Passing an array of unknown size of structs from c# to c++ dll and back

Question

I asked a similar question yesterday, but this is slightly different. I am having problems passing arrays of struct from c# to c++, and getting this back again.

Here is the c++ code. Firstly, the struct:

struct Tri
{
   public:
     int v1, v2, v3;
}

and now the c++ dll part:

extern "C" __declspec(dllexport) void Dll_TriArray(Tri *tri)
{
   int num = 10;
   tri = new Tri[num];

   for (int i = 0; i < num; i++)
   {
      tri[i].v1 = i + 5;
      tri[i].v2 = i + 10;
      tri[i].v3 = i + 25;
   }
}

and here's the c# code, again starting with the struct:

[StructLayout(LayoutKind.Sequential)]
public struct Tri
{
   public int v1, v2, v3;
}

public class Testing
{
   [DllImport("testing.dll")]
   static extern void Dll_TriArray(out Tri[] tryArray);

   public GetTriArray()
   {
      Tri[] triArray;
      Dll_TriArray(out triArray);
   }
}

So the triArray i get when calling the GetTriArray method will come back as null. I have thought about passing an IntPtr in as the argument, but then how does one marshal an intptr into/from an array of struct?

BTW - at this stage, i'm not interested in memory leaks.

BTW - at this stage, i'm not interested in memory leaks. That's a dangerous stance to take at any point in your development. — DonBoitnott
– DonBoitnott, Commented Jun 14, 2013 at 10:32
This might be of interest:stackoverflow.com/questions/1086294/… — Matthew Watson
– Matthew Watson, Commented Jun 14, 2013 at 10:33

Bbrado · Accepted Answer · 2013-06-14 12:15:57Z

1

I'm not an expert (by any means) in C# but the C++ part gets passed a pointer to Tri-struct, which in C++ can be used like an dynamic array you allocate and fill that correctly but you don't have a way to get it back, because from C-perspective you'd need to modify the caller's (C#) pointer but you only get a copy and not a reference to the original.

In C++ the closest thing to what you are tying to do, would be to change the prototype to void Dll_TriArray(Tri *&tri) (call by ref, not call by copy) but I'm not sure how to interface that with C# (probably Dll_TriArray(ref triArray); ).

answered Jun 14, 2013 at 12:15

Bbrado

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