Why my CheckBox.Checked Property will reset?

Question

Using: .NET 3.5SP1, VS2008

I was editting someone else asp.net script, he did the Data retreving at the Page_Load while Page is not postback.

I could see the data was populated into the DropDownList properly even after I refresh, navigates, postback in the page.

I added couples more DropDownList and some CheckBoxes into the script, only the DropDownList I added got populated properly. But not the CheckBox.

So I do a test in a new project, which is similar to its script structure:

ASPX:

<form id="form1" runat="server">
    <div>
        <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="true">
            <asp:ListItem>Item1</asp:ListItem>
            <asp:ListItem>Item2</asp:ListItem>
        </asp:DropDownList>
        <% 
            if (DropDownList1.SelectedValue == "Item2")
            {                
        %>
        <asp:CheckBox ID="CheckBox1" runat="server" Text="CheckBox 1" />
        <asp:TextBox ID="TextBox1" runat="server" Text="" />
        <asp:DropDownList ID="DropDownList2" runat="server" AutoPostBack="true">
            <asp:ListItem>Item1</asp:ListItem>
            <asp:ListItem>Item2</asp:ListItem>
        </asp:DropDownList>
        <%
            }
        %>
    </div>
    </form>

Code-Behind:

 public partial class _Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!Page.IsPostBack)
            {
                this.CheckBox1.Checked = true;
                this.CheckBox1.Text = "Hello CheckBox";
                this.TextBox1.Text = "Hello TextBox";
                this.DropDownList2.SelectedValue = "Item2";
            }
        }
    }

So as you see the code, when the page first load, the CheckBox1's text will change, Checked will be true, so as other TextBox and DropDownList2

After I select DropDownList1's item to Item2, when the CheckBox1, TextBox1, DropDownList2 nothing got setted, except the CheckBox1.Text.

Why is this happen?

EDIT:

I tried to put them into Panel, in this way it work. But the problem is the program I am editting is using the format above.. So I am not allow to change them all to Panel.

<form id="form1" runat="server">
    <div>
        <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="true">
            <asp:ListItem>Item1</asp:ListItem>
            <asp:ListItem>Item2</asp:ListItem>
        </asp:DropDownList>
        <% 
            if (DropDownList1.SelectedValue == "Item2")
            {
                this.MyPanel.Visible = true;
            }
            else
            {
                this.MyPanel.Visible = false;
            }
        %>
        <asp:Panel ID="MyPanel" runat="server" Visible="false" >
            <asp:CheckBox ID="CheckBox1" runat="server" Text="CheckBox 1" />
            <asp:TextBox ID="TextBox1" runat="server" Text="" />
            <asp:DropDownList ID="DropDownList2" runat="server" AutoPostBack="true">
                <asp:ListItem>Item1</asp:ListItem>
                <asp:ListItem>Item2</asp:ListItem>
            </asp:DropDownList>
        </asp:Panel>

    </div>
    </form>

Answer 1

Try out setting EnableViewState and ViewStateMode, perhaps some of the parent controls has disabled it so default Inherit value has been applied.

MSDN:

The default value of the ViewStateMode property for a Web server control in a page is Inherit. As a result, if you do not set this property at either the page or the control level, the value of the EnableViewState property determines view-state behavior.

<asp:CheckBox ID="CheckBox1" runat="server" Text="CheckBox 1" 
              EnableViewState="true"
              ViewStateMode="Enabled" />