Creating a System::String object from a BSTR in Managed C++ - is this way a good idea???
- by Eli
My co-worker is filling a System::String object with double-byte characters from an unmanaged library by the following method:
RFC_PARAMETER aux;
Object* target;
RFC_UNICODE_TYPE_ELEMENT* elm;
elm = &(m_coreObject->m_pStructMeta->m_typeElements[index]);
aux.name = NULL;
aux.nlen = 0;
aux.type = elm->type;
aux.leng = elm->c2_length;
aux.addr = m_coreObject->m_rfcWa + elm->c2_offset;
GlobalFunctions::CreateObjectForRFCField(target,aux,elm->decimals);
GlobalFunctions::ReadRFCField(target,aux,elm->decimals);
Where GlobalFunctions::CreateObjectForRFCField creates a System::String object filled with spaces (for padding) to what the unmanaged library states the max length should be:
static void CreateObjectForRFCField(Object*& object, RFC_PARAMETER& par, unsigned dec)
{
switch (par.type)
{
case TYPC:
object = new String(' ',par.leng / sizeof(_TCHAR));
break;
// unimportant afterwards.
}
}
And GlobalFunctions::ReadRFCField() copies the data from the library into the created String object and preserves the space padding:
static void ReadRFCField(String* target, RFC_PARAMETER& par)
{
int lngt;
_TCHAR* srce;
switch (par.type)
{
case TYPC:
case TYPDATE:
case TYPTIME:
case TYPNUM:
lngt = par.leng / sizeof(_TCHAR);
srce = (_TCHAR*)par.addr;
break;
case RFCTYPE_STRING:
lngt = (*(_TCHAR**)par.addr != NULL) ? (int)_tcslen(*(_TCHAR**)par.addr) : 0;
srce = *(_TCHAR**)par.addr;
break;
default:
throw new DotNet_Incomp_RFCType2;
}
if (lngt > target->Length) lngt = target->Length;
GCHandle gh = GCHandle::Alloc(target,GCHandleType::Pinned);
wchar_t* buff = reinterpret_cast<wchar_t*>(gh.AddrOfPinnedObject().ToPointer());
_wcsnset(buff,' ',target->Length);
_snwprintf(buff,lngt,_T2WFSP,srce);
gh.Free();
}
Now, on occasion, we see access violations getting thrown in the _snwprintf call. My question really is: Is it appropriate to create a string padded to a length (ideally to pre-allocate the internal buffer), and then to modify the String using GCHandle::Alloc and the mess above.
And yes, I know that System::String objects are supposed to be immutable - I'm looking for a definitive "This is WRONG and here is why".
Thanks,
Eli.
