Windows Thumbnail Handler Never Gets Called - windows

Question

I'm writing a thumbnail handler for a custom file type. When I register it, it returns with a success message. The "thumbnail" is shown as completely blank, but it does not say "no thumbnail available". The problem is, the actual thumbnail handler never gets called. I know this, because I put a series of statements in my handler under handler::initialize, handler::queryinterface and handler::getthumbnail. I have gone through most of this documentation., but all I can find is that I need to use initialize and getthumbnail. Here is my code:
Header:
#pragma once
#include <windows.h>
#include <thumbcache.h> // For IThumbnailProvider
#include <wincodec.h> // Windows Imaging Codecs
#include <fstream>
#include <iostream>
#pragma comment(lib, "windowscodecs.lib")
class ThumbnailProvider :
public IInitializeWithStream,
public IThumbnailProvider
{
public:
// IUnknown
IFACEMETHODIMP QueryInterface(REFIID riid, void **ppv);
IFACEMETHODIMP_(ULONG) AddRef();
IFACEMETHODIMP_(ULONG) Release();
// IInitializeWithStream
IFACEMETHODIMP Initialize(IStream *pStream, DWORD grfMode);
// IThumbnailProvider
IFACEMETHODIMP GetThumbnail(UINT cx, HBITMAP *phbmp, WTS_ALPHATYPE *pdwAlpha);
ThumbnailProvider();
protected:
~ ThumbnailProvider();
private:
// Reference count of component.
long m_cRef;
// Provided during initialization.
IStream *m_pStream;
std::ofstream output;
void stripImageFrom (IStream *stream, HBITMAP *phbmp);
};
Body:
#include " ThumbnailProvider.h"
#include <Shlwapi.h>
#include <Wincrypt.h> // For CryptStringToBinary.
#include <msxml6.h>
#include <atlimage.h>
#include <fstream>
#pragma comment(lib, "Shlwapi.lib")
#pragma comment(lib, "Crypt32.lib")
#pragma comment(lib, "msxml6.lib")
extern HINSTANCE g_hInst;
extern long g_cDllRef;
ThumbnailProvider:: ThumbnailProvider() : m_cRef(1), m_pStream(NULL)
{
std::ofstream st;
st.open("C:\\Users\\labs\\Desktop\\Output\\out.txt", std::ios_base::app);
st << "Made provider";
st.close();
InterlockedIncrement(&g_cDllRef);
}
ThumbnailProvider::~ ThumbnailProvider()
{
InterlockedDecrement(&g_cDllRef);
}
#pragma region IUnknown
// Query to the interface the component supported.
IFACEMETHODIMP ThumbnailProvider::QueryInterface(REFIID riid, void **ppv)
{
std::ofstream st;
st.open("C:\\Users\\labs\\Desktop\\Output\\out.txt", std::ios_base::app);
st << "Querying interface";
st.close();
static const QITAB qit[] =
{
QITABENT( ThumbnailProvider, IThumbnailProvider),
QITABENT( ThumbnailProvider, IInitializeWithStream),
{ 0 },
};
return QISearch(this, qit, riid, ppv);
}
// Increase the reference count for an interface on an object.
IFACEMETHODIMP_(ULONG) ThumbnailProvider::AddRef()
{
return InterlockedIncrement(&m_cRef);
}
// Decrease the reference count for an interface on an object.
IFACEMETHODIMP_(ULONG) ThumbnailProvider::Release()
{
ULONG cRef = InterlockedDecrement(&m_cRef);
if (0 == cRef)
{
delete this;
}
return cRef;
}
#pragma endregion
#pragma region IInitializeWithStream
// Initializes the thumbnail handler with a stream.
IFACEMETHODIMP ThumbnailProvider::Initialize(IStream *pStream, DWORD grfMode)
{
std::ofstream st;
st.open("C:\\Users\\labs\\Desktop\\Output\\out.txt", std::ios_base::app);
st << "Got to initialization";
st.close();
// A handler instance should be initialized only once in its lifetime.
HRESULT hr = HRESULT_FROM_WIN32(ERROR_ALREADY_INITIALIZED);
if (m_pStream == NULL)
{
// Take a reference to the stream if it has not been initialized yet.
hr = pStream->QueryInterface(&m_pStream);
}
return hr;
}
#pragma endregion
#pragma region IThumbnailProvider
// Gets a thumbnail image and alpha type. The GetThumbnail is called with the
// largest desired size of the image, in pixels. Although the parameter is
// called cx, this is used as the maximum size of both the x and y dimensions.
// If the retrieved thumbnail is not square, then the longer axis is limited
// by cx and the aspect ratio of the original image respected. On exit,
// GetThumbnail provides a handle to the retrieved image. It also provides a
// value that indicates the color at of the image and whether it has
// valid alpha in ation.
IFACEMETHODIMP ThumbnailProvider::GetThumbnail(UINT cx, HBITMAP *phbmp,
WTS_ALPHATYPE *pdwAlpha) {
std::ofstream st;
st.open("C:\\Users\\labs\\Desktop\\Output\\out.txt", std::ios_base::app);
st << "Getting thumbnail";
st.close();
ThumbnailProvider::stripImageFrom (m_pStream, phbmp);
cx = 1024 * 1024;
*pdwAlpha = WTSAT_UNKNOWN;
return S_OK;
}
#pragma endregion
#pragma region Helper Functions
// The PNG signature is 137 80 78 71 13 10 26 10. This does not make sense to do backward. I will do it forward.
void ThumbnailProvider::stripImageFrom (IStream *stream, HBITMAP *phbmp) {
unsigned long numBytes = 0;
unsigned long *numBytesPtr = &numBytes;
char *chptr = nullptr;
byte vals[8] = { 0 };
STATSTG *stat = nullptr;
DWORD temp = NULL;
stream->Stat(stat, temp);
unsigned long long length = stat->cbSize.QuadPart;
unsigned long long i;
for (i = 0; i<length; i++) {
stream->Read(chptr, 1, numBytesPtr);
if (*chptr == 137) {
vals[0] = 1;
}
else if (*chptr == 80 && vals[0]) {
vals[1] = 1;
}
else if (*chptr == 78 && vals[1]) {
vals[2] = 1;
}
else if (*chptr == 71 && vals[2]) {
vals[3] = 1;
}
else if (*chptr == 13 && vals[3]) {
vals[4] = 1;
}
else if (*chptr == 10 && vals[4] && !vals[5]) {
vals[5] = 1;
}
else if (*chptr == 26 && vals[5]) {
vals[6] = 1;
}
else if (*chptr == 10 && vals[6]) {
vals[7] = 1;
i -= 7;
break;
}
else {
memset(vals, 0, 8 * sizeof(vals[0]));
}
}
if (vals[7]) {
IStream *imgstream = nullptr;
stream->Read(imgstream, length-i, numBytesPtr);
CImage *img = nullptr;
img->Load(imgstream);
*phbmp = *img;
}
}
#pragma endregion
Everything else is just edited from one of Microsoft's examples, so I am pretty sure that is ok. The example is here.

Answer 1

If you're having this problem, make sure you're building for the right architecture. That solved it for me.