C++/CLI : How do I hold a reference to Native pointer in a Ref Class

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Russell Mangel - 24 Dec 2006 17:04 GMT

/*

Hi,

I am trying to hold a reference to un-managed pointer IStorage.
The client/callers will make many accesses to IStorage, but
only in-directly. For performance reasons IStorage needs to be
opened once, and a reference held by MyClass for duration of session.

1. Should I use a native class to do the native stuff?
2. Should I use pin_ptr?
3. Is it possible to correct the error message in MyClass?

Please suggest to me the best way to accomplish this.

Russell Mangel
Las Vegas, NV

PS.
Thanks for your assistance.

This project is a C++/CLI CLR Class Library, a DLL for use by C#, and VB.Net
clients.

When you create a CLR Class Library project do the following:
Under Linker, Input, Additional Dependencies, remove $(NoInherit).

*/

#pragma once
using namespace System;

#define WIN32_LEAN_AND_MEAN
#include <stdio.h>
#include <tchar.h>
#include <windows.h>
#include <ole2.h>

namespace MyNameSpace
{
public ref class MyClass
{
private:
// This line causes an error:
IStorage *pStorage;
// C2440: 'reinterpret_cast' : cannot convert from 'cli::interior_ptr<Type>'
to 'IStorage **'

public:
int Open()
{
HRESULT hr = S_OK;
// IStorage *pStorage = NULL;
hr = StgOpenStorage(
L"C:\\AnyMSOfficeDocument.doc",
NULL,STGM_READ | STGM_SHARE_DENY_WRITE,
NULL,
NULL,
reinterpret_cast <IStorage**>(&pStorage) );

return hr;
}
};
}

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Carl Daniel [VC++ MVP] - 24 Dec 2006 23:05 GMT

> /*
>
[quoted text clipped - 6 lines]
>
> 1. Should I use a native class to do the native stuff?

Most of it, yes.

> 2. Should I use pin_ptr?

No. That's for converting a managed pointer to a native one. To hold onto
a native pointer in a managed class, simply cast it to a suitably-sized
integer (int on x86 or __int64 on x64/IA64).

> 3. Is it possible to correct the error message in MyClass?

Yes.

int pStorage;

int Open()
{
HRESULT hr = S_OK;
hr = StgOpenStorage(
L"C:\\AnyMSOfficeDocument.doc",
NULL,STGM_READ | STGM_SHARE_DENY_WRITE,
NULL,
NULL,
reinterpret_cast <IStorage**>(&pStorage)
);

return hr;
}

Naturally, when you need to access the storage, you'll need to
reinterpret_cast the int back to IStorage*. Be sure you remember to Release
that pointer! That probably means that your managed class should be both
IDisposable and have a finalizer. That's automatic in C++/CLI if you simply
declare a destructor.

-cd

Reply to this Message

Russell Mangel - 25 Dec 2006 06:15 GMT

Carl, thanks for your reply.

I tried to compile #3, but I get the same error message:
Error 1 error C2440: 'reinterpret_cast' : cannot convert from
'cli::interior_ptr<Type>' to 'IStorage **'

MyClass needs to hold a reference to IStorage for the duration of the
session.
MyClass will have several other methods, which need the IStorage pointer
to do more work.

Example: C# Client:

// C# Client Code
MyClass myClass = new MyClass("C:\\14.msg");

// Client calls Open()
Int32 hr = myClass.Open();

// Client now calls GetRootElements()
// This method uses the private IStorage pointer
// which was previously created by Open()
foreach (Element element in myClass.GetRootElements())
{
Console.WriteLine(element.Name);
}

// Cleanup IStorage
myClass.Dispose();

What am I missing here?

Russell Mangel
Las Vegas, NV

Reply to this Message

Carl Daniel [VC++ MVP] - 25 Dec 2006 07:44 GMT

> Carl, thanks for your reply.
>
> I tried to compile #3, but I get the same error message:
> Error 1 error C2440: 'reinterpret_cast' : cannot convert from
> 'cli::interior_ptr<Type>' to 'IStorage **'

Oh sorry - you have to use a local variable as an intermediary - the
compiler won't let you cast the address of a member variable, not even for
the duration of a single function call. You can probably do it with a
pin_ptr as well, but that still takes an intermediary, so I don't see how
that's any better.

public ref class MyClass
{
private:
int m_pStorage;

~MyClass()
{
if (m_pStorage)
reinterpret_cast<IStorage*>(m_pStorage)->Release();
}

public:
int Open()
{
int pStorage; // local intermediary
HRESULT hr = S_OK;
hr = StgOpenStorage(
L"C:\\AnyMSOfficeDocument.doc",
NULL,STGM_READ | STGM_SHARE_DENY_WRITE,
NULL,
NULL,
reinterpret_cast <IStorage**>(&pStorage)
);

if (SUCCEEDED(hr))
m_pStorage = pStorage;

return hr;
}
};

-cd

Reply to this Message

Russell Mangel - 25 Dec 2006 15:51 GMT

Holy Christmas, its working!

1. As far as my brain can think, there shouldn't be any problem with the
garbage collector moving around the pointer:
m_pStorage; right? Even if the GC moves the managed int (pointer) around it
will still point to the Native memory. All I have to do
is make sure that I cleanup the Native memory so I don't cause a memory
leak, right?

2. Why do I have to use a System::Int32 for the pointer. I tried to use a
System::Intptr for this , I couldn't make it work, but maybe I did something
wrong?

Thanks for your help.

Russell Mangel
Las Vegas, NV

PS

Merry Christmas!

Additonally:
I can't think of another way to do this, than the solution you have given
me. Reason: Structured Storage documents are just like a file
system right? And so they can have any number of Storages (Directories), and
so I am forced to hold a reference to the IStorage native memory. I have
another version of this contraption working using a Native Class, this was
pretty easy, but it falls apart once you try to open nested IStorage
Storages, I would be forced to track all the IStorage native pointers in the
Native class, and this seems harder to do.

Reply to this Message

Carl Daniel [VC++ MVP] - 26 Dec 2006 05:10 GMT

> Holy Christmas, its working!
>
[quoted text clipped - 4 lines]
> is make sure that I cleanup the Native memory so I don't cause a
> memory leak, right?

Right.

> 2. Why do I have to use a System::Int32 for the pointer. I tried to
> use a System::Intptr for this , I couldn't make it work, but maybe I
> did something wrong?

I would think that IntPtr should work as well - it's just an integer type
sized to match the size of a native pointer on the host platform.

> Thanks for your help.
>
[quoted text clipped - 15 lines]
> all the IStorage native pointers in the Native class, and this seems
> harder to do.

Yes, you need to keep a reference to the outermost IStorage open - closing
it will close the file and cause any IStorage interfaces you may be holding
on nested storages to become invalid.

-cd

Reply to this Message

Ben Voigt - 26 Dec 2006 17:13 GMT

> Holy Christmas, its working!
>
> 1. As far as my brain can think, there shouldn't be any problem with the
> garbage collector moving around the pointer:
> m_pStorage; right? Even if the GC moves the managed int (pointer) around
> it

Except while StgOpenStorage is running, because it writes to m_pStorage.
You must give StgOpenStorage the address of a pointer in non-GC-managed
memory. This means either pinned, or on the stack.

> will still point to the Native memory. All I have to do
> is make sure that I cleanup the Native memory so I don't cause a memory
[quoted text clipped - 3 lines]
> System::Intptr for this , I couldn't make it work, but maybe I did
> something wrong?

Carl has given you 3 horribly broken solutions to your simple problem.
Casting is not the answer. Use a local variable of the correct type:

IStorage *pStorage = NULL;
hr = StgOpenStorage(
L"C:\\AnyMSOfficeDocument.doc",
NULL,STGM_READ | STGM_SHARE_DENY_WRITE,
NULL,
NULL,
&pStorage);
this->pStorage = pStorage;

> Thanks for your help.
>
[quoted text clipped - 14 lines]
> IStorage Storages, I would be forced to track all the IStorage native
> pointers in the Native class, and this seems harder to do.

Reply to this Message

Russell Mangel - 28 Dec 2006 01:54 GMT

Hi, it was very nice of you to help me with my question, thanks.

I changed my code to use an IStorage pointer, and it seems to be working.

However, my source code has changed a little from the beginning of this post
could you take a look at it now and see if I have any problems. As you can
see by my source code I simply open a docfile, and then I pass the managed
IStorage pointer to the constuctor of the Managed Class StorageInfo, this
Manged class has a destructor that releases the managed pointer.

I am still concerned because your previous statement.
Ben Voigt <Wrote>
"You must give StgOpenStorage the address of a pointer in non-GC-managed
memory. This means either pinned, or on the stack."

SSCore::IStorageInfo ^SSCore::Functions::StgOpenStorage(String ^fileName)
{
pin_ptr<const wchar_t> pinnedFIlename = PtrToStringChars(fileName);

HRESULT hr = S_OK;
IStorage *pNativeStorage = NULL;

hr = ::StgOpenStorage(
pinnedFIlename,
NULL,
STGM_READ | STGM_SHARE_DENY_WRITE,
NULL,
0,
reinterpret_cast <IStorage**>(&pNativeStorage) );

if(hr != S_OK)
{
Error::ThrowException(hr, "StgOpenStorage");
}
else
{
StorageInfo ^storageInfo = gcnew StorageInfo(pNativeStorage);
return storageInfo;
}
}

//
// Here is destructor for StorageInfo
//
SSCore::StorageInfo::~StorageInfo()
{
// m_pStorage is a Native IStorage type which was passed into the
constructor
// by the previous code.
if(m_pStorage)
{
m_pStorage->Release();
}
}

// End Source Code

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C++/CLI : How do I hold a reference to Native pointer in a Ref Class

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