I am currently building a project that makes use of DllImport in C# to call a method in a C++ dll.

The method I am trying to call is defined as follows:

extern "C" TESTSYSTEM BSTR setLSystem1(BSTR& i_settings)

I believe i_settings is a file path to a file in the project containing the DLL.

I am trying to call this method in C# using this code:

 public static class DllWrapper
 {
     [DllImport("path/to/dll/dll", CallingConvention = CallingConvention.Cdecl, SetLastError = true)]
     [return: MarshalAs(UnmanagedType.BStr)]
     public static extern string setLSystem1([MarshalAs(UnmanagedType.BStr)] ref string s);

     public static void SetLSystem(string s)
     {
         var res = setLSystem1(ref s);
     }
 }

Though when I call SetLSystem, I get the following error:

System.Runtime.InteropServices.SEHException: ‘External component has thrown an exception.’

I have also tried to run this version of the code removing the ref keyword:

 public static class DllWrapper
 {
     [DllImport("path/to/dll.dll", CallingConvention = CallingConvention.Cdecl, SetLastError = true)]
     [return: MarshalAs(UnmanagedType.BStr)]
     public static extern string setLSystem1([MarshalAs(UnmanagedType.BStr)] string s);

     public static void SetLSystem(string s)
     {
         var res = setLSystem1(s);
     }
 }

though that produces:

System.AccessViolationException: ‘Attempted to read or write protected memory. This is often an indication that other memory is corrupt.’

Unfortunately both errors are quite vague, and I am not very familiar with c++, and can’t find much information online about passing strings from C# as BSTR or BSTR& to c++. Does anyone have any guidance as to how to achieve this?