Which to use, AllocCoTaskMem or AllocHGlobal

microsoft.public.dotnet.languages.vb

2 Jul 2005 8:24 PM

**Developer**

If I need to allocate memory, maybe because I'm going to:
Marshal.StructureToPtr
Does it matter which of the following I use?

Marshal.AllocCoTaskMem
Marshal.AllocHGlobal
I know the arguments are different which would make one more convient in a
given situation, but other that that does it make any signifficant
difference.

Thanks

5 Jul 2005 5:16 AM

Jay B. Harlow [MVP - Outlook]

**Developer**,
AllocCoTaskMem is used to allocate COM memory, unless I knew I needed to
allocate COM memory (memory that another COM object/API needed to release) I
would use AllocHGlobal.

In other words I use AllocHGlobal, as I have yet needed to use
AllocCoTaskMem.

Hope this helps
Jay

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" **Developer**" <REMOVEdevelo***@a-znet.com> wrote in message
news:uwAWFQ0fFHA.1960@TK2MSFTNGP10.phx.gbl...
| If I need to allocate memory, maybe because I'm going to:
| Marshal.StructureToPtr
| Does it matter which of the following I use?
|
| Marshal.AllocCoTaskMem
| Marshal.AllocHGlobal
| I know the arguments are different which would make one more convient in a
| given situation, but other that that does it make any signifficant
| difference.
|
|
|
| Thanks
|
|
|
|

5 Jul 2005 12:39 PM

**Developer**

I've seen a lot of examples that use AllocCoTaskMem (see below)

Will I know that I need COM memory? What I mean is I will be obvious donig
"COM" things not simply calling some Windows API and not knowing I'm dealing
with COM. Somehow, I wonder about the FORMATRANGE example becaues I know
the RichTextBox windows control uses COM at times.

Thanks again

'Allocate memory for the FORMATRANGE struct and

'copy the contents of our struct to this memory

Dim lParam As IntPtr

lParam = Marshal.AllocCoTaskMem(Marshal.SizeOf(lFr))

Marshal.StructureToPtr(lFr, lParam, False)

Dim lPointerToDevMode As IntPtr = Marshal.AllocCoTaskMem(lDevModeDataSize)

lRet = WinSpool.DocumentProperties(IntPtr.Zero, lPrinterHandle, printerName,
lPointerToDevMode, IntPtr.Zero, GDI.DM_OUT_BUFFER)

If (lRet < 0) Then

MsgBox("Cannot get the DEVMODE structure.")

GoTo cleanup

End If

lPointerToDevMode = Marshal.AllocCoTaskMem(lSizeOfDevMode)

lFlag = WinSpool.DocumentProperties(IntPtr.Zero, lPrinterHandle,
printerName, lPointerToDevMode, IntPtr.Zero, GDI.DM_OUT_BUFFER)

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"Jay B. Harlow [MVP - Outlook]" <Jay_Harlow_***@msn.com> wrote in message
news:OaWhQDSgFHA.3512@TK2MSFTNGP10.phx.gbl...
> **Developer**,
> AllocCoTaskMem is used to allocate COM memory, unless I knew I needed to
> allocate COM memory (memory that another COM object/API needed to release)
> I
> would use AllocHGlobal.
>
> In other words I use AllocHGlobal, as I have yet needed to use
> AllocCoTaskMem.
>
> Hope this helps
> Jay
>
> " **Developer**" <REMOVEdevelo***@a-znet.com> wrote in message
> news:uwAWFQ0fFHA.1960@TK2MSFTNGP10.phx.gbl...
> | If I need to allocate memory, maybe because I'm going to:
> | Marshal.StructureToPtr
> | Does it matter which of the following I use?
> |
> | Marshal.AllocCoTaskMem
> | Marshal.AllocHGlobal
> | I know the arguments are different which would make one more convient in
> a
> | given situation, but other that that does it make any signifficant
> | difference.
> |
> |
> |
> | Thanks
> |
> |
> |
> |
>
>

5 Jul 2005 1:41 PM

Jay B. Harlow [MVP - Outlook]

**Developer**,

| Will I know that I need COM memory?

Yes! At least I would hope one would, by being familiar with the API that
you are calling to know if it requires the COM allocator or not.

Looking at the DocumentProperties API I do not see that any of its
parameters require the COM allocator, ergo I would not use AllocCoTaskMem, I
would use AllocHGlobal instead. My choice to use AllocHGlobal of course may
be wrong as I quickly looked at the DocumentProperties API on MSDN just now.
For example I know the Structured Storage API & MAPI "requires" COM
allocator, however I don't see where on MSDN that Structured Storage
requires the COM allocator... MAPI has its own API calls for memory that I
understand uses an IMalloc.

When reviewing AllocCoTaskMem & AllocHGlobal it helps to understand the
CoTaskMemAlloc API:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/com/html/c4cb588d-9482-4f90-a92e-75b604540d5c.asp

And GlobalAlloc (LocalAlloc):
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/memory/base/globalalloc.asp

My understanding is that AllocCoTaskMem (aka CoTaskMemAlloc) will allocate
memory via the standard implementation of IMalloc. What I don't know is
where CoTaskMemAlloc allocates its memory out of (if its the "directly" the
same "pool" as GlobalAlloc (if it simply calls GlobalAlloc) or if it
allocates a Heap & allocates from that (HeapCreate & HeapAlloc APIs).

| Will I know that I need COM memory?

I think the question might really be: does it hurt calling AllocCoTaskMem
instead of AllocHGlobal? I suspect calling AllocCoTaskMem when you don't
need to is not as bad as not calling AllocCoTaskMem when you do need to. (if
that made any sense ;-))

Or: can FreeHGlobal (GlobalFree API) really safely be used interchangable
with FreeCoTaskMem (CoTaskMemFree API)? If you are allocating & freeing the
memory it really doesn't matter; if you are allocating the memory & someone
else frees it then I would say it does matter.

I would expect Adam Nathan's book to address this, however I'm not seeing it
in his book right now.

Hope this helps
Jay

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5 Jul 2005 1:59 PM

**Developer**

I'm waiting for your book. You really get to the bottom of it.
Thanks

PS

> | Will I know that I need COM memory?
> I think the question might really be: does it hurt calling AllocCoTaskMem
> instead of AllocHGlobal? I suspect calling AllocCoTaskMem when you don't
> need to is not as bad as not calling AllocCoTaskMem when you do need to.
> (if
> that made any sense ;-))
>

How about:
need to is not as bad as calling AllocHGlobal when you should call
AllocCoTaskMem when you do need to.

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>
> Hope this helps
> Jay
>
..