How do I load an IStream into a TMemoryStream - delphi

Question

How do I use a TComInterface<IStream> object and read it into a TMemoryStream? Can I take TComInterface<IStream> and use it as a TStream* somehow to use the TMemoryStream LoadFromStream call?

Answer 1

One way is to Read() the IStream data into a buffer that you then Write() to the TMemoryStream, eg:
TComInterface<IStream> pIStrm;
TMemoryStream *pMStrm;
...
STATSTG stat = {0};
OleCheck(pIStrm->Stat(&stat, STATFLAG_NONAME));
LARGE_INTEGER li;
li.QuadPart = 0;
ULARGE_INTEGER ul;
OleCheck(pIStrm->Seek(li, STREAM_SEEK_CUR, &ul));
unsigned __int64 ulPos = ul.QuadPart;
BYTE buffer[1024];
while (ulPos < stat.cbSize.QuadPart)
{
ULONG ulRead;
OleCheck(pIStrm->Read(buffer, min(stat.cbSize.QuadPart - ulPos, sizeof(buffer)), &ulRead));
pMStrm->WriteBuffer(buffer, ulRead);
ulPos += ulRead;
}
...
Another option is to write a TStream-derived class that accesses the IStream internally (similar to how the RTL's TStreamAdapter class wraps a TStream so it can be passed around as an IStream), eg:
class TIStreamWrapper : public TStream
{
private:
TComInterface<IStream> pIStrm;
protected:
virtual __int64 __fastcall GetSize();
virtual void __fastcall SetSize(const __int64 NewSize);
public:
__fastcall TIStreamWrapper(IStream *Strm);
virtual int __fastcall Read(void *Buffer, int Count);
virtual int __fastcall Write(const void *Buffer, int Count);
virtual __int64 __fastcall Seek(const __int64 Offset, TSeekOrigin Origin);
};
__fastcall TIStreamWrapper::TIStreamWrapper(IStream *Strm)
: pIStrm(Strm, true)
{
}
__int64 __fastcall TIStreamWrapper::GetSize()
{
STATSTG stat = {0};
OleCheck(pIStrm->Stat(&stat, STATFLAG_NONAME));
return stat.cbSize.QuadPart;
}
void __fastcall TIStreamWrapper::SetSize(const __int64 NewSize)
{
ULARGE_INTEGER ul;
ul.QuadPart = NewSize;
OleCheck(pIStrm->SetSize(ul));
}
int __fastcall TIStreamWrapper::Read(void *Buffer, int Count)
{
ULONG ulRead;
OleCheck(pIStrm->Read(Buffer, Count, &ulRead));
return ulRead;
}
int __fastcall TIStreamWrapper::Write(const void *Buffer, int Count)
{
ULONG ulWritten;
OleCheck(pIStrm->Write(Buffer, Count, &ulWritten));
return ulWritten;
}
static const DWORD IStreamSeekOrigin[] = {STREAM_SEEK_SET, STREAM_SEEK_CUR, STREAM_SEEK_END};
__int64 __fastcall TIStreamWrapper::Seek(const __int64 Offset, TSeekOrigin Origin)
{
LARGE_INTEGER li;
li.QuadPart = Offset;
ULARGE_INTEGER ul;
OleCheck(pIStrm->Seek(li, IStreamSeekOrigin[Origin], &ul));
return ul.QuadPart;
}
TComInterface<IStream> pIStrm;
TMemoryStream *pMStrm;
...
TIStreamWrapper *pWrapper = new TIStreamWrapper(pIStrm);
try
{
pMStrm->LoadFromStream(pWrapper);
// or: pMStrm->CopyFrom(pWrapper, 0);
}
__finally
{
delete pWrapper;
}
...