Void pointer to managed object type

[vijay.gandhi]

Hi,

I am trying to convert some unmanaged code (C++) to managed code
(using C++/CLI).

1) One of the functions used returns a void* which I need to cast into
a handle of a managed object. Can somebody please tell me how.

Actually, the function that returns a void* is a part of a MFC class -
CPtrList, used in my unmanaged code. I was searching of its equivalent
in .NET, but couldn't find it. Is there any equivalent?

2) Also, is there any equivalent of double pointers **, in .NET
handles?

Thanks in advance,
Vijay.

 

[vijay.gandhi]

I received an answer in another forum:

You can't cast a void* to unmanaged memory to a managed object
reference. To turn this into managed memory, you'd have to use
Marshal.Copy() or Marshal.PtrToStructure(). That will of course only
work if you know the type of the data that the void* points to.

To handle double pointers, you'd declare the member or function
argument with the MarshalAs(UnmanagedType.LPArray) attribute. Or
LPStruct if it points to a structure or LPTStr if it points to a
string. In the case of an array, you'll also have to use the
SizeConst argument to tell the marshaler how long the array is.

Hans Passant.

 

[Tamas Demjen]

1) One of the functions used returns a void* which I need to cast into
a handle of a managed object. Can somebody please tell me how.

Typically you can solve that problem with the IntPtr type. Of course C#
won't be able to interpret the meaning of IntPtr, but it can store its
value, and when you pass it back to a C++ function, you can get the
native pointer out of it. This is how handles (GDI or file handles) are
implemented in .NET -- they are simply stored as IntPtr members.

Note that the value of an IntPtr is not managed, it just contains a
navite pointer address. The .NET framework doesn't know what to do with
that address, and doesn't keep track of native objects where those
IntPtrs point to. IntPtr is just as unsafe as a void*, because it
contains no type information, and it may point to a dead object.

Actually, the function that returns a void* is a part of a MFC class -
CPtrList, used in my unmanaged code. I was searching of its equivalent
in .NET, but couldn't find it. Is there any equivalent?

Perhaps you could use List<IntPtr>, which is roughly analogous to

2) Also, is there any equivalent of double pointers **, in .NET
handles?

The short answer is no, but depending on what you are doing, there are
alternative solutions to that problem.

There's no such type as a handle to a handle, and you can't call the %
operator to obtain a handle to a handle:

MyRefClass^ c = gcnew MyRefClass;
MyRefClass^^ p = %c; // error!!!

You can, however, generate a tracking reference to a handle. Here are
two examples:

bool Allocate(Object^% obj)
{
obj = gcnew String("hello");
return true;
}

void Swap(Object^% one, Object^% two)
{
Object^ temp = one;
one = two;
two = temp;
}

In a similar way, you can get a tracking pointer to a handle too:

ref class C
{
};

void TestTrackingPointer()
{
C^ c = gcnew C;
interior_ptr<C^> p = &c;
}

There are restrictions regarding tracking references and pointers. For
example, they can't be declared as class members. They must be either
function arguments or automatic (local) variables. When the garbage
collector moves an object around in the memory, it updates all the
tracking references and pointers to it. Tracking pointers can't be used
in verifiable assemblies, because they can be freely incremented, and
therefore they can point outside of the object's memory bounds. On the
other hand, they're very fast. Unlike array indexing, interior_ptr is
not bounds checked, and it is excellent for fast image processing routines.

And finally, if you want to pass the address of a managed object to an
unmanaged function, you can pin it using the pin_ptr keyword. While an
object is pinned, the garbage collection won't move it around in the
memory. That's yet another way of obtaining a pointer to managed memory.

Tom

 

[Ben Voigt]

Typically you can solve that problem with the IntPtr type. Of course C#
won't be able to interpret the meaning of IntPtr, but it can store its
value, and when you pass it back to a C++ function, you can get the native
pointer out of it. This is how handles (GDI or file handles) are
implemented in .NET -- they are simply stored as IntPtr members.

That's if you need to store a void* in a managed type. What it sounds like
the OP needs, is to return a managed handle in an untyped way, like void*.
You'd use System::Object^ for that, every managed handle can cast to and
from Object^. There still needs to be a distinction between managed and
unmanaged though, to keep the garbage collector informed. Specifically,
storing the address of a managed object in a void* will leave you with a
dangling pointer.

Note that the value of an IntPtr is not managed, it just contains a navite
pointer address. The .NET framework doesn't know what to do with that
address, and doesn't keep track of native objects where those IntPtrs
point to. IntPtr is just as unsafe as a void*, because it contains no type
information, and it may point to a dead object.


Perhaps you could use List<IntPtr>, which is roughly analogous to


The short answer is no, but depending on what you are doing, there are
alternative solutions to that problem.

There's no such type as a handle to a handle, and you can't call the %

Sure there is: a ref class.

generic<typename To>
ref struct Handle
{
To Target;
};

triple indirection on string: Handle<Handle<Handle<System::String^>^>^>

mmm, syntax might be wrong, might need To^ Target and leave off the ^ when
using.... I always use templates, not generics, when in C++/CLI.