在C＃中使用cpp代码（返回自定义结构和std：string）

Question

I'm trying to build a wrapper to use cpp code inside c# code, and I want to return custom struct (Class) as output for method2, and std::string for method1, and this is my code in cpp

extern "C" __declspec(dllexport)  std::string method1()
{
  std::string s;
  //Some Code/////////
  return s;
}

and this is the method that should return custom struct (or class)

extern "C" __declspec(dllexport)  MyStruct method2()
{
  MyStruct s;
  //Some Code//////
  return s;
}

and I tried to write c# code for both of these methods and here is my c# code

[DllImport("<dllPath>", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.StdCall)]
[return: MarshalAs(UnmanagedType.LPStr)]
public static extern string  method1(); //Exception

[DllImport("<DllPath>")]
public static extern MyStruct method2(); //Compile Error

Now when I'm trying to run the c# code I get MarshalDirectiveException for method1, and compiling error for method2?

Answer 1

In the first PInvoke, the C++ method should return const char* (s.c_str()). You should remove "[return [...]]" and replace string by IntPtr. Then, you can convert the IntPtr to string with Marshal.PtrToStringAnsi(ptr). http://msdn.microsoft.com/en-US/library/s9ts558h(v=vs.110).aspx can help you.

In the second PInvoke, you should define in C# the MyStruct (but I can't be more precise because I have no information about MyStruct). This link can help you : http://www.codeproject.com/Articles/66243/Marshaling-with-C-Chapter-Marshaling-Compound-Ty

EDIT : Thanks to hvd's comment, it is better sometimes to use BSTR , because the use of char* can be dangerous !