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Asked: 2026-05-13T18:03:59+00:00

I am using some unmanaged code that is returning pointers (IntPtr) to large image

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I am using some unmanaged code that is returning pointers (IntPtr) to large image objects. I use the references but after I am finished with the images, I need to free that memory referenced by the pointers. Currently, the only thing that frees the memory is to shutdown my entire app. I need to be able to free that memory from inside my application.

Here is the call to that allocates the memory. hbitmap is the pointer that is returned and needs to be deallocated.

[DllImport("twain_32.dll", EntryPoint = "#1")]
public static extern TwainResult DsImageTransfer(
    [In, Out] Identity origin, [In] Identity dest, DataGroup dg, 
    DataArgumentType dat, Message msg, ref IntPtr hbitmap);
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1 Answer

  1. Editorial Team

    That would depend on how that memory was allocated. The Marshal class has methods for deallocating memory allocated through the common interop allocation patterns, like FreeCoTaskMem. If the unmanaged code uses a non Interop compatible way of allocating, then you cannot interop with it.

    Updated

    If I would venture a guess, the function #1 you invoke in twain_32.dll is the DS_ENTRY function in a TWAIN provider. The Twain specifications call out the memory resource management protocol:

    Memory Management in TWAIN 2.0 and
    Higher
    TWAIN requires Applications and
    Sources to manage each other’s memory.
    The chief problem is guaranteeing
    agreement on the API’s to use. TWAIN
    2.0 introduces four new functions that are obtained from the Source Manager
    through DAT_ENTRYPOINT.

    TW_HANDLE PASCAL DSM_MemAllocate (TW_UINT32)
    PASCAL DSM_MemFree (TW_HANDLE)
    TW_MEMREF PASCAL DSM_MemLock(TW_HANDLE)
    void PASCAL DSM_MemUnlock(TW_HANDLE)

    These functions correspond to the
    WIN32 Global Memory functions
    mentioned in previous versions of the
    TWAIN Specification: GlobalAlloc,
    GlobalFree, GlobalLock, GlobalUnlock
    On MacOS/X these functions call
    NewPtrClear and DisposePtr. The lock
    and unlock functions are no-ops, but
    they still must be called. TWAIN 2.0
    compliant Applications and Sources
    must use these calls on all platforms
    (Windows, MacOS/X and Linux). The
    Source Manager takes the
    responsibility to make sure that all
    components are using the same memory
    management API’s.

    So to free resources you’re supposed to call DSM_MemFree, which supposedly on Win32 platforms would be implemented through GlobalFree, or Marshal.FreeHGlobal.

    Since this is mostly speculation on my part, you better validate with the specs of the specific TWAIN implementation you use.

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