Marshalling a struct containing c-strings

Question

I have a C++ struct

struct UnmanagedStruct
{
   char* s;
};

and a C# struct

struct ManagedStruct {
   [MarshalAs(UnmanagedType.LPStr)]
   string s;
}

the C++ library exposes

extern "C" UnmanagedStruct __declspec(dllexport) foo( char* input );

And it is imported like

  [DllImport("SomeDLL.dll", CharSet = CharSet.Ansi)]
  static extern ManagedStruct foo( string input );

Answer 1

Edit & Correction: For return values of p/invoke calls, the "normal" method doesn't work. I was so used to the normal, ref, and out behaviors related to method parameters that I assumed return values would work in a similar fashion. Here is the link for a solution to the return value problem:
PInvoke error when marshalling struct with a string in it

You only need to use a StringBuilder if you are passing the struct to the C++ method as a byref parameter and the string is a buffer that the method will alter. For a return value, you just need to specify the type of string, which in this case is:

struct ManagedStruct
{
    [MarshalAs(UnmanagedType.Lpstr)]
    string s;
}

Rememer to add a property to expose the string, since s is private here (which is OK, fields should be private).