Returning values C#/COM to C++ client? - c++

Question

We have an old C++ COM .dll that a customer calls from their C++ client.
We are trying to replace our old .dll with a new COM registered one written in .NET.
The C++ client can call our new .dll but crashes after a particular method call to us.
It seems we are returning something wrong in our variable "outNoOfChildren" or in the outChildList array below.
(The error message in the client reads: "Unknown exception caught.
Number of childs Measured: -2147467259 Limits: = 2", it expects 2 which we are trying to return.)
Strange thing is that the method call in our new .NET .dll seems to work from another test client we have (VB6).
This is the function in the C++ .dll that we are trying to replace:
STDMETHODIMP marcomBase::XgetChildren(VARIANT inUser,
VARIANT inId,
VARIANT *outNoOfChildren,
VARIANT *outChildList,
VARIANT *outErrMsg,
VARIANT *status)
long stat= 1;
char user[255+1];
char id[255+1];
long noOfChildren = 0;
char errMsg[255+1] = "";
_bstr_t temp_bstr;
long inparamType;
long dumInt;
SAFEARRAY *pArray;
long ix[2];
VARIANT var;
SAFEARRAYBOUND rgsabound[2];
ChildT childList;
ChildT *childListTemp = NULL;
ChildT *childListTempNext = NULL;
childList.nextChild = NULL;
getInParam( inUser, &inparamType, user, &dumInt);
if (inparamType != VT_BSTR)
{
strcpy(errMsg, "Parameter 1 incorrect, type must be VT_BSTR");
stat = 0;
}
if (stat == 1)
{
getInParam( inId, &inparamType, id, &dumInt);
if (inparamType != VT_BSTR)
{
strcpy(errMsg, "Parameter 2 incorrect, type must be VT_BSTR");
stat = 0;
}
}
if (stat == 1)
{
stat = barApiObj.getChildren(user, id, &noOfChildren, childList, errMsg);
}
outNoOfChildren->vt = VT_I4;
outNoOfChildren->lVal = noOfChildren;
rgsabound[0].lLbound = 1;
rgsabound[0].cElements = noOfChildren;
rgsabound[1].lLbound = 1;
rgsabound[1].cElements = 3;
pArray = ::SafeArrayCreate(VT_VARIANT, 2, rgsabound);
outChildList->vt = VT_ARRAY|VT_VARIANT;
outChildList->parray = pArray;
ix[0] = 1;
childListTemp = childList.nextChild;
while (childListTemp != NULL)
{
temp_bstr = childListTemp->child;
var.vt = VT_BSTR;
var.bstrVal = temp_bstr.copy();
ix[1] = 1;
::SafeArrayPutElement(pArray, ix, &var);
temp_bstr = childListTemp->prodNo;
var.vt = VT_BSTR;
var.bstrVal = temp_bstr.copy();
ix[1] = 2;
::SafeArrayPutElement(pArray, ix, &var);
temp_bstr = childListTemp->rev;
var.vt = VT_BSTR;
var.bstrVal = temp_bstr.copy();
ix[1] = 3;
::SafeArrayPutElement(pArray, ix, &var);
ix[0] = ix[0] + 1;
childListTempNext = childListTemp->nextChild;
delete childListTemp;
childListTemp = childListTempNext;
}
temp_bstr = errMsg;
outErrMsg->vt = VT_BSTR;
outErrMsg->bstrVal = temp_bstr.copy();
status->vt = VT_I4;
status->lVal = stat;
return S_OK;
}
This is the .NET .dll that we have written:
.NET Interface:
...
[DispId(12)]
[Description("method XgetChildren")]
object XgetChildren(object inUser, object inId, out object outNoOfChildren, out object outChildList, out object outErrMsg);
...
.NET Class
public object XgetChildren(object inUser, object inId, out object outNoOfChildren, out object outChildList, out object outErrMsg)
{
outNoOfChildren = 0;
outChildList = null;
outErrMsg = "";
List<IndividualInfo> children = null;
try
{
PrevasMesExternalServiceClient client = getClient();
children = client.GetChildren(new SerialNo() { Number = inId.ToString() });
}
catch (Exception ex)
{
return Constants.STATUS_FAIL;
}
int[] myLengthsArray = { children.Count, 3 }; //Length of each array
int[] myBoundsArray = { 1, 1 }; //Start index of each array
Array myArray = Array.CreateInstance(typeof(string), myLengthsArray, myBoundsArray); //Create 1-based array of arrays
for (int i = 0; i < children.Count; i++)
{
myArray.SetValue(Utils.getStrValue(children[i].SerialNumber, Constants.pmesIDNO_LENGTH), i+1, 1);
myArray.SetValue(Utils.getStrValue(children[i].ProductNumber, Constants.pmesPRODUCTNO_LENGTH), i+1, 2);
myArray.SetValue(Utils.getStrValue(children[i].Revision, Constants.pmesRSTATE_LENGTH), i+1, 3);
}
outChildList = myArray;
outNoOfChildren = children.Count;
return Constants.STATUS_OK;
}
Update:
The original C++ .dll looks like this in OLE/COM Object Viewer:
[id(0x0000000c), helpstring("method XgetChildren")]
HRESULT XgetChildren(
[in] VARIANT inUser,
[in] VARIANT inId,
[out] VARIANT* outNoOfChildren,
[out] VARIANT* outChildList,
[out] VARIANT* outErrMsg,
[out, retval] VARIANT* status);
After registering our new .NET .dll it looks like this in OLE/COM Object Viewer:
[id(0x0000000c), helpstring("method ")]
HRESULT XgetChildren(
[in] VARIANT inUser,
[in] VARIANT inId,
[out] VARIANT* outNoOfChildren,
[out] VARIANT* outChildList,
[out] VARIANT* outErrMsg,
[out, retval] VARIANT* pRetVal);
Update 2: I'm beginning to suspect the error may not be in "outNoOfChildren" but in the array "outChildList". I have updated the question and modified the code samples for this.
Any ideas much appreciated!

Answer 1

Measured: -2147467259 Limits: = 2"
That's a magic number. Convert it to hex and you get 0x80004005. That's an error code, the infamous E_FAIL or "Unspecified error". So somewhere in the dots that we can't see, you are interpreting an error return value as a number. Possibly a variant of type vtError and ignoring the variant type. That's all that can be concluded from the question, review the error handling in your code.