Fill description textbox on the basis of value selected in partnumber

Question

I have 2 textboxes txtPartNum and txtDesc. partnumber is autocomplete from database

 

i have the partnumber and description saved in list _partNum. how do i get the description text box filled on the basis of partnumber selected on textChanged or keyDown event. code suggestion please.? 

public class PartNumbers
{
    public string parts { get; set; }
    public string descr { get; set; }
}

List<PartNumbers> _partNum = new List<PartNumbers>();

public void partNumberLoad()
{
    try
    {
        string query = "select itmPartNumber, itemDescr from vstItems";
        SqlCommand cmd = new SqlCommand(query, MainClass.con);
        if (MainClass.con.State == ConnectionState.Closed)
            MainClass.con.Open();
        SqlDataReader sdr = cmd.ExecuteReader();
        AutoCompleteStringCollection autoPart = new AutoCompleteStringCollection();
        while (sdr.Read())
        {
            autoPart.Add(sdr["itmPartNumber"].ToString());
            PartNumbers partNumbers = new PartNumbers { parts = sdr["itmPartNumber"].ToString(), descr = sdr["itemDescr"].ToString() };
                    _partNum.Add(partNumbers);
        }
        sdr.Close();
        cmd.Connection.Close();
        txtPartNum.AutoCompleteMode = AutoCompleteMode.Suggest;
        txtPartNum.AutoCompleteSource = AutoCompleteSource.CustomSource;
        txtPartNum.AutoCompleteCustomSource = autoPart;
    }
}