for rsdf files you should use:

Code: 
    class Program
    {
        [DllImport("DLMF.dll", EntryPoint = "RSDFToPlain", CallingConvention = CallingConvention.StdCall)]
        public static extern string RSDFToPlain(string RSDFFile);

        static void Main(string[] args)
        {
            string file = Environment.CurrentDirectory + "\\Simpsons_Staffel_4_Komplett.rsdf";
            string data = RSDFToPlain(file);

            Console.WriteLine(data);
            Console.ReadKey(true);
        }
    }
i don't understand why u use 2 slashes here:
Code: 
string file = Environment.CurrentDirectory + "\\Simpsons_Staffel_4_Komplett.rsdf";

// i would use it this way
string file = Environment.CurrentDirectory + "\Simpsons_Staffel_4_Komplett.rsdf";