generate automatic empcode in the textbox by company wise when the page is load

1. You will have to set AutoPostBack = true for the DropDowbList and also add a SelectedIndexChanged event and the you can code as follows

protected void ddlCustomers_SelectedIndexChanged(object sender, EventArgs e)
{
    String strConnString = ConfigurationManager
                          .ConnectionStrings["conString"].ConnectionString;

    String strQuery = "select * from Customers where" +
                      " CustomerID = @CustomerID";

    SqlConnection con = new SqlConnection(strConnString);
    SqlCommand cmd = new SqlCommand();
    cmd.Parameters.AddWithValue("@CustomerID", ddlCustomers.SelectedItem.Value);     
    cmd.CommandType = CommandType.Text;
    cmd.CommandText = strQuery;
    cmd.Connection = con;
    try
    {
       con.Open();
        SqlDataReader sdr= cmd.ExecuteReader();
        while (sdr.Read())
        {
           lblName.Text = sdr["ContactName"].ToString();
           lblCountry.Text = sdr["Country"].ToString(); 
        }
    }
    catch (Exception ex)
    {
        throw ex;
    }
    finally
    {
        con.Close();
        con.Dispose();
    }
}