Converting from c# string to c++ string

I'm using a Dll created from a C++ file. When I either put the .dll and .lib files in my Unity-project folder or when I use the function that I need, Unity crashes and I can't open the project untile I remove the .dll or delete the function from the c# script.

This function works well on C++, both in Visual Studio and in Dev-C++

PS: Assets/alzBraccioCorretto.json is the file that I need to read

I've tried the same procedure for more simple dlls (just functions with int or double variables) and it worked fine, so I don't know what I'm missing with this one, probably UNICODE

In the Unity script I wrote

 [DllImport("QuartaLibreria.dll", CharSet = CharSet.Unicode)]

 static extern string LeggiFile(string nomeFile);

 Text testo;

 unsafe void Start()
 {
 testo = GetComponent<Text>();

 temp = LeggiFile("Assets/alzBraccioCorretto.json");
 testo.text = temp;
 }

In the header of the library I have

 #define QUARTALIBRERIA_API __declspec(dllexport)

 //all the other headers and #include

 string leggiFile(char* nomeFile);

 extern "C" {
 QUARTALIBRERIA_API wstring LeggiFile(wstring nomeFile);}

In the cpp of the library I have

 string leggiFile(string nomeFile) {

 ifstream _stream(nomeFile.c_str());
 string temp;
 _stream >> temp >> temp;
 _stream.close();
 return temp;
 }


 QUARTALIBRERIA_API wstring LeggiFile(wstring nomeFile){
 std::string str(nomeFile.begin(), nomeFile.end());
 string daRit = leggiFile(str);
 std::wstring WdaRit(daRit.length(), L' '); // Make room for characters

 std::copy(daRit.begin(), daRit.end(), WdaRit.begin());

 return WdaRit;
 }