Calling c++ delegate from c# out of memory exception

Question

I have created ac# wrapper for a c++ dll,and I don't have the source code of the c++ dll. Now the c++ has a delegate function,I have created the delegate function in c# wrapper and provide it with the necessary parameters. The issue that am facing is whenever the delegate function is complete I receive an out of memory exception and I have also discovered that the delegate uses a new thread. I will demonstrate the code that I have below:

1) C# wrapper

public struct WISCN_RUN_OPTS
{
    public uint Version;
    public WISCN_CALLBACK_CODELINE_DONE CodelineDoneCallback;
}


[UnmanagedFunctionPointer(CallingConvention.Cdecl, CharSet = CharSet.Unicode)]
public delegate bool WISCN_CALLBACK_CODELINE_DONE(
    uint doc_index,
    uint user_data,
    uint codelines_count,
    WISCN_CODELINE[] codelines,ref
    WISCN_CODELINE_DOC_CTRL p_doc_ctrl);

[DllImport(@"wiscn.dll", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Cdecl, SetLastError = true)]
public static extern WISCN_ERROR WiScn_Run(_WISCN_HINST_STRUCT hinst, uint num_of_docs, ref uint p_docs_done, ref WISCN_RUN_OPTS p_opts);

---

2) C# windows application

private void ButtonProcessDocsWithOcr_Click(object sender, EventArgs e)
{
    var run_opts = new DllLoad.WISCN_RUN_OPTS
    {
        Version = DefineConstants.WISCN_STRUCT_VERSION_RUN_OPTS
        CodelineDoneCallback = DocDoneCallback;
    };

    //The callback delegate is called when this method is triggered
    WiScn_Run(hinst, 0, ref docNum, ref run_opts);
}

bool DocDoneCallback(uint doc_index, uint user_data,
    uint codelines_count,
    WISCN_CODELINE[] codelines,ref
    WISCN_CODELINE_DOC_CTRL p_doc_ctrl)
{
    return false;
}//After this line i receive the out of memory exception 
 // when it tries to resume ButtonProcessDocsWithOcr_Click event.

---

3) C++ wrapper header file

typedef struct 
{
    DWORD   Version;
    WISCN_CALLBACK_CODELINE_DONE    CodelineDoneCallback;                                                              
}
WISCN_RUN_OPTS;

typedef BOOL (*WISCN_CALLBACK_CODELINE_DONE)(DWORD doc_index, 
    DWORD user_data, DWORD codelines_count, const WISCN_CODELINE codelines[],
    WISCN_CODELINE_DOC_CTRL *p_doc_ctrl);

typedef WISCN_ERROR (WISCN_API *WISCN_RUN)(WISCN_HINST hinst, DWORD num_of_docs, LPDWORD p_docs_done, const WISCN_RUN_OPTS *p_opts);

WISCN_ERROR WISCN_API WiScn_Run(WISCN_HINST hinst, DWORD num_of_docs, LPDWORD p_docs_done, const WISCN_RUN_OPTS *p_opts);

4) c++ sample

BOOL DocDoneCallback(DWORD doc_index, DWORD user_data, 
    DWORD codelines_count, const WISCN_CODELINE codelines[], 
     WISCN_CODELINE_DOC_CTRL *p_doc_ctrl)
{
    return FALSE;
}
void main()
{
    WISCN_RUN _WiScn_Run;
    WISCN_RUN_OPTS run_opts;
    _WiScn_Run = (WISCN_RUN)GetProcAddress(hmod, "WiScn_Run");
    run_opts.Version = WISCN_STRUCT_VERSION_RUN_OPTS;
    run_opts.DocDoneCallback = DocDoneCallback;
    _WiScn_Run(hinst, 0, NULL, &run_opts);
}

Answer 1

You mentioned that a new thread is created by the unmanaged code. It seems plausible to me that the callback is called in the context of the unmanaged thread. Check that by calling GetCurrentThreadId .

[DllImport("kernel32.dll")]
static extern uint GetCurrentThreadId();

Call this before you call WiScn_Run and then in DocDoneCallback . I hypothesise that you will discover that your callback runs in a different thread.

If I am right with this hunch then you can never call that library from C#. What you'd have to do would be to wrap the C++ code in a mixed mode C++/CLI wrapper so that you can implement your callback function in unmanaged code.