question about autopostback and viewstate

Question

Hi

This code gives (with selected value 'c') : ok c

My question is: why is the value of variable id lost after postback and why is the SelectedValue not lost after postback, despite the fact that ViewState is enabled? What is the function in this case of ViewState?

Thanks

aspx file:

<%@ Page Language="C#" AutoEventWireup="true" EnableViewState="true" CodeBehind="WebForm1.aspx.cs" Inherits="mot.WebForm1" %>
<asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" />
<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="true" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
                <asp:ListItem>a</asp:ListItem>
                <asp:ListItem>b</asp:ListItem>
                <asp:ListItem>c</asp:ListItem>
 </asp:DropDownList>
 <asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>

code-behind:

using System;
namespace mot
{
    public partial class WebForm1 : System.Web.UI.Page
    {
        internal string id="ok";
        
        protected void Button1_Click(object sender, EventArgs e)
        {
            Label1.Text = id + " " + DropDownList1.SelectedValue;
        }

        protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
        {
            id = DropDownList1.SelectedValue;
        }
    }