want to return more than one piece of information per item. (I.e., each IENUMITEM will pertain to a single network card. And for each network card, we want to let the script know the card's name and its MAC address.) The best way to approach this is to create a "sub-object" which we'll arbitrarily call a INetwork object. We'll put two functions in this object, Name and Address. The Name function will return a BSTR of the network card's name, and the Address function will return a BSTR of the MAC address.

We're going to create an INetwork object for each network card in the computer. Then, we're going to create an IENUMITEM in our list for that card. We'll stuff the INetwork pointer into the IENUMITEM VARIANT's punkVal field, and set the vt field to VT_DISPATCH. This list of IENUMITEMs (containing INetwork objects) is created in allocNetworkObjectsCollection (in NetCards.c).

In order to be accessible from VBScript, we'll need to include the IDispatch functions in our INetwork's VTable. Of course, this means that we'll need an ITypeInfo for our INetwork's VTable. So, we'll have to generate a new GUID for its VTable. And, we'll have to call GetTypeInfoOfGuid, passing that new GUID, to get an ITypeInfo for it. We'll save this in a global variable named NetTypeInfo. All this code is in NetCards.c. This code should look remarkably similar to what you saw with our collection and IExample5 objects, because those also have IDispatch functions and need their ITypeInfo objects.

And to make our INetwork's extra functions (i.e., Name and Address) directly callable from C/C++, we'll need to declare its VTable as "dual" in our IDL file. We'll also have to include its VTable definition in IExample5.h so a C/C++ app knows exactly what order and arguments those extra functions involve. I've added the definition of our INetwork's VTable to both IExample5.h and IExample5.idl. Note that it looks remarkably similar to our IExample5 object. Both contain IDispatch functions. Both are declared dual. Their only differences are in their extra functions. But like our IEnumVARIANT object, our INetwork object itself doesn't need to be declared in our IDL. Only its VTable needs to be declared. After all, our INetwork is going to look like a standard IDispatch object to an app, except that its extra functions will be in its VTable, and a C/C++ app can call them directly.

By giving our IENUMLIST a count field, we can determine when all the collection and IEnumVARIANT objects are done with its list, and therefore delete the list whenever we want. (I.e., unlike the previous example, we aren't restricted to deleting the list only when our DLL terminates.) In fact, you'll notice that we don't actually create our list of INetwork objects until an app actually asks us for our collection of network cards. And then, we delete the list of INetwork objects as soon as the last collection/IEnumVARIANT using that list is Release()d.

In the IExample5 directory is a VBScript example that does a For Each loop to gain access to each one of our INetwork objects, and then calls the Name and Address functions to display the card's name and MAC address.

Add/Remove items

Sometimes, we may want to give a script/app the ability to add or remove items from our list. The traditional approach for this is to put Add and Remove functions in our collection object. Because each list may contain different kinds of items, requiring different kinds of data, you'll have to define another collection object specific to a particular list. You'll define its VTable, and put these extra two functions (Add and Remove) in it. The Add function will have to be written so that the script/app can pass in whatever data is needed to create the new item. (And, the function may first search the list to see if there's already a matching item, so that duplicate items are avoided). The Remove function should be passed some arg(s) that allow you to locate the desired item to delete.

Of course, you'll need to generate a GUID for this new collection's VTable. And, you'll need to create an ITypeInfo for it by passing its GUID to GetTypeInfoOfGuid, and storing the ITypeInfo in a global that is used by Invoke, GetTypeInfo, and GetIDsOfNames.

The good news is that you can use many of the same functions (in IEnumVariant.c) that our original collection (ICollection) and IEnumVARIANT uses. So, there is not as much new code to write as you may expect.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Jeff Glatt
United States United States
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Comments and Discussions

QuestionIn the IExample4, function "next" may less some sentences Pin
ligand27-Jan-13 14:59
Memberligand27-Jan-13 14:59 
GeneralMy vote of 5 Pin
yao200725-Nov-10 22:00
Memberyao200725-Nov-10 22:00 
GeneralMy vote of 5 Pin
yao200722-Nov-10 23:12
Memberyao200722-Nov-10 23:12 
GeneralTrivial typos Pin
Hou Tianze10-Dec-09 16:08
MemberHou Tianze10-Dec-09 16:08 
GeneralNothing of examples works properly Pin
Member 166982026-May-09 2:10
MemberMember 166982026-May-09 2:10 
GeneralRe: Nothing of examples works properly Pin
Hou Tianze10-Dec-09 16:12
MemberHou Tianze10-Dec-09 16:12 
GeneralLink error:error LNK2005: _IID_ICollection already defined in IExample3.obj Pin
Member 429892926-Aug-08 2:11
MemberMember 429892926-Aug-08 2:11 
GeneralRe: I solve this problem Pin
Member 429892926-Aug-08 20:33
MemberMember 429892926-Aug-08 20:33 
GeneralIExample4 lost something Pin
scz30-Aug-07 22:04
Memberscz30-Aug-07 22:04 
GeneralRe: IExample4 lost something Pin
while loop29-Jul-15 19:10
Memberwhile loop29-Jul-15 19:10 
GeneralAbout of freeCollectionTypeInfo Pin
Black.Stone21-Aug-06 22:52
MemberBlack.Stone21-Aug-06 22:52 
GeneralRe: About of freeCollectionTypeInfo Pin
Jeff Glatt22-Aug-06 8:44
MemberJeff Glatt22-Aug-06 8:44 
GeneralDefinition of a collection Pin
Vlad Vissoultchev8-May-06 2:09
MemberVlad Vissoultchev8-May-06 2:09 
GeneralRe: Definition of a collection Pin
Jeff Glatt8-May-06 7:20
MemberJeff Glatt8-May-06 7:20 
Yes, I do know that there is no requirement for Add or Remove (and I never described them as "required" in my article). I never mentioned a Clear method at all. As far as the other comments:

I refer you to[^]

Note the URL. And pertinent excerpts from the article:

"The simplest standard way to access the data [in a collection] is to expose a property called Count and a method called Item."

"Collections that are compatible with For Each must have a method called _NewEnum that has the dispid DISPID_NEWENUM. Note that the leading underscore in the name _NewEnum indicates that the method should be hidden from object browsers."

Here's another URL (and again note where it comes from):[^]

And pertinent quotes:

"Collection objects have a Count property that allows users to determine the number of items in a collection. You should provide your collection object with this property."

"Collection objects have an Item(Index) property."

"In order to support [a For Each loop], a collection object needs to have a hidden method that is defined in IDL as follows:

[id(0xfffffffc)] HRESULT _NewEnum([out, retval] IUnknown** ppunk);

The method name must be '_NewEnum'."

Here's an except about ATL collections. Granted, an ATL collection is not necessarily a COM collection, but since ATL is very, very often used to create COM components, it does tend to speak to the design of a COM collection.[^]

"At a minimum, collection interfaces must provide a Count property that returns the number of items in the collection, an Item property that returns an item from the collection based on an index, and a _NewEnum property that returns an enumerator for the collection."

I did a search and did not come up with any MSDN document about COM collections that supports your assertations. In fact, as shown above, there is MSDN documentation that contradicts such, and supports what I stated in my article. Now, it certainly may be the case that there are script engines and applications out there whose only requirement of a COM Collection is that it meet your minimum requirements. (After all, why would an engine need both a collection's Item function as well as an IEnumVARIANT's Next?) And there may certainly be Microsoft components that adhere only to those minimum standards. But MSDN documentation confirms that your minimum requirements are not the "standard" design, and does not state the opposite.

Ironically, it seems that you've come up with more examples where Microsoft is schizophrenic when it comes to COM software design (one group states something should be done as so, and another group does exactly the opposite. That's bound to happen at such a large software house).

I therefore do stand by my comments/complaints, and disagree that they are "baseless".
GeneralRe: Definition of a collection Pin
Vlad Vissoultchev9-May-06 4:40
MemberVlad Vissoultchev9-May-06 4:40 

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