Fill description textbox on the basis of value selected in partnumber

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 _partNum = new List();
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;
}
}