library Project1;

uses
  System.SysUtils,
  System.Classes;

type

  IStringFunctions = interface
    ['{240B567B-E619-48E4-8CDA-F6A722F44A71}']
    function GetMethodValueAsString():PAnsiChar; stdcall;
  end;

  TStringFunctions = class(TInterfacedObject, IStringFunctions)
  public
    function GetMethodValueAsString():PAnsiChar; stdcall;
  end;

{$R *.res}

function  TStringFunctions.GetMethodValueAsString():PAnsiChar; stdcall;
begin
  Result := 'test';
end;


procedure GetImplementation(out instance:IStringFunctions); stdcall; export;
begin
  instance := TStringFunctions.Create;
end;

exports GetImplementation;

begin
end.

using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

namespace ConsoleApplication1
{
    [ComVisible(true)]
    [ComImport, InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("240B567B-E619-48E4-8CDA-F6A722F44A71")]
    public interface IStringFunctions
    {
        [MethodImplAttribute(MethodImplOptions.PreserveSig)]
        [return: MarshalAs(UnmanagedType.AnsiBStr)]
        string GetMethodValueAsString();
    }

    class Program
    {
        [DllImport("kernel32.dll", EntryPoint = "LoadLibrary", CallingConvention = CallingConvention.StdCall)]
        static extern int LoadLibrary([MarshalAs(UnmanagedType.LPStr)] string lpLibFileName);

        [DllImport("kernel32.dll", EntryPoint = "GetProcAddress", CallingConvention = CallingConvention.StdCall, CharSet = CharSet.Ansi)]
        static extern IntPtr GetProcAddress(int hModule, [MarshalAs(UnmanagedType.LPStr)] string lpProcName);

        [DllImport("kernel32.dll", EntryPoint = "FreeLibrary", CallingConvention = CallingConvention.StdCall)]
        static extern bool FreeLibrary(int hModule);

        [UnmanagedFunctionPointer(CallingConvention.StdCall, CharSet = CharSet.Ansi)]
        delegate void GetImplementation([MarshalAs(UnmanagedType.Interface)] out IStringFunctions instance);

        static void Main(string[] args)
        {
            const string dllName = "Project1.dll";
            const string functionName = "GetImplementation";

            int libHandle = LoadLibrary(dllName);
            if (libHandle == 0) throw new Exception(string.Format("Could not load library "{0}"", dllName));

            var delphiFunctionAddress = GetProcAddress(libHandle, functionName);
            if (delphiFunctionAddress == IntPtr.Zero) throw new Exception(string.Format("Can't find function "{0}" in library "{1}"", functionName, dllName));

            GetImplementation getImplementation = (GetImplementation)Marshal.GetDelegateForFunctionPointer(delphiFunctionAddress, typeof(GetImplementation));

            if (getImplementation != null)
            {
                IStringFunctions instance = null;
                getImplementation(out instance);

                if (instance != null)
                {
                    //!!! don't return value !!!!
                    String result = instance.GetMethodValueAsString();
                    Console.WriteLine(result);
                }
            }
            Console.ReadLine();
        }
    }
}