When you have a SAFEARRAY, you need to know what it is a SAFEARRAY *of*

Raymond Chen

A customer had a problem with SAFEARRAY, or more specifically, with CComSafeArray.

CComSafeArray<VARIANT> sa;
GetAwesomeArray(&sa);

LONG lb = sa.GetLowerBound();
LONG ub = sa.GetUpperBound();
for (LONG i = lb; i <= ub; i++) {
 CComVariant item = sa.GetAt(i);
 … use the item …
}

The GetAt method returns a VARIANT&, and when it is copy-constructed into item, the DISP_E_BAD­VAR­TYPE exception is raised.

On the other hand, if the offending line is changed to

CComQIPtr<IAwesome> pAwesome = sa.GetAt(i).punkVal;

then the problem goes away.

Your initial reaction to this code would be that there is an off-by-one error in the loop control, but it turns out that there isn’t because SAFEARRAY uses inclusive upper bounds rather than exclusive.

The first step in debugging this is seeing what is in the bad variant that makes the copy constructor think it’s not a valid variant type.

Inspecting in the debugger shows that the variant returned by GetAt has a valid punk, but the vt is 0x1234. Well, that’s not a valid variant type, so that’s the proximate cause of the problem.

How did an invalid variant type get into your SAFEARRAY?

At this point the customer realized that maybe their code to create the array was faulty, so they offered to share it.

void GetAwesomeArray(SAFEARRAY **ppsa)
{
 SAFEARRAY *psa = SafeArrayCreateVector(VT_UNKNOWN, 0, m_count);
 for (LONG i = 0; i < m_count; i++) {
  CComPtr<IAwesome> spAwesome;
  CreateAwesomeThing(i, &spAwesome);
  SafeArrayPutElement(psa, &i, spAwesome);
 }
 *ppsa = psa;
}

Okay, now all the pieces fell into place.

The Get­Awesome­Array function is creating an array of VT_UNKNOWN, but the code fragment that calls Get­Awesome­Array treats it as an array of VT_VARIANT.

Your array of IUnknown* is being misinterpreted as an array of VARIANT. That explains all the symptoms: The vt is wrong, because it’s really just the low-order word of the first IUnknown*. Ignoring the vt and going straight for the punk seems to work because that’s where the second IUnknown* happens to be. (Or third, if you are compiling as 32-bit.)

In other words, it’s as if you did a reinterpret_cast<VARIANT&>(punkArray[0]).

If you had used regular C-style arrays or a C++ collection, then the compile-time type checking would have told you that you mismatched the producer and consumer. But since you went through a SAFEARRAY, that compile-time type information is lost, since a SAFEARRAY is a polymorphic array. It now becomes your job to keep track of what you have an array of, and its dimensions and bounds.

You can keep track of this information via documentation, “This function returns a 1-dimensional SAFEARRAY of VT_IUNKNOWN, with lower bound 0 and variable upper bound.” Or you can check at runtime, by calling Safe­Array­Get­Vartype to see what the base type is, and Safe­Get­Dim to see how many dimensions the array has, and Safe­Array­Get­LBound and Safe­Array­Get­UBound to obtain the upper and lower bounds for those dimensions.

The code above seemed not to be sure which model it wanted to use. It trusted the base type and the dimension, but checked the upper and lower bounds.

Anyway, assuming we are going with the “keep track via documentation” approach, the solution for the original problem is to have the producer and consumer agree on exactly what kind of SAFEARRAY is being handed around. Either produce an array of VT_UNKNOWN and consume it as a CComSafeArray<IUnknown*> or produce an array of VT_VARIANT and consume it as a CComSafeArray<VARIANT>.

Raymond Chen

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