C#/C++ interop and struct

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Luca Beretta - 04 Oct 2005 11:39 GMT

i've a C++ dll with a function that return a pointer to a struct, and i need to
call that function from a C# windows app.
i already try some ways founded on the MSDN and on the web but i cannot map
returned struct to a managed struct.

// C++ Code

struct NKAF
{
    char* SectorCode;
    char* CountryCode;
    char* AgencyCode;
    char* TaxNumber;
};

// function prototype
extern "C" __declspec(dllexport) void Netlink_Read_NKAF(NKAF*);

// C# code
[StructLayout(LayoutKind.Sequential,CharSet=CharSet.Ansi)]
public struct NKAF
{
[MarshalAs(UnmanagedType.ByValTStr,SizeConst=16)] string SectorCode;
[MarshalAs(UnmanagedType.ByValTStr,SizeConst=16)] string CountryCode;
[MarshalAs(UnmanagedType.ByValTStr,SizeConst=16)] string AgencyCode;
[MarshalAs(UnmanagedType.ByValTStr,SizeConst=16)] string TaxNumber;
}

[DllImport("scgx.dll")]
public static extern void Netlink_Read_NKAF(IntPtr FreeData);

private void frmMain_Load(object sender, System.EventArgs e)
{
    NKAF dati = new NKAF();
    GCHandle hGC = new GCHandle();
    hGC = GCHandle.Alloc(dati,GCHandleType.Pinned);
    Netlink_Read_NKAF(hGC.AddrOfPinnedObject());
}

when i execute i get this exception

"System.ArgumentException" in mscorlib.dll
Additional information: Object contains non-primitive or non-blittable data

i think the problem is the managed string datatype; if i replace them with int
all works fine (but obviusly i get a nullreference exception after the call
to the function)

any idea or suggestion ? are there any other ways to do this ?

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Mattias Sjögren - 04 Oct 2005 15:52 GMT

Luca,

>i've a C++ dll with a function that return a pointer to a struct, and i need to
>call that function from a C# windows app.

[...]

>// function prototype
>extern "C" __declspec(dllexport) void Netlink_Read_NKAF(NKAF*);

There's no way the function can return a pointer since the only
parameter is passed by value, and the function has a void return type.

I'll assume that the function simply populates the fields of the
existing struct you pass in.

>// C++ Code
>
[quoted text clipped - 5 lines]
> char* TaxNumber;
>};

[...]

>// C# code
>[StructLayout(LayoutKind.Sequential,CharSet=CharSet.Ansi)]
[quoted text clipped - 5 lines]
>[MarshalAs(UnmanagedType.ByValTStr,SizeConst=16)] string TaxNumber;
>}

Since the strings in the C++ declaration aren't fixed-size, you should
remove the MarshalAs attirbutes in the C# declaration.

>[DllImport("scgx.dll")]
>public static extern void Netlink_Read_NKAF(IntPtr FreeData);

Not sure why you chose IntPtr as the parameter type. It would be much
easier with

public static extern void Netlink_Read_NKAF(ref NKAF FreeData);

Mattias

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Mattias Sjögren [MVP] mattias @ mvps.org
http://www.msjogren.net/dotnet/ | http://www.dotnetinterop.com
Please reply only to the newsgroup.

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Luca Beretta - 04 Oct 2005 16:40 GMT

Mattias,
first thanks you a lot for your answer; then, my function in C++ dll do
following things:

extern "C" __declspec(dllexport) void Netlink_Read_NKAF(NKAF *FreeData)
{
    NKAF ret;
    /* initialize structure */
    ret.Address = '\0';
    ret.AgencyCode = '\0';
    ret.BirthDate = '\0';

// populate all the fields reading a smartcard (sample)
ret.DoctorCode = new char[dwSubSub84L];
    for(DWORD idx = 0; idx < dwSubSub84L; idx++)
{
    ret.DoctorCode[idx] = (BYTE)NKAF_Stream_Data[jdx+idx+1];
    }
    ret.DoctorCode[idx] = '\0';

FreeData = &ret;
}

in my original dll i've a different function prototype, that receive no
parameters and return directly the pointer, but i've some problem in the
interop with C#, so reading MSDN i founded some samples like mine actual
version.

i need to change my c++ code ? which is the best way ?

now i changed my C# code to use ref like this:

[StructLayout(LayoutKind.Sequential,CharSet=CharSet.Ansi)]
public struct NKAF
{
    string SectorCode;
    string CountryCode;
    string AgencyCode;
}

[DllImport("scgx.dll")]
public static extern void Netlink_Read_NKAF(ref NKAF FreeData);

NKAF dati = new NKAF();
Netlink_Read_NKAF(ref dati);

but i get a System.NullReferenceException

> Luca,
>
[quoted text clipped - 42 lines]
>
> Mattias

Reply to this Message

Mattias Sjögren - 07 Oct 2005 23:11 GMT

Luca,

>extern "C" __declspec(dllexport) void Netlink_Read_NKAF(NKAF *FreeData)

[...]

> FreeData = &ret;
>}

This wont work. You're only changing the local pointer value of the
argument, not what it points to. So no changes will propagate back to
the caller.

Instead of populating a new NKAF struct(ret) in the function code, you
could modify the existing one:

FreeData->Address = '\0';

and so on. But you may have to change the C# declarations to use
IntPtr instead of string if you chose to do it this way.

Also if you return pointers to buffers allocated with new, like in

ret.DoctorCode = new char[dwSubSub84L];

you have to provide a way for the caller to free that memory.

Mattias

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Mattias Sjögren [MVP] mattias @ mvps.org
http://www.msjogren.net/dotnet/ | http://www.dotnetinterop.com
Please reply only to the newsgroup.

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C#/C++ interop and struct

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