Returning Array of Structure from C++ DLL to C# Application

Question

I am trying to return array of structure from the C++ DLL to C# application. I could return structure from DLL Also could able to send list of structure from c# application to C++ DLL and print its data. But not able to fill and return array of structure from DLL to C# application. Could anybody help on this. Below is the code on which I am currently working:

C# Code:

[StructLayout(LayoutKind.Sequential)]
public struct DATA
{
    [MarshalAs(UnmanagedType.I4)]
    public int id;
    [MarshalAs(UnmanagedType.LPWStr)]
    public string Name;
};

[DllImport("PassStruct.dll")]
private static extern void PrintListOfStructData(IntPtr[] pD, int nSize);

[DllImport("PassStruct.dll")]
private static extern IntPtr ReturnStruct();

public static DATA ReturnStructure()
{
    DATA oRData = new DATA();
    IntPtr pD = new IntPtr();

    pD = ReturnStruct();
    oRData = (DATA)Marshal.PtrToStructure(pD, typeof(DATA));

    FreeStruct(pD);
    return oRData;
}

public static void PrintListOfStructData(List<NativeDLLHelper.DATA> data)
{
    int NumberOfElements = data.Count;
    DATA oData = new DATA();
    IntPtr[] MemPtr = new IntPtr[NumberOfElements];
    for (int i = 0; i < NumberOfElements; i++)
    {
        MemPtr[i] = Marshal.AllocHGlobal(Marshal.SizeOf(oData));
        Marshal.StructureToPtr(data[i], MemPtr[i], false);
    }

    PrintListOfStructData(MemPtr, NumberOfElements);

    for (int i = 0; i < NumberOfElements; i++)
    {
        Marshal.FreeHGlobal(MemPtr[i]);
    }
}

C++ DLL Code:

typedef struct _DATA
{
    int          nID;
    wchar_t      *sName;
}DATA;

extern "C" DLLEXPORT DATA* ReturnStruct()
{
    DATA *obj = new DATA();

    obj->nID = 100;
    wstring sName = L"String from DLL";
    obj->sName = ::SysAllocString(sName.c_str());

    return obj;
}

extern "C" DLLEXPORT void PrintListOfStructData(DATA **pD, int nSize)
{
    for(int i=0; i<nSize; i++)
    {
        wcout<<"ID: "<<pD[i]->nID<<endl;
        wcout<<"Name: "<<pD[i]->sName<<endl;
    }
}

extern "C" DLLEXPORT void FreeStruct(DATA *obj)
{
    delete obj;
}

Sean Hall · Accepted Answer · 2014-04-22 04:25:07Z

1

Don't declare PrintListOfStructData as taking an IntPtr array:

[DllImport("PassStruct.dll")]
private static extern void PrintListOfStructData(IntPtr pD, int nSize);

Then, declare all of the structs into the same block of memory:

int size = Marshal.SizeOf(typeof(DATA));
IntPtr MemPtr = Marshal.AllocHGlobal(NumberOfElements * size);
try
{
    for (int i = 0; i < NumberOfElements; i++)
    {
        Marshal.StructureToPtr(data[i], IntPtr.Add(MemPtr, i * size), false);
    }

    PrintListOfStructData(MemPtr, NumberOfElements);
}
finally
{
    if (MemPtr != IntPtr.Zero) Marshal.FreeHGlobal(MemPtr);
}

answered Apr 22, 2014 at 4:25

Sean Hall

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