I'm working on retrieving some info from windows task scheduler.
MSDN indicates there are several types of action. I want to deal with them separately. I tried:
IAction* pAction = NULL;
pActionCollection->get_Item(_variant_t(i), &pAction);
if (IExecAction* pExecAction = dynamic_cast<IExecAction*>(pAction)) { /*my work...*/ }
if (IComHandlerAction* pComHandlerAction = dynamic_cast<IComHandlerAction*>(pAction)) { /*my work...*/ }
if (IEmailAction* pEmailAction = dynamic_cast<IEmailAction*>(pAction)) { /*my work...*/ }
if (IShowMessageAction* pShowMessageAction = dynamic_cast<IShowMessageAction*>(pAction)) { /*my work...*/ }
But this program throws exception at the first dynamic_cast.
Exception thrown at 0x00007FFB516365A5 (vcruntime140d.dll) in myProgram.exe: 0xC0000005: Access violation reading location 0x00000130BAFEDB04.
The definition in taskschd.h shows IExecAction is a derived class from IAction.
This works well:
if (IExecAction* pExecAction = ((IExecAction*)pAction)) { /*my work...*/ }
But what if I want to do some type checking?
How could I use it properly?
for get pointer of com interface from another com interface on the same object we need use only QueryInterface method, which is always implemented by any interface. so code in your case need to be next:
IAction* pAction;
IExecAction* pExecAction;
IEmailAction* pEmailAction;
HRESULT hr;
if (SUCCEEDED(hr = pAction->QueryInterface(IID_PPV_ARGS(&pExecAction))))
{
// use pExecAction
pExecAction->Release();
}
if (SUCCEEDED(hr = pAction->QueryInterface(IID_PPV_ARGS(&pEmailAction))))
{
// use pExecAction
pEmailAction->Release();
}
even if one interface inherit from another use c/c++ cast is always wrong. for example
pExecAction = static_cast<IExecAction*>(pAction);
pEmailAction = static_cast<IEmailAction*>(pAction);
this code is correct from c++ syntax, because both IExecAction : IAction and IEmailAction : IAction inherit from IAction. and this cast (if take to account layout of this 3 interfaces) give you equal binary values for pExecAction and pEmailAction. but pExecAction can not have the same binary value as pEmailAction. must be
assert((void*)pEmailAction != (void*)pExecAction);
why ? because have pEmailAction and pExecAction have different virtual functions at the same position in vtable. for example on the 10-th position in the table of IExecAction must be pointer to get_Path method. from another side on the 10-th position in the table of IEmailAction must be pointer to get_Server method. if (void*)pEmailAction == (void*)pExecAction - they will be have the same pointers to vtable. but pointer to which function - get_Path or get_Server will be in the 10-th position ? as result pointer to this 2 interfaces can not be the same (point to the same memory). so how minimum one static_cast here (may be and both) give wrong result. for understand how QueryInterface work and why pointers to pExecAction and pEmailAction wiil be different - we need look for implementation. implementation of interfaces - this is some class, which inherit (usually) from all this interfaces and implement it like this:
class CAction : IExecAction, IEmailAction
{
virtual ULONG STDMETHODCALLTYPE AddRef( );
virtual HRESULT STDMETHODCALLTYPE QueryInterface( REFIID riid, void **ppvObject)
{
PVOID pvObject;
if (riid == __uuidof(IAction))
{
pvObject = static_cast<IExecAction*>(this);
// or can be
pvObject = static_cast<IEmailAction*>(this);
}
else if (riid == __uuidof(IExecAction))
{
pvObject = static_cast<IExecAction*>(this);
}
else if (riid == __uuidof(IEmailAction))
{
pvObject = static_cast<IExecAction*>(this);
}
else
{
*ppvObject = 0;
return E_NOINTERFACE;
}
*ppvObject = pvObject;
AddRef();
return S_OK;
}
};
look that static_cast<IExecAction*>(this); will always give another binary value compare static_cast<IEmailAction*>(this); - the CAction will be containing 2 different vtables - one for IExecAction and one for IEmailAction. they have common initial part (9 entries) but then different. and static_cast<IExecAction*>(this); and static_cast<IEmailAction*>(this); return 2 different (always) pointers to this 2 different vtables. when for IAction* we select return or first or second vtable pointer. both will be correct. what pointer return implementation - we can not know (the layout of actual class which implement IExecAction, IEmailAction is unknown for us)
Related
I have been poking around with WRL at the ABI layer for the last couple of weeks and have run into this problem.
I have an interface defined in IDL as follows:
namespace Async{
[uuid(f469e110-7ef5-41df-a237-9ddef9aed55c), version(1.0)]
interface IDownloader : IInspectable
{
HRESULT GetFeed([in] HSTRING url,[out, retval] Windows.Web.Syndication.SyndicationFeed ** feed);
[propget]HRESULT Feed([out, retval]Windows.Web.Syndication.SyndicationFeed ** feed);
}
[version(1.0), activatable(1.0)]
runtimeclass Downloader
{
[default] interface IDownloader;
}
}
Which I have defined in my header file like so:
#pragma once
#include "Async_h.h"
namespace ABI {
namespace Async {
class Downloader : public Microsoft::WRL::RuntimeClass<ABI::Async::IDownloader>
{
InspectableClass(L"Async.Downloader", BaseTrust);
public:
Downloader();
STDMETHOD(GetFeed)(HSTRING url, ABI::Windows::Web::Syndication::ISyndicationFeed ** feed);
STDMETHOD(get_Feed)(ABI::Windows::Web::Syndication::ISyndicationFeed ** feed);
private:
//Microsoft::WRL::ComPtr<ABI::Windows::Foundation::Uri> feedUrl;
Microsoft::WRL::ComPtr<ABI::Windows::Web::Syndication::ISyndicationFeed> m_feed;
};
ActivatableClass(Downloader);
}
}
In my cpp file I implement the functions:
STDMETHODIMP Downloader::GetFeed(HSTRING url, ISyndicationFeed** feed)
{
HRESULT hr;
RoInitializeWrapper ro(RO_INIT_MULTITHREADED);
ComPtr<IUriRuntimeClass> uri;
ComPtr<IUriRuntimeClassFactory> uriFactory;
hr = GetActivationFactory(HStringReference(RuntimeClass_Windows_Foundation_Uri).Get(), &uriFactory);
hr = uriFactory->CreateUri(url, uri.GetAddressOf());
ComPtr<ISyndicationClient> client;
ComPtr<IInspectable> inspectable;
RoActivateInstance(HStringReference(RuntimeClass_Windows_Web_Syndication_SyndicationClient).Get(), &inspectable);
hr = inspectable.As(&client);
Event timerCompleted(CreateEventEx(nullptr, nullptr, CREATE_EVENT_MANUAL_RESET, WRITE_OWNER | EVENT_ALL_ACCESS));
auto callback = Callback<IAsyncOperationWithProgressCompletedHandler<SyndicationFeed*,RetrievalProgress>>([&](IAsyncOperationWithProgress<SyndicationFeed*,RetrievalProgress> *op, AsyncStatus status) ->HRESULT
{
auto error = GetLastError();
if (status == AsyncStatus::Completed)
{
hr = op->GetResults(m_feed.GetAddressOf());
*feed = m_feed.Get();
}
return S_OK;
});
ComPtr<IAsyncOperationWithProgress<SyndicationFeed*,RetrievalProgress>> operation;
hr = client->RetrieveFeedAsync(uri.Get(), operation.GetAddressOf());
operation->put_Completed(callback.Get());
return S_OK;
}
STDMETHODIMP Downloader::get_Feed(ISyndicationFeed** feed)
{
*feed = m_feed.Get();
return S_OK;
}
The property works as expected it is projected to c++/cx as it should be. However,in the GetFeed method, when I attempt to set the feed parameter to the retrieved feed I get an access violation. Obviously I know that the memory is bad but the way I understand COM properties, they are essentially function calls and the property method and the GetFeed method are doing exactly the same thing minus the retrieval part.
Here are my questions:
What is the difference between COM property methods and regular interface methods in terms of the projected return value if any?
Why is the parameter to the property method initialized to nullptr and the parameter to the GetFeed Method not when they are described exactly the same in IDL?
If the out parameters in property methods are initialized, what part of the COM runtime is doing that for me and is that controllable? IE is there a way to get memory that I can write to passed to me?
I know that I could probably design that away but that is not the point. I am just trying to learn how it all works.
Thanks.
In your lambda you are capturing by reference with [&]. You need to capture the feed parameter by value, since the stack frame is long gone by the time your lambda executes.
The bigger issue is that the client has no idea when they can retrieve the results since you don't provide that information. (I see you create an unused Win32 Event object, so maybe there's some other code to make that work that you've deleted).
This should be quite simple. ID2D1LinearGradientBrush derives from ID2D1Brush with a valid vtable. I realize QueryInterface would work here, however my question pertains to dynamic_cast.
[definition from d2d1.h]
interface DX_DECLARE_INTERFACE("2cd906ab-12e2-11dc-9fed-001143a055f9")
ID2D1LinearGradientBrush : public ID2D1Brush
{
// ....
}
However, given the over-simplified example functions ...
bool ClampToItem(ID2D1Brush *brush, SizeF itemSize)
{
// As expected, works when a linear gradient brush is the parameter.
ID2D1LinearGradientBrush *linearGradientBrush = static_cast<ID2D1LinearGradientBrush *>(brush);
linearGradientBrush->SetStartPoint(D2D1_POINT_2F{ itemSize.Width, 0.0f });
linearGradientBrush->SetEndPoint(D2D1_POINT_2F{ itemSize.Width, itemSize.Height });
return true;
}
bool ClampToItem2(ID2D1Brush *brush, SizeF itemSize)
{
// this dynamic cast FAILS EVERY TIME with an access violation
ID2D1LinearGradientBrush *linearGradientBrush = dynamic_cast<ID2D1LinearGradientBrush *>(brush);
if (!linearGradientBrush) // <-- never gets here
return false;
linearGradientBrush->SetStartPoint(D2D1_POINT_2F{ itemSize.Width, 0.0f });
linearGradientBrush->SetEndPoint(D2D1_POINT_2F{ itemSize.Width, itemSize.Height });
return true;
}
Since I will not be certain that a ID2D1LinearGradientBrush will be provided as a parameter, I would like to use a dynamic_cast. I must be missing something simple. Do these COM objects not contain RTTI info? Thank you for your help.
// For clarification, this works as expected
bool ClampToItem3(ID2D1Brush *brush, SizeF itemSize)
{
ID2D1LinearGradientBrush *linearGradientBrush;
HRESULT hr = brush->QueryInterface(__uuidof(ID2D1LinearGradientBrush), (void **)&linearGradientBrush);
if (hr == S_OK)
{
linearGradientBrush->SetStartPoint(D2D1_POINT_2F{ itemSize.Width, 0.0f });
linearGradientBrush->SetEndPoint(D2D1_POINT_2F{ itemSize.Width, itemSize.Height });
linearGradientBrush->Release();
linearGradientBrush = nullptr;
}
return true;
}
EDIT:
Tracing into the dynamic cast (into rtti.cpp):
extern "C" PVOID __CLRCALL_OR_CDECL __RTDynamicCast(
PVOID inptr, // Pointer to polymorphic object
LONG VfDelta, // Offset of vfptr in object
PVOID SrcType, // Static type of object pointed to by inptr
PVOID TargetType, // Desired result of cast
BOOL isReference) // TRUE if input is reference, FALSE if input is ptr
throw(...)
is called, inptr is valid, vfDelta is 0, SrcType and TargetType look great, isRef says false.
Tracing further, the memory access violation occurs here:
// Ptr to CompleteObjectLocator should be stored at vfptr[-1]
_RTTICompleteObjectLocator *pCompleteLocator =
(_RTTICompleteObjectLocator *) ((*((void***)inptr))[-1]);
char *pCompleteObject = (char *)inptr - COL_OFFSET(*pCompleteLocator);
since *pCompleteLocator was moved to an invalid location.
Standard COM interfaces from Microsoft use __declspec(novtable) as part of their definition; if you look at the definition of DX_DECLARE_INTERFACE you'll see this is the case here. This means that the base interface classes don't have vtables, only the concrete implementation has one. RTTI doesn't have the necessary information for dynamic_cast to work properly.
For COM interfaces you should always use QueryInterface to do dynamic casting.
If I have two COM interface pointers (i.e. ID3D11Texture2D), and I want to check if they are the same underlying class instance, can I compare the two pointers directly for equality? I have seen code where we cast it to something else before the comparison is done, so wanted to confirm.
BOOL IsEqual (ID3D11Texture2D *pTexture1, ID3D11Texture2D *pTexture2)
{
if (pTexture1 == pTexture2)
{
return true;
}
else
{
return false;
}
}
Thanks.
The correct COM way to do this is to query interface with IUnknown. A quote from the remarks here in MSDN:
For any one object, a specific query for the IUnknown interface on any
of the object's interfaces must always return the same pointer value.
This enables a client to determine whether two pointers point to the
same component by calling QueryInterface with IID_IUnknown and
comparing the results. It is specifically not the case that queries
for interfaces other than IUnknown (even the same interface through
the same pointer) must return the same pointer value.
So the correct code is
BOOL IsEqual (ID3D11Texture2D *pTexture1, ID3D11Texture2D *pTexture2)
{
IUnknown *u1, *u2;
pTexture1->QueryInterface(IID_IUnknown, &u1);
pTexture2->QueryInterface(IID_IUnknown, &u2);
BOOL areSame = u1 == u2;
u1->Release();
u2->Release();
return areSame;
}
Update
Added a call to Release so decrease reference counts. Thanks for the good comments.
You can also use ComPtr for this job. Please look in MSDN.
I am trying to specify a vector in an IDL specification of property getter.
I am new to C++ so bear with me. The object IThis is a hypothetical object of anything.
I am getting a compile error at the id declaration expecting a type specifier. Thank you very much!!!!!!!!!!!!!!!
STDMETHOD(get_ThisList)(vector<IThis*> *value)
{
*value = mThisList;
return S_OK;
}
vector<IThis*> mThisList;
[propget] IDLAPI ThisList([out,retval] vector<IThis*>* value);
You can't use vector in a COM interface, with IDL or otherwise. You could return a safearray of interface pointers; it would look like this:
// In IDL
[propget]
HRESULT ThisList([out,retval] SAFEARRAY(IThis*)* value);
// In C++
HRESULT get_ThisList(SAFEARRAY** value);
Other alternatives include a conformant array (though this is inadvisable for an automation interface, as yours appears to be), and a separate collection object that represents a list of objects.
An implementation for get_ThisList might look something like this:
STDMETHODIMP MyObject::get_ThisList(SAFEARRAY** value) {
if (!value) return E_POINTER;
SAFEARRAYBOUND bound = {mThisList.size(), 0};
*value = SafeArrayCreate(VT_UNKNOWN, 1, &bound);
IUnknown** data;
SafeArrayAccessData(*value, (void**)&data);
for (int i = 0; i < mThisList.size(); ++i) {
(data[i] = mThisList[i])->AddRef();
}
SafeArrayUnaccessData(*value);
return S_OK;
}
Error handling is left as an exercise for the reader.
std::vector is a C++ class; IDL (interface definition language - different language) has no concept of it.
"expecting a type specifier" means the IDL compiler doesn't recognize std::vector as a type.
You will have to return a pointer to array of IThis as the return value, and put it into a smarter object at the caller's site.
I came across a problem in my code today where an access violation was being caused, AFAICT, by casting a COM object of mine to an IUnknown**. The function it was passed into executed without a problem but when calling one of my object's functions it would execute some random function and corrupt the stack then die.
Indicative code (just ignore why it's done this way - I know it's bad and I know how to fix it but this is a question of why problems like this may occur):
void MyClass2::func(IMyInterface* pMyObj)
{
CComPtr<IMyInterface2> pMyObj2;
HRESULT hRes = pMyObj->GetInternalObject((IUnknown**)&pMyObj2);
if (SUCCEEDED(hRes))
pMyObj2->Function(); // corrupt stack
}
void MyClass::GetInternalObject(IUnknown** lpUnknown)
{
pInternalObject->QueryInterface(IID_IMyInterface2, (void**)lpUnknown);
}
I have always been a bit suspicious of using C/C++ casts on COM objects but I've never encountered (possibly through undefined behaviour) any problems until now.
I had a quick look and from what I can tell casting to IUnknown is technically valid so long as there is no multiple interitance in the inheritance chain, however it is not considered best practice - I should really pass an IUnknown to MyClass::GetInternalObject(IUnknown** lpUnknown) and then query the return value for the interface I want.
My question is, are there rules as to when C/C++ casts can be used on COM objects, and aside from multiple inheritance and the adjustor thunks they bring, how can casting COM objects result in surprises like access violations? Please be detailed.
Edit: They're all good examples of how it should be done properly but what I was hoping for was a technical explanation of why you shouldn't cast COM objects (assuming one exists) e.g. casting will return pMyObj2-4 in situation x but QueryInterface will return pMyObj2-8 because of y...or is casting COM objects simply a matter of bad practice/style?
TIA
I'd just use CComPtr and CComQIPtr to manage COM interfaces, instead of writing code with C-style casts that to me seem inappropriate in the context of COM:
void MyClass2::Func(IMyInterface* pMyObj)
{
// Assuming:
// HRESULT IMyInterface::GetInternalObject( /* [out] */ IUnknown** )
CComPtr<IUnknown> spUnk;
HRESULT hr = pMyObj->GetInternalObject(&spUnk);
if (SUCCEEDED(hr))
{
// Get IMyInterface2 via proper QueryInterface() call.
CComQIPtr<IMyInterface2> spMyObj2( spUnk );
if ( spMyObj2 )
{
// QueryInterface() succeeded
spMyObj2->Function();
}
}
}
Moreover, I'm not a COM expert, but I see with suspicion your code:
void MyClass::GetInternalObject(IUnknown** lpUnknown)
{
pInternalObject->QueryInterface(IID_IMyInterface2, (void**)lpUnknown);
}
If you are QueryInterface()'ing IID_MyInterface2, you should store that in an IMyInterface2*, not in an IUnknown*.
If your method returns an IUnknown*, then I'd QueryInterface() an IID_IUnknown:
// NOTE on naming convention: your "lpUnknown" is confusing.
// Since it's a double indirection pointer, you may want to use "ppUnknown".
//
void MyClass::GetInternalObject(IUnknown** ppUnknown)
{
pInternalObject->QueryInterface(IID_IUnknown, (void**)ppUnknown);
}
or better use IID_PPV_ARGS macro:
void MyClass::GetInternalObject(IUnknown** ppUnknown)
{
IUnknown* pUnk = NULL;
HRESULT hr = pInternalObject->QueryInterface(IID_PPV_ARGS(&pUnk));
// Check hr...
// Write output parameter
*ppUnknown = pUnk;
}
COM style casts have a specific name: QueryInterface().
I think the issue is that because a cast from IMyInterface* to IUnknown* is OK (in COM everything inherits from IUknown right?) you think that a cast from IMyInterface** to IUnknown** is also OK. But that's not true in C++, and I doubt it's true in COM either.
To me the following looks more logical, apologies if this isn't strictly correct, my COM is very rusty, but hopefully you get the idea.
CComPtr<IUnknown> pMyObj2;
HRESULT hRes = pMyObj->GetInternalObject(&pMyObj2);
if (SUCCEEDED(hRes))
{
CComPtr<IMyInterface> pMyObj3 = (IMyInterface*)pMyObj2;
pMyObj3->Function();
}
I.e. get an IUnknown object first, and then down cast that to your actual type.
I don't see any issues in your code snippets, the stack corruption perhaps has its cause but its somewhere else.
I don't think it is your actual code because GetInternalObject should be of HRESULT type and yours is not, so you lost something during copy/pasting.
To stay safer, just avoid direct QueryInterface calls because together with casting they might misinterpret interfaces. Casting to and from IUnknown* might be inevitable though. If the callee cannot be trusted to return proper interface casted to IUnknown, on the caller side you might prefer to QI once again to make sure you hold the interface of your interest.
Provided that GetInternalObject is a COM interface method on its own, you could have it like this:
void MyClass2::func(IMyInterface* pMyObj)
{
CComPtr<IUnknown> pMyObj2Unknown;
pMyObj->GetInternalObject((IUnknown**)&pMyObj2Unknown);
CComQIPtr<IMyInterface2> pMyObj2 = pMyObj2Unknown; // This is only needed if callee is not trusted to return you a correct pointer
if (pMyObj2)
pMyObj2->Function(); // corrupt stack
}
STDMETHODIMP MyClass::GetInternalObject(IUnknown** lpUnknown) // COM method is typically both HRESULT and __stdcall
{
CComQIPtr<IMyInterface2> pMyInterface2 = pInternalObject;
if(!pMyInterface2)
return E_NOINTERFACE;
*lpUnknown = pMyInterface2.Detach(); // *lpUnknown will have to me IMyInterface2 this way
return S_OK;
}
PS If GetInternalObject was a native method, not COM, you would avoid casting to IUnknown* at all.