C++ API return an array of a struct

You can't return an array from a function in the way you are probably thinking but you can return a pointer to the array. You will also need to return the length of the array so that you can properly marshal the data to the appropriate C# type. This requires a slight change to the function signature to instead return the information through parameters passed to the function.

extern "C" EXPORT_API bool GetCharacterPositions(CharacterInformation** data, int *length)
{
    CharacterInformation *characterInformationArray = new CharacterInformation[arraysize];

    // initialize contents of array.

    // ... snip ...

    *length = arraysize;
    *data = characterInformationArray;

    return true;
}

In this case you will need to add a default constructor to CharacterInformation and use two-phase initialization (i.e. an Init() function).

Be aware that you should not return a pointer or reference to a non-static local variable as the contents of the variable will be destroyed when it goes out of scope (i.e. when the function returns).

To marshal the data you can try something like the following. This is untested but should give you a push in the right direction.

[DllImport("Dllname.dll", 
    CallingConvention = CallingConvention.Winapi, 
    CharSet = CharSet.Auto)]
[return: MarshalAs(UnmanagedType.I1)]
public static extern bool GetCharacterPositions(out IntPtr arrayPtr, out int size);

public static List<CharacterInformation> GetCharacterPositions()
{
  var arrayValue = IntPtr.Zero;
  var size = 0;
  var list = new List<CharacterInformation>();

  if ( !GetCharacterPositions(out arrayValue, out size))
  {
    return list; 
  }

  var dataEntrySize = Marshal.SizeOf(typeof(CharacterInformation));
  for ( var i = 0; i < size; i++)
  {  
    var cur = (CharacterInformation )Marshal.PtrToStructure(arrayValue, typeof(CharacterInformation ));
    list.Add(cur);
    arrayValue = new IntPtr(arrayValue.ToInt32() + dataEntrySize);
  }

  return list;
}

You will need to add an additional call in order to properly delete the data in C++ otherwise you will end up with a memory leak.

You can also allocate an array before the call to function and pass the buffer and its size. Your C function will fill the array in this case.

extern "C" EXPORT_API void GetCharacterPositions(CharacterInformation *pArr, int size)
{
    for ( int i = 0; i < size; ++ i )
    {
       //fill pArr[i]   
    }

}

you can consider my code. I use the reference

#include<iostream>

using namespace std;

struct CharacterInformation {
  int id;
  double x;
  double y;
  CharacterInformation(int ID, double X, double Y) : id(ID), x(X), y(Y) {}
};

extern "C" EXPORT_API void GetCharacterPositions(CharacterInformation &ci) {
  CharacterInformation ciTmp(0,1,2);
  ci.id = ciTmp.id;
  ci.x = ciTmp.x;
  ci.y = ciTmp.y;
}

int main()
{
    CharacterInformation ci(0,0,0);
    GetCharacterPositions(ci);
    cout<<ci.id<<ci.x<<ci.y<<endl;
    return 0;
}