The Trace prints "????????" not "test"

C++

extern "C" char* CSharp_Test_ReturnString()
{
  string s = "test";
  int len = s.length();
  char* comem = (char*)CoTaskMemAlloc(len+1);
  strcpy_s(comem, len+1, (const char*)ba);
  return(comem);
}

C#
[DllImport("__Internal", EntryPoint = "CSharp_Test_ReturnString")]
public extern static string CSharp_Test_ReturnString();


void TestInterop()
{
  string tr = CSharp_Test_ReturnString();
  Trace.WriteLine(string.Format("ReturnString: {0}", tr));
}