using System;
using System.Collections.Generic;
using System.IO;
using System.Xml.Serialization;

class Test
{
    static void Main()
    {
        XmlSerializer xs = new XmlSerializer(typeof(Instruments));
        Instruments instruments;

        using(Stream s = File.OpenRead("gsvirdi.xml"))
        {
           instruments = (Instruments)xs.Deserialize(s);                
        }
      
        // check it worked
        Console.Clear();

        foreach(Instrument inst in instruments.InstList)
        {
           Console.WriteLine("Ins : {0}", inst.Ins); 
           foreach(string typ in inst.TypList)
           {
              Console.WriteLine("typ : {0}", typ); 
           }
           foreach(string mak in inst.MakList)
           {
              Console.WriteLine("mak : {0}", mak);
           } 
           Console.WriteLine();
        }

        Console.ReadKey(); 
    }
}

[XmlRoot("instruz")]
public class Instruments
{
   [XmlElement("inst")]
   public List<Instrument> InstList = new List<Instrument>();
}

public class Instrument
{
   string ins;
     
   [XmlElement("Ins")]
   public string Ins
   {
      get { return ins; }
      set { ins = value; }
   } 

   [XmlElement("typ")]
   public List<string> TypList;
 
   [XmlElement("mak")]
   public List<string> MakList;
}

private Instruments instruments;

private void Form1_Load(object sender, EventArgs e)
{
    XmlSerializer xs = new XmlSerializer(typeof(Instruments));
    
    using (Stream s = File.OpenRead("gsvirdi.xml"))
    {
        instruments = (Instruments)xs.Deserialize(s);
    }

    cb1.DataSource = instruments.InstList;
    cb1.DisplayMember = "Ins";
    
}

// add handler from designer
private void cb1_SelectedIndexChanged(object sender, EventArgs e)
{
    int index = cb1.SelectedIndex;
    if (index > -1)
    {
        cb2.DataSource = instruments.InstList[index].TypList;
        cb3.DataSource = instruments.InstList[index].MakList;
    }

}