AutoPostBack and parameter value after browser Back - asp.net

Question

When using postback and then browser Back button, previous content is shown correctly, but the parameter that triggered postback has the latest value - it is not returned to the value it had before it triggered postback. This creates an invalid state.
In the example, dropdown has AutoPostBack set to true.
After I type "a2" and change dropdown value to "text2":
After I type "a3" and change dropdown value to "text3":
After Back:
Turning of browser cache programmatically didn't help, as it gives "Document Expired" on browser Back action.
After going Back, I wish that parameters match the content which is generated with those parameters, specifically the ones that are AutoPostBack. Is there a desired way of getting that behavior?
Code:
aspx
<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="true">
<asp:ListItem Text="text1" />
<asp:ListItem Text="text2" />
<asp:ListItem Text="text3" />
</asp:DropDownList>
<div>Server content - <asp:Label ID="ServerContent" runat="server" /> </div>
aspx.cs
protected void Page_Load(object sender, EventArgs e)
{
ServerContent.Text = string.Format(
"received values: {0}, {1}", TextBox1.Text, DropDownList1.SelectedValue);
}

Answer 1

It is still not very clear what you are trying to do. The DropDownList changes, the server does a form post, you get a response and then you want that response in the previous state (before form post)? Seems like a weird rquirement. If a user goes back because he made a mistake, he changes the value and then does a PostBack again. No harm done.
But if you really want to set the value to it's default, you need to set the value of the DropDownList with JavaScript when the page has not done a PostBack. Take a look at my snippet below, it gives the desired results. It sets the SelectedIndex at 2 when the page is loaded, but does not when a PostBack is performed so the new value is still visible. Now when the user goes back the script is still present and will set the DropDownList to it's default value again.
<asp:DropDownList ID="DropDownList1" runat="server" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged" AutoPostBack="true">
<asp:ListItem Text="aaa" Value="aaa"></asp:ListItem>
<asp:ListItem Text="bbb" Value="bbb"></asp:ListItem>
<asp:ListItem Text="ccc" Value="ccc"></asp:ListItem>
</asp:DropDownList>
Code behind
protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
{
ScriptManager.RegisterStartupScript(Page, Page.GetType(), "setValue", "document.getElementById('" + DropDownList1.ClientID + "').selectedIndex = 2;", true);
}
}

Answer 2

I had to keep values for certain fields in additional hidden fields and use them on the client. The same example with "solution":
aspx
<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="true">
<asp:ListItem Text="text1" />
<asp:ListItem Text="text2" />
<asp:ListItem Text="text3" />
</asp:DropDownList>
<asp:HiddenField runat="server" ID="DropDownList1ValueCarrier" ClientIDMode="Static" />
<div>Server content - <asp:Label ID="ServerContent" runat="server" /> </div>
aspx.cs
protected void Page_PreRenderComplete(object sender, EventArgs e)
{
DropDownList1ValueCarrier.Value = DropDownList1.Text;
}
js
$('#DropDownList1').val($('#DropDownList1ValueCarrier').val());