I have a COM interface with a following method definition (IDL notation):
SCODE GetText( [in, out] ULONG* pcwcBuffer,
[out, size_is(*pcwcBuffer)] WCHAR* awcBuffer );
Typelib marshaling is used for COM+, the type library is registered, other methods of the interface work allright when called through COM+, but not this method.
The server side copies an array of WCHARs into the awcBuffer and its length into pwcBuffer, no buffer overrun ever occurs.
static const wchar_t* Text = L"Sample";
STDMETHODIMP CImpl::GetText( ULONG* bufferLength, WCHAR* buffer )
{
const int length = wcslen( Text );
*bufferLength = length;
memcpy( buffer, Text, length * sizeof( WCHAR ) );
return S_OK;
}
When the client calls this method through COM+ the buffer contents gets lost. Specifically only the first wide char is preserved - if the server copies "Sample" wide character string, the client only receives "S" string. The return value on the client size is S_OK, the buffer length returned to the client is exactly the same as what the server copied.
I finally switched to BSTR to workaround this problem but it's really interesting why the whole valid looking construct doesn't work.
What's the possible reason of the described behaviour?
IIRC, the typelib marshaller ignores the size_is attribute -- thus, only 1 char is marshaled.
J. Passing is right. For the typelib marshaller to work, the COM interface must be OLE Automation compatible. The typelib marshaller is implemented in oleaut32.dll, so I guess there's a clue in the name.
[size_is] is perfectly valid IDL, and compiles into a valid typelib, but the typelib marshaler can only handle a subset of valid interfaces. That subset is usually referred to as OLE Automation. As an aside, VB6 clients can only speak OLE Automation, so they wouldn't be able to consume your interface either.
Try marking your interface with the [oleautomation] attribute in your IDL. It should give you a warning or error message that might point you to more information on the subject.
In "normal" COM, you could generate a proxy/stub DLL from your IDL to do the marshalling, but I'm afraid I don't remember whether COM+ would use your custom marshalling code even if you bothered to build it.
Update: in Juval Lowy's book "COM and .NET Component Services", I found this statement: "...configured components cannot use interfaces that require custom marshaling". So I guess that interface will never work in COM+. If you can, re-write to use a BSTR instead.
Couple of questions:
Why aren't you using BSTR?
Do you have the sources of the GetText function?
What is the size of the buffer returned by the function?
Related
I'm trying to marshal an interface to another thread.
Windows provides the convenient CoMarshalInterThreadInterfaceInStream helper function to take care of the boilerplate code associated with using CoMarshalInterface directly.
const Guid CLSID_Widget = "{F8383852-FCD3-11d1-A6B9-006097DF5BD4}";
const Guid IID_IWidget = "{EBBC7C04-315E-11D2-B62F-006097DF5BD4}";
//Create our widget
HRESULT hr = CoCreateInstance(CLSID_Widget, null,
CLSCTX_INPROC_SERVER | CLSCTX_LOCAL_SERVER,
IID_IWidget, out widget);
OleCheck(hr);
//Marshall the interface into an IStream
IStream stm;
hr = CoMarshalInterThreadInterfaceInStream(IID_IWidget, widget, out stm);
OleCheck(hr);
Except that the call to CoMarshalThreadInterfaceInStream fails with:
REGDB_E_IIDNOTREG (0x80040155)
Interface not registered
Go directly to CoMarshalInterface
The COM API function CoMarshalInterThreadInterfaceInStream provides a simple wrapper around CreateStreamOnHGlobal and CoMarshalInterface, as shown here:
// from OLE32.DLL (approx.)
HRESULT CoMarsha1InterThreadInterfaceInStream(
REFIID riid, IUnknown *pItf, IStream **ppStm)
{
HRESULT hr = CreateStreamOnHGlobal(0, TRUE, ppStm);
if (SUCCEEDED(hr))
hr = CoMarshalInterface(*ppStm, riid, pItf,
MSHCTX_INPROC, 0, MSHLFLAGS_NORMAL);
return hr;
}
So we can try ourselves.
IStream stm = new Stream()
hr = CoMarshallInterface(stm, IID_IWidget, widget,
MSHCTX_INPROC, // destination context is in-process/same host
NULL, // reserved, must be null
MSHLFLAGS_NORMAL // marshal once, unmarshal once
);
OleCheck(hr);
But that fails with:
REGDB_E_IIDNOTREG (0x80040155)
Interface not registered
Use standard marshaling
My class does not implement IMarhsal interface. This is right and normal.
By default, when CoMarshalInterface is first called on an object, the object is asked whether it wishes to handle its own cross-apartment communications. This question comes in the form of a QueryInterface request for the IMarshal interface. Most objects do not implement the IMarshal interface and fail this
QueryInterface request, indicating that they are perfectly happy to let COM
handle all communications via ORPC calls. Objects that do implement the
IMarshal interface are indicating that ORPC is inappropriate and that the object implementor would prefer to handle all cross-apartment communications
via a custom proxy. When an object implements the IMarshalinterface all references to the object will be custom marshaled.
When an object does not implement the IMarshal interface, all references to the object will be standard marshaled. Most objects elect to use standard marshaling.
So the the question becomes why is the standard COM marshaler having so much problems? What is the source of the error Interface not registered?
The interface is, in fact, not registered
The requires for COM are undocumented, but i can tell you that my interface GUID does not exist in:
HKEY_CLASSES_ROOT\Interface\{EBBC7C04-315E-11D2-B62F-006097DF5BD4}
The reason for this will be explained at the end.
I know that Windows provides you the CoRegisterPSClsid function that allows you to register an interface inside your process, so that the standard marshaler will be able to marshal it:
Enables a downloaded DLL to register its custom interfaces within its running process so that the marshaling code will be able to marshal those interfaces.
HRESULT CoRegisterPSClsid(
_In_ REFIID riid,
_In_ REFCLSID rclsid
);
Parameters:
riid [in]: A pointer to the IID of the interface to be registered.
rclsid [in]: A pointer to the CLSID of the DLL that contains the proxy/stub code for the custom interface specified by riid.
Which i can try calling, but what clsid do i use?
CoRegisterPSClsid(IID_IWidget, ???);
What is the CLSID of the DLL that contains the proxy/stub code for the custom interface specified by riid? Do i use my class itself?
CoRegisterPSClsid(IID_IWidget, CLSID_Widget);
That doesn't sound right; but i don't understand the COM standard marshaler well enough. Doesn't that CLSID have to be one of the standard COM marsharling classes; implementing IPSFactoryBuffer?
Either way, it doesn't work. I still get the error "Interface not registered".
Register the interface
I of course can register my interface in the registry:
HKEY_CURRENT_USER\Software\Classes\Interface\{EBBC7C04-315E-11D2-B62F-006097DF5BD4}
(default) = "IWidget"
But that doesn't fix it. Spelunking through the Interface registry keys, i notice that many specify a ProxyStubClsid32 entry.
When a new interface is requested on an object, the proxy and stub managers
must resolve the requested IID onto the CLSID of the interface marshaler.
Under Windows NT 5.0, the class store maintains these mappings in the NT
directory, and they are cached at each host machine in the local registry. The
machine-wide IID-to-CLSID mappings are cached at
HKEY_CLASSES_ROOT\Interface
and the per-user mappings are cached at
HKEY_CURRENT_USER\Software\Classes\Interface
One or both of these keys will contain a subkey for each known interface. If the interface has an interface marshaler installed, there will be an additional
subkey (ProxyStubClsid32) that indicates the CLSID of the interface
marshaler.
Except what class implements marshaling? I don't have a marshaler.
Can COM automatically marshal based on a TypeLibrary
Is it possible that if i register a Type Library with my Interface, that COM's standard marshaler will be able to bootstrap a proxy class on the fly?
I registered my interface above. Now i manually include the TypeLibrary:
HKEY_CURRENT_USER\Software\Classes\Interface\{EBBC7C04-315E-11D2-B62F-006097DF5BD4}\TypeLib
(default) = "{38D528BD-4948-4F28-8E5E-141A51090580}"
And if i monitor the registry during the call to CoMarshalInterface i see that it attempts, and does find, my Interface IID:
Operation: RegOpenKey
Path: HKCR\WOW6432Node\Interface\{EBBC7C04-315E-11D2-B62F-006097DF5BD4}
Result: SUCCESS
It then tries to look for a ProxyStubClsid32, and fails:
Operation: RegOpenKey
Path: HKCR\WOW6432Node\Interface\{668790E3-83CC-47E0-907F-A44BA9A99C8D}\ProxyStubClsid32
Result: NAME NOT FOUND
My hope would then be that the standard COM marshaler attempts to look for:
Operation: RegOpenKey
Path: HKCR\WOW6432Node\Interface\{668790E3-83CC-47E0-907F-A44BA9A99C8D}\TypeLib
But it doesn't.
The OLE Automation marshaler
According to Don Box, the Ole Automation marshaler (PSOAInterface - {00020424-0000-0000-C000-000000000046}) is able to build a stub/proxy out of a type library:
The Type Library Marshaler
When these specially annotated interfaces are encountered by RegisterTypeLib (or LoadTypeLib in legacy mode), COM adds ProxyStubClsid32 entries for the interface with the value {00020424-0000-0000-C0000-000000000046}. This GUID corresponds to the class PSOAInterface that is registered as living in OLEAUT32.DLL, the OLE automation DLL. Because of the DLL that it lives in, this marshaler is sometimes called the [oleautomation] marshaler, although it is also called the type library marshaler or the universal marshaler. I'll refer to it as the type library marshaler, since it really has very little to do with IDispatch. (In fact, it is common to use the [oleautomation] attribute on interfaces that don't derive from IDispatch directly or indirectly.)
The class factory for the type library marshaler does something very tricky in its CreateProxy and CreateStub routines. Rather than return a statically compiled vtable (which is impossible given the fact that the requested interface didn't exist when OLEAUT32.DLL was built as part of the OS), the type library marshaler actually builds an /Oicf-style proxy and stub based on the type library for the interface. Because there is no efficient way to find the ITypeInfo for an arbitrary interface, the LIBID and version of the interface's type library must be stored under the:
HKCR\Interface\{XXX}\TypeLib
registry key.
I tried to set PSOAInterface as my interface's ProxyStub class:
Register standard COM class PSOAInterface as our proxy stub clsid
HKEY_CURRENT_USER\Software\Classes\Interface\{EBBC7C04-315E-11D2-B62F-006097DF5BD4}\ProxyStubClsid32
(default) = "{00020424-0000-0000-C0000-000000000046}"
Register our type library for our interface
HKEY_CURRENT_USER\Software\Classes\Interface\{EBBC7C04-315E-11D2-B62F-006097DF5BD4}\TypeLib
(default) = "{38D528BD-4948-4F28-8E5E-141A51090580}"
Version = "1.0"
The type library itself is already registered:
HKEY_CURRENT_USER\Software\TypeLib\{38D528BD-4948-4F28-8E5E-141A51090580}\1.0\0\win32
(default) = "D:\Junk\ComLibraryProxyTest\Win32\Project1.dll"
But it still fails.
it reads HKCR\Interface\[IID_IWidget]
it reads HKCR\Interface\[IID_IWidget]\ProxyStubClsid32
But it:
never reads: HKCR\Interface\[IID_IWidget]\TypeLib
So it fails to proxy the object.
Questions
Is it possible for the standard COM "Ole Automation" marshaler to build a proxy class out of a Type Library at runtime?
Is it possible for me to build a proxy class out of a Type Library at runtime?
Background
CoMarshalInterface takes an interface pointer on input and writes the
serialized representation of the pointer to a caller-provided byte stream. This byte stream can then be passed to another apartment, where the
CoUnmarshalInterface API function uses the byte stream to return an interface
pointer that is semantically equivalent to the original object yet can be
legally accessed in the apartment that executes the CoUnmarshalInterface
call. When calling CoMarshalInterface, the caller must indicate how far away
the importing apartment is expected to be. COM defines an enumeration for
expressing this distance:
enum MSHCTX {
MSHCTX_INPROC = 4, // in-process/same host
MSHCTX_LOCAL = 0, // out-of-process/same host
MSHCTX_NOSHAREDMEM = 1, //16/32 bit/same host
MSHCTX_DIFFERENTMACHINE = 2, // off-host
};
It is legal to specify a greater distance than required, but it is more efficient
to use the correct MSHCTX when possible. CoMarshalInterface also allows
the caller to specify the marshaling semantics using the following marshal flags:
enum MSHLFLAGS {
MSHLFLAGS_NORMAL, // marshal once, unmarshal once
MSHLFLAGS_TABLESTRONG, // marshal once, unmarshal many
MSHLFLAGS_TABLEWEAK, // marshal once, unmarshal many
MSHLFlAGS_NOPING = 4 // suppress dist. garbage collection
Normal marshaling (sometimes called call marshaling) indicates that the
marshaled object reference must be unmarshaled only once, and if additional
proxies are needed, additional calls to CoMarshalInterface are required. Table
marshaling indicates that the marshaled object reference may be unmarshaled
zero or more times without requiring additional calls to CoMarshalInterface.
I think that all COM object type libraries, when compiled by MIDL, can automatically create a proxy/stub factory. But in my case:
if the COM standard marshaler can't find a proxy/stub factory for an interface, it returns the error REGDB_E_IIDNOTREG.
It may be the case that i have to either:
use CreateProxyFromTypeInfo and CreateStubFromTypeInfo to create my own proxy
let the standard COM marshaler automatically create a proxy/stub if there's a typeinfo chunk associated with the interface GUID.
Bonus Reading
Old New Thing: What are the rules for CoMarshalInterThreadInterfaceInStream and CoGetInterfaceAndReleaseStream?
Old New Thing: Why do I get the error REGDB_E_IIDNOTREG when I call a method that returns an interface?
Old New Thing: Why do I get E_NOINTERFACE when creating an object that supports that interface?
Don Box - MSJ: Standard Marshalling in COM (archive)
Mysterious disappearing acts by samples from Microsoft SDKs
What CoMarshalInterface actually needs is IMarshal implementation (pointer) for the given interface, so that API could request the marshaling magic from it and, in particular, request IMarshal::GetUnmarshalClass in order to obtain information who is going to do the reverse magic afterwards:
This method is called indirectly, in a call to CoMarshalInterface, by whatever code in the server process is responsible for marshaling a pointer to an interface on an object. This marshaling code is usually a stub generated by COM for one of several interfaces that can marshal a pointer to an interface implemented on an entirely different object.
You don't have IMarshal implemented on your widget, so you are going to get it somewhere from.
As you started your question mentioning that you "want to marshal an interface to another thread" and the code comment says "Create our widget" there is a chance that you can utilize IMarhsal implementation of free threaded marshaler. The question does not provide information to tell whether it is possible and/or acceptable solution.
Back to the challenge of obtaining a marshaler, you are trying to work this around by utilizing a "standard" marshaler by "registering the interface":
why is the standard COM marshaler having so much problems
[...]
I of course can register my interface in the registry
Well, this is not how things actually work.
Interface registration is not just a registry key that "okay, this IID has its own key in the registry". The purpose of such registration is to point where to look for proxy/stub pair for this interface. Your manual creating of registry entries cannot help here if you don't have a proxy/stub DLL for the interface in question. If you had it, you would just regsvr32 it the usual way to create the registry keys.
So called standard marshaler, your next try, is not supposed to marshal any interface and your IWidget in particular. OLE supplies so called "PSOAInterface" marshaler, which is capable of supplying proxy/stub pairs for OLE Automation compatible interfaces. Just for them! The marshaler does not have so much problems, it actually has just one: your IWidget is unlikely to be compatible or you would not have the problem in first place.
If your IWidget was compatible, had an associated type library, where it was marked as [oleautomation], the type library registration process would have automatically created the registry keys referencing PSOAInterface and supplying ProxyStubClsid32. Then marshaling API would have picked PSOAInterface for your widget, it would have picked up the registered type library, load details of the interface, then provided standard proxy/stub pair for it and that's it. Standard marshaler works just within these contraints and not just for any interface pointed to.
That is, your options are:
implement IMarshal on the widget server
make IWidget OLE Automation compatible, with type library, so that type library registration process activates "standard marshaler" PSOAInterface for your interface
build, if possible and applicable, proxy/stub implementation for your interface automatically generated by MIDL complier (you can check standard ATL DLL project, created from Visual Studio template on how it can be done - it creates additional project with "PS" suffix, for a supplementary proxy/stub DLL)
implement separately IMarshal in a standalone DLL, and register it with the registry and your IWidget interface, so that marshaling API would pick it up when it attempts to marshal the interface pointer (I suppose it's the only option here if your IWidget is OLE incompatible and you have reasons to not alter the original implementation)
use free threaded marshaler in the widget server if its restrictions are acceptable
Oh wait, hold on - there is another weird one. If you don't want or cannot afford, or you are reluctant to change the COM server (widget, that is) but you can modify client side code as you like, you can create a thin wrapper that implements two interfaces IWidget (all methods forward calls to real server) and IMarshal on client side, and pass its IWidget to the CoMarshalInterThreadInterfaceInStream API. This will force COM to use your marshaling without altering the original server. You are on your own, of course, to do the actual marshaling afterwards. It is unlikely that it matches your actual need, and it is just an abstract note to the discussion (which mostly consists of attempts to do impossible without details on interface itself and available options on modification of server implementation).
TL;DR: Actual questions:
Is it possible for the standard COM "Ole Automation" marshaler to build a proxy class out of a Type Library at runtime?
Short answer: yes.
Is it possible for me to build a proxy class out of a Type Library at runtime?
Short answer: yes, with the type library marshaler and IPSFactoryBuffer. Or if you're willing to use the undocumented CreateProxyFromTypeInfo and CreateStubFromTypeInfo.
I wonder why you'd want to this.
This question is riddled with red herrings.
I'm trying to marshal an interface to another thread.
(...) the call to CoMarshalThreadInterfaceInStream fails with:
REGDB_E_IIDNOTREG (0x80040155)
Interface not registered
A useful error code, in this case.
Go directly to CoMarshalInterface
(...) that fails with:
REGDB_E_IIDNOTREG (0x80040155)
Interface not registered
It's not by switching API calls that do essentially the same thing that you could solve your issue.
Use standard marshaling
My class does not implement IMarhsal interface.
Can you confirm it doesn't implement INoMarshal, IStdMarshalInfo and IAgileObject either, just to be exhaustive?
The interface is, in fact, not registered
This was expected.
I know that Windows provides you the CoRegisterPSClsid
(...)
Which i can try calling, but what clsid do i use?
CoRegisterPSClsid(IID_IWidget, ???);
If you don't have a proxy/stub, why would you go through this?
But to answer this question, a standard marshalers' CLSID is usually the same GUID as the first IID found in an IDL file.
Register the interface
I of course can register my interface in the registry:
HKEY_CURRENT_USER\Software\Classes\Interface\{EBBC7C04-315E-11D2-B62F-006097DF5BD4}
(default) = "IWidget"
But that doesn't fix it. Spelunking through the Interface registry keys, i notice that many specify a ProxyStubClsid32 entry.
You should read the documentation, instead of relying on what other registry entries contain.
(...) I don't have a marshaler.
This should be the actual problem. If this object is yours, how do you intend to marshal it? Please see the bottom section of my answer.
Can COM automatically marshal based on a TypeLibrary
This is clearly rhetoric. Why would you think of a type library just now, instead of to begin with? The rest of your question corroborates.
Now i manually include the TypeLibrary:
HKEY_CURRENT_USER\Software\Classes\Interface\{EBBC7C04-315E-11D2-B62F-006097DF5BD4}\TypeLib
(default) = "{38D528BD-4948-4F28-8E5E-141A51090580}"
You mention this twice.
But it:
never reads: HKCR\Interface[IID_IWidget]\TypeLib
The type library marshaler has its own cache of interface->type library. To clear the cache, try recreating the registry entries, logging off and logging on, or ultimately, reboot.
The object may implement IProvideClassInfo, I don't know if the type library marshaler actually cares to QueryInterface for this to fetch a runtime ITypeInfo to work with.
These are the main types of marshalers:
Standard marshalers, which are usually compiled into a DLL from C/C++ source code generated by MIDL (which consists mainly of declarations on how to marshal types)
The type library marshaler, which can marshal types at runtime based on automation-compatible type information
The free-threaded marshaler aggregated with CoCreateFreeThreadedMarshaler, which avoids marshaling between threads within the same process
Custom marshalers, which do whatever the developer wants, most commonly to implement marshal-by-value, or a free-threaded marshaler back when CoCreateFreeThreadedMarshaler didn't exist
The standard marshalers generated by MIDL consist mainly of declarations on how to marshal types in and out, so the way they work is in the same vein as the type library marshaler. According to Don Box, the result is very similar.
However, the actual declarations are very different. The type library marshaler works on type information that is meant to work in VB6 and (excluding certain things, such as user-defined types) scripting languages (mainly VBScript and JScript) and is intended to be consumed by IDEs (e.g. VB6, VBA, Delphi, OleView) and interop code generators (e.g. VC++ #import, tlbimp.exe, Delphi's "Import Type Library", Lisp 1 2), etc.
I've been using COM few years, but never having to write an interface, so that gets me little confused.
I have an interface, let it be called IFoo, defined as
import "unknwn.idl";
[
object,
uuid( // guidgen generated one // ),
] interface IFoo : IClassFactory
{
HRESULT Bar(const BYTE * pb, ULONG cb );
};
which I have compiled using MIDL-generated files and registered into the Windows registry. I also have made a local COM server, essentially which implements IFoo. In this I followed "Microsoft SDKs\Windows\v7.0\Samples\com\fundamentals\dcom\simple\" sample code replacing IStream by my own IFoo.
I can create an instance of IFoo in my client code and all that, but the member function 'Bar' just doesn't work. I cannot figure out why, see, when I call
IFoo->Bar( 0x42A110, 352 ); // address just an example
the arguments are seen by the server side as
Bar( 0x560E20, 352 );
Address-values are not important, but what is this translation here?? Help appreciated :) Currently, I just pass a pointer to beginning of a std::vector's data which might just be inaccessible across process boundaries, but that doesn't explain the translation.
Arguments passed across COM boundaries are usually marshalled. This is done because the COM boundary may be a machine boundary (in the case of DCOM) or a process boundary (in the case of out of process COM components). The COM infrastructure does this marshalling on your behalf and what's done will depend on the data types involved. Read up on Appartments, Marshalling and Proxies and Stubs (or take a look at COM IDL & Interface Design) if you're interested in the details.
Given that marshalling will be required for your arguments you next need to understand how IDL declares the arguments to your function. The IDL you've written doesn't pass enough information to the MIDL compiler for the proxy and stub to be generated correctly. That BYTE *pb could be a single byte or an array and the value could be passed in to the function or returned from it... Now, I know that it's an array because I can see you have a length specified after it, but MIDL doesn't know that. Likewise I'm guessing it's data being passed into the function as cb isn't a pointer and so you can't be telling the caller how much data you've returned but only how large the array being passed in is... To inform MIDL that it needs to marshal a number of bytes rather than just one you need to tell it that pb is an array. You do this like this:
HRESULT Bar(
[in, size_is(cb)] BYTE *pb,
[in] ULONG cb);
What we're doing here is telling MIDL that the data in both pb and cb needs to be marshalled IN to the function and that pb represents an array of BYTE which is cb bytes long.
i'm faced with implementing an IDispatch interface. There are four methods, and fortunately 3 of them are easy:
function TIEEventsSink.GetTypeInfoCount(...): HResult;
{
Result := E_NOTIMPL;
}
function TIEEventsSink.GetTypeInfo(...): HResult;
{
Result := E_NOTIMPL;
}
function TIEEventsSink.GetIDsOfNames(...): HResult;
{
Result := E_NOTIMPL;
}
It's the last method, Invoke that is difficult. Here i am faced with having to actually case the DispID, and call my appropriate method; unmarhsalling parameters from a variant array.
function Invoke(
dispIdMember: DISPID;
riid: REFIID;
lcid: LCID;
wFlags: WORD;
var pDispParams: DISPPARAMS;
var pVarResult: VARIANT;
var pExcepInfo: EXCEPINFO;
var puArgErr: DWORD
): HRESULT;
Not wanting to have to write all the tedious boilerplate code, that i'm sure will have bugs, i went googling - rather than doing any work.
i found this snippit on the MSDN Documentation of IDispatch.Invoke:
Generally, you should not implement Invoke directly.
Excellent! i didn't want to implement it anyway! Continuing reading:
Instead, use the dispatch interface to create functions CreateStdDispatch and DispInvoke. For details, refer to CreateStdDispatch, DispInvoke, Creating the IDispatch Interface and Exposing ActiveX Objects.
The Creating the IDispatch Interface link says:
You can implement IDispatch by any of the following means:
[snip]
Calling the CreateStdDispatch function. This approach is the simplest, but it does not provide for rich error handling or multiple national languages.
[snip]
Excellent, CreateStdDispatch it is:
Creates a standard implementation of the IDispatch interface through a single function call. This simplifies exposing objects through Automation.
HRESULT CreateStdDispatch(
IUnknown FAR* punkOuter,
void FAR* pvThis,
ITypeInfo FAR* ptinfo,
IUnknown FAR* FAR* ppunkStdDisp
);
i was going to call it as:
CreateStdDispatch(
myUnk, //Pointer to the object's IUnknown implementation.
anotherObject, //Pointer to the object to expose.
nil //Pointer to the type information that describes the exposed object (i has no type info)
dispInterface //the IUnknown of the object that implements IDispatch for me
);
What i cannot figure out is how the Windows API implemention of CreateStdDispatch knows what methods to call on my object - especially since CreateStdDispatch doesn't know what object-oriented language i'm using, or its calling conventions.
How will CreateStdDispatch know
what method to call for a given dispid?
the calling convention of my language?
how to handle exceptions from the language that my object oriented object is written in?
Note: i have no choice but to implement a dispinterface; i didn't define the interface. i wish it was a simple early bound IUnknown, but it tisn't.
Doesn't the ITypeInfo parameter passed into CreateStdDispatch expose all of the method information?
So you'd create type info first calling CreateDispTypeInfo and pass that through to CreateStdDispatch which can then use the type information to work out which method to call since CreateDispTypeInfo requires INTERFACEDATA which contains all this information
I could be way wrong since I don't have time to look into it but that would make sense to me.
I'll investigate this later and update the answer.
The short answer to your question is: neither CreateStdDispatch() nor the IDispatch implementation it creates knows anything at all about the methods to be called.
The object that you get back simply stores the parameters that you passed to CreateStdDispatch(), and for all IDispatch methods it only turns around and makes the corresponding calls on the ITypeInfo that you gave it. That is all.
If you pass nil for ptinfo as shown in your code then you only get E_INVALIDARG, since the implementing object cannot do anything at all without an ITypeInfo to which to delegate all the work.
If you inspect the code for CStdDisp in oleaut32.dll then you will find that it calls API functions like DispInvoke() (which also live in that DLL) instead of invoking the ITypeInfo methods directly, but these functions are all simple wrappers for calls to the ITypeInfo methods, without any further functionality.
In case anyone wonders: neither CreateStdDispatch() nor CStdDisp performs any additional magic; all they do is give you an IDispatch that does whatever the ITypeInfo that you passed in can do. Think of it as a kind of an adapter that allows you to plug an ITypeInfo into an IDispatch socket.
It is true that TAutoIntfObject.Create() needs a type library. However, all that the constructor does is call GetTypeInfoOfGuid() on it in order to get a type info pointer, to which the object then delegates most of the work related to dispatch things.
Borland in their wisdom made the member variable for the type info pointer private, which means that you really need to hand the constructor some type library or other that contains the interface in question, instead of simply writing another constructor or overriding some virtual function. On the other hand it shouldn't be too hard to load the type library via the registry or to dump parts of it to a TLB file. Inspecting a TLB with OleView gives you actual compilable IDL which is often also Borland-compilable RIDL.
CreateStdDispatch() does not know anything about exceptions either. The catching of exceptions thrown from COM methods and their conversion to HRESULT and/or IErrorInfo is compiler magic induced by Delphi's safecall keyword on the implementing method.
The same goes for the translation of HRESULTs to exceptions when calling COM methods specified as safecall in their interface declarations. The compiler simply inserts a call to #CheckAutoResult after every invocation of a safecall method; this function checks the HRESULT and throws EOleSysError if appropriate.
Simply switch the Delphi debugger to disassembly ('CPU view') to inspect all the magic that the compiler does for you!
What is a "Handle" when discussing resources in Windows? How do they work?
It's an abstract reference value to a resource, often memory or an open file, or a pipe.
Properly, in Windows, (and generally in computing) a handle is an abstraction which hides a real memory address from the API user, allowing the system to reorganize physical memory transparently to the program. Resolving a handle into a pointer locks the memory, and releasing the handle invalidates the pointer. In this case think of it as an index into a table of pointers... you use the index for the system API calls, and the system can change the pointer in the table at will.
Alternatively a real pointer may be given as the handle when the API writer intends that the user of the API be insulated from the specifics of what the address returned points to; in this case it must be considered that what the handle points to may change at any time (from API version to version or even from call to call of the API that returns the handle) - the handle should therefore be treated as simply an opaque value meaningful only to the API.
I should add that in any modern operating system, even the so-called "real pointers" are still opaque handles into the virtual memory space of the process, which enables the O/S to manage and rearrange memory without invalidating the pointers within the process.
A HANDLE is a context-specific unique identifier. By context-specific, I mean that a handle obtained from one context cannot necessarily be used in any other aribtrary context that also works on HANDLEs.
For example, GetModuleHandle returns a unique identifier to a currently loaded module. The returned handle can be used in other functions that accept module handles. It cannot be given to functions that require other types of handles. For example, you couldn't give a handle returned from GetModuleHandle to HeapDestroy and expect it to do something sensible.
The HANDLE itself is just an integral type. Usually, but not necessarily, it is a pointer to some underlying type or memory location. For example, the HANDLE returned by GetModuleHandle is actually a pointer to the base virtual memory address of the module. But there is no rule stating that handles must be pointers. A handle could also just be a simple integer (which could possibly be used by some Win32 API as an index into an array).
HANDLEs are intentionally opaque representations that provide encapsulation and abstraction from internal Win32 resources. This way, the Win32 APIs could potentially change the underlying type behind a HANDLE, without it impacting user code in any way (at least that's the idea).
Consider these three different internal implementations of a Win32 API that I just made up, and assume that Widget is a struct.
Widget * GetWidget (std::string name)
{
Widget *w;
w = findWidget(name);
return w;
}
void * GetWidget (std::string name)
{
Widget *w;
w = findWidget(name);
return reinterpret_cast<void *>(w);
}
typedef void * HANDLE;
HANDLE GetWidget (std::string name)
{
Widget *w;
w = findWidget(name);
return reinterpret_cast<HANDLE>(w);
}
The first example exposes the internal details about the API: it allows the user code to know that GetWidget returns a pointer to a struct Widget. This has a couple of consequences:
the user code must have access to the header file that defines the Widget struct
the user code could potentially modify internal parts of the returned Widget struct
Both of these consequences may be undesirable.
The second example hides this internal detail from the user code, by returning just void *. The user code doesn't need access to the header that defines the Widget struct.
The third example is exactly the same as the second, but we just call the void * a HANDLE instead. Perhaps this discourages user code from trying to figure out exactly what the void * points to.
Why go through this trouble? Consider this fourth example of a newer version of this same API:
typedef void * HANDLE;
HANDLE GetWidget (std::string name)
{
NewImprovedWidget *w;
w = findImprovedWidget(name);
return reinterpret_cast<HANDLE>(w);
}
Notice that the function's interface is identical to the third example above. This means that user code can continue to use this new version of the API, without any changes, even though the "behind the scenes" implementation has changed to use the NewImprovedWidget struct instead.
The handles in these example are really just a new, presumably friendlier, name for void *, which is exactly what a HANDLE is in the Win32 API (look it up at MSDN). It provides an opaque wall between the user code and the Win32 library's internal representations that increases portability, between versions of Windows, of code that uses the Win32 API.
A HANDLE in Win32 programming is a token that represents a resource that is managed by the Windows kernel. A handle can be to a window, a file, etc.
Handles are simply a way of identifying a particulate resource that you want to work with using the Win32 APIs.
So for instance, if you want to create a Window, and show it on the screen you could do the following:
// Create the window
HWND hwnd = CreateWindow(...);
if (!hwnd)
return; // hwnd not created
// Show the window.
ShowWindow(hwnd, SW_SHOW);
In the above example HWND means "a handle to a window".
If you are used to an object oriented language you can think of a HANDLE as an instance of a class with no methods who's state is only modifiable by other functions. In this case the ShowWindow function modifies the state of the Window HANDLE.
See Handles and Data Types for more information.
A handle is a unique identifier for an object managed by Windows. It's like a pointer, but not a pointer in the sence that it's not an address that could be dereferenced by user code to gain access to some data. Instead a handle is to be passed to a set of functions that can perform actions on the object the handle identifies.
So at the most basic level a HANDLE of any sort is a pointer to a pointer or
#define HANDLE void **
Now as to why you would want to use it
Lets take a setup:
class Object{
int Value;
}
class LargeObj{
char * val;
LargeObj()
{
val = malloc(2048 * 1000);
}
}
void foo(Object bar){
LargeObj lo = new LargeObj();
bar.Value++;
}
void main()
{
Object obj = new Object();
obj.val = 1;
foo(obj);
printf("%d", obj.val);
}
So because obj was passed by value (make a copy and give that to the function) to foo, the printf will print the original value of 1.
Now if we update foo to:
void foo(Object * bar)
{
LargeObj lo = new LargeObj();
bar->val++;
}
There is a chance that the printf will print the updated value of 2. But there is also the possibility that foo will cause some form of memory corruption or exception.
The reason is this while you are now using a pointer to pass obj to the function you are also allocating 2 Megs of memory, this could cause the OS to move the memory around updating the location of obj. Since you have passed the pointer by value, if obj gets moved then the OS updates the pointer but not the copy in the function and potentially causing problems.
A final update to foo of:
void foo(Object **bar){
LargeObj lo = LargeObj();
Object * b = &bar;
b->val++;
}
This will always print the updated value.
See, when the compiler allocates memory for pointers it marks them as immovable, so any re-shuffling of memory caused by the large object being allocated the value passed to the function will point to the correct address to find out the final location in memory to update.
Any particular types of HANDLEs (hWnd, FILE, etc) are domain specific and point to a certain type of structure to protect against memory corruption.
A handle is like a primary key value of a record in a database.
edit 1: well, why the downvote, a primary key uniquely identifies a database record, and a handle in the Windows system uniquely identifies a window, an opened file, etc, That's what I'm saying.
Think of the window in Windows as being a struct that describes it. This struct is an internal part of Windows and you don't need to know the details of it. Instead, Windows provides a typedef for pointer to struct for that struct. That's the "handle" by which you can get hold on the window.,
Is it possible to have a COM method which passes a HWND? With the following method in my object CoCreateInstance returns DISP_E_BADVARTYPE (0x80020008):
STDMETHODIMP ShowDialog(HWND hWndParent);
So far, I'm getting round this problem by passing an OLE_HANDLE then casting it but it feels like a cludge:
STDMETHODIMP ShowDialog(OLE_HANDLE hWndParent);
I think that HWND is a pointer to a struct thats why you can't use it in the IDL.
If you look at Microsoft Typelibs you will see all sort of variation on how to pass a handle (From int to long to HANDLE).
Your interface is probably registered as "dual", and HWND is not one of the types supported by OLE automation. Does your interface need to be IDispatch-compatible (do you need to call it from scripting or late-bound languages)? If not, deriving from IUnknown rather than IDispatch and not registering as dual will help you out.
NB: Casting is okay as long as you are only using the method in-process.