const int WSADESCRIPTION_LEN = 257;
    const int WSASYS_STATUS_LEN = 128;
    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
    public struct WSAData
    {
      public ushort wVersion;
      public ushort wHighVersion;
      [MarshalAs(UnmanagedType.ByValTStr, SizeConst = WSADESCRIPTION_LEN + 1)]
      public string szDescription;
      [MarshalAs(UnmanagedType.ByValTStr, SizeConst = WSASYS_STATUS_LEN + 1)]
      public string szSystemStatus;
      ushort iMaxSockets;
      ushort iMaxUdpDg;
      [MarshalAs(UnmanagedType.LPStr)]
      string lpVendorInfo;
    };

    unsafe public struct servent
    {
      public char* s_name;
      public char** s_aliases;
      public ushort s_port;
      public char* s_proto;
    };

    [DllImport("ws2_32.dll", SetLastError = true)]
    static extern ushort htons(ushort hostshort);

    [DllImport("ws2_32.dll", SetLastError = true)]
    unsafe static extern servent* getservbyname(string name, string proto);

    [DllImport("ws2_32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    public static extern int WSAStartup(ushort wVersionRequested, out WSAData lpWSAData);

    [DllImport("ws2_32.dll", SetLastError = true)]
    public static extern int WSACleanup();

    unsafe static ushort GetPortByName(string PortName, string PortType)
    {
      servent* ServEnt = getservbyname(PortName, PortType);
      if (ServEnt != null)
        return htons(ServEnt->s_port);
      return 0;
    }

    private void button1_Click(object sender, EventArgs e)
    {
      WSAData D;
      if (WSAStartup(0x101, out D) == 0) 
      {
        try
        {
          int Port = GetPortByName("ftp", "tcp");
          MessageBox.Show("Port = " + Port); //Порт 21
        }
        finally
        {
          WSACleanup();
        }
      }
    }

    [StructLayout(LayoutKind.Sequential)]
    public struct servent
    {
      public IntPtr s_name;    //public char* s_name;
      public IntPtr s_aliases; //char** s_aliases;
      public ushort s_port;
      public IntPtr s_proto; //char* s_proto;
    };

    [DllImport("ws2_32.dll", SetLastError = true)]
    static extern IntPtr getservbyname(string name, string proto);

    static ushort GetPortByName(string PortName, string PortType)
    {
      IntPtr ptr = getservbyname(PortName, PortType);
      if (ptr != null)
      {
        servent ServEnt = (servent)Marshal.PtrToStructure(ptr, typeof(servent));
        return htons(ServEnt.s_port);
      }
      return 0;
    }

    [StructLayout(LayoutKind.Sequential)]
    public struct servent32
    {
      public string s_name;    //public char* s_name;
      public IntPtr s_aliases; //char** s_aliases;
      public ushort s_port;
      public string s_proto; //char* s_proto;
    };

    [StructLayout(LayoutKind.Sequential)]
    public struct servent64
    {
      public string s_name;    //public char* s_name;
      public IntPtr s_aliases; //char** s_aliases;
      public string s_proto; //char* s_proto;
      public ushort s_port;
    };

    static ushort GetPortByName(string PortName, string PortType)
    {
      ushort Result = 0;
      WSAData D;
      if (WSAStartup(0x101, out D) == 0) 
      {
        try
        {
          IntPtr ptr = getservbyname(PortName, PortType);
          if (ptr != null)
          {
            int Bits = Marshal.SizeOf(typeof(IntPtr)) * 8; 
            switch (Bits)
            {
              case 32:
                servent32 ServEnt32 = (servent32)Marshal.PtrToStructure(ptr, typeof(servent32));
                Result = htons(ServEnt32.s_port);
                break;
              case 64:
                servent64 ServEnt64 = (servent64)Marshal.PtrToStructure(ptr, typeof(servent64));
                Result = htons(ServEnt64.s_port);
                break;
              default:
                throw new Exception(Bits + "-bit operating system is not supported yet");
            }
          }
        }
        finally
        {
          WSACleanup(); 
        }
      }
      return Result;
    }