C#

extern "C" __declspec(dllexport) char** foo(int *nSize);

// implementation
__declspec(dllexport) char** foo(int *nSize)
{
  *nSize = 2;
  ::MessageBox(NULL,"unmanaged foo call","message",MB_OK);

  char **lpstr = (char**) CoTaskMemAlloc( 2*sizeof(LPSTR) );
  lpstr[0] = (char*) CoTaskMemAlloc( 100 ) ;
  strcpy(lpstr[0], "string1");
  lpstr[1] = (char*) CoTaskMemAlloc( 100 ) ;
  strcpy(lpstr[1], "string2");

  return lpstr;
}

// declaration
[DllImport(@"..\bin\foo.dll", CharSet=CharSet.Auto)]
static public extern IntPtr foo(ref int nSize); // 

// implementation
int nSize = 0;
IntPtr pData = foo(ref nSize);
for (int i=0; i<nSize; i++)
{
  IntPtr pString = Marshal.ReadIntPtr(pData, 4*i);
  MessageBox.Show( Marshal.PtrToStringAnsi(pString) );
}