protected void TextBox1_TextChanged(object sender, EventArgs e)
{
   var record = MyStaticDataClass.GetMyTableRecord(TextBox1.Text);
   if (record != null)
   {
      TextBox2.Text = record.Field2;
      TextBox3.Text = record.Field3;
   }
}

public void RaiseCallbackEvent(string eventArgument)
    {
        System.Text.StringBuilder sb = new System.Text.StringBuilder();
        using (SqlConnection cn = new SqlConnection(ConfigurationManager.ConnectionStrings["TNConnectionString"].ConnectionString))
        {
            ///Выводим SQL Запрос из таблицы dbo.Шасси
            string query = @"Select НомерШасси from Шасси WHERE ВЧ ='" + Request["DropDownList1"] + "'";
            SqlCommand cmd = new SqlCommand(query, cn);
            cn.Open();

            SqlDataReader reader = cmd.ExecuteReader();
            // Занесение Описания в тексбокс
            while (reader.Read())
            {
                string word = reader["НомерШасси"].ToString();
                sb = sb.Append(word);
                sb = sb.Append("\t");
            }
            reader.Close();
            cn.Close();

            CallbackResult = sb.ToString();               
      }