i have geeting data from database in combobox, but after selecting any item from combobox dosent get related record in related textbox

objConn1.Open();
SqlDataAdapter d = new SqlDataAdapter("select Itemno from ITdata", objConn1);
DataSet dt = new DataSet();
d.Fill(dt);
comboBox4.DataSource = dt.Tables[0]; /// assing the first table of dataset
comboBox4.DisplayMember = "Itemno";
objConn1.Close();