将DropDown的SelectedIndex设置为0，并将AutoPostBack设置为true

Question

I am using a DropDown button with AutoPostBack set to true. I'm trying to reset the DropDown to the default selection on page postback -- DropDown.SelectedIndex = 0;. The DropDown does reset to the default selection but I'm unable to make a selection as it continuously reverts to the default position "Make a Selection" immediately after selecting another option. I'm not sure if it matters but this is placed inside an update panel.

One of the reasons I need this to set back to 0 is that when I click the back button on the browser, the last selection is selected and if the user tries to go back to the same selection, that particular selection in the DropDown doesn't work unless I refresh the page. For example. Choice 1 redirects me to page2. If I click back, Choice 1 is still selected. If I click the drop down to select Choice 1 again, nothing happens unless I refresh the page. Here is my DropDown HTML:

<asp:DropDownList ID="DropDown" runat="server" EnableViewState="true" 
AppendDataBoundItems="true" AutoPostBack="true" 
OnSelectedIndexChanged="DropDown_SelectedIndexChanged">
                    <asp:ListItem>Make a Selection</asp:ListItem>
                    <asp:ListItem>Choice 1</asp:ListItem>
                    <asp:ListItem>Choice 2</asp:ListItem>
                </asp:DropDownList>

And C#:

if (IsPostBack)
        {                
            DropDown.SelectedIndex = 0;

        }

Answer 1

Based on what you are experiencing, I think it's your browser what's restoring your control state when you use the browser back button.

A way to avoid this behavior is by adding the autocomplete="off" to your <form> element, for example:

<form id="form1" runat="server" autocomplete="off">
...
</form>

If adding the autocomplete="off" alone is not enough, also tell the browser to not cache the page by sending the following headers, for example, from your Page_Load method:

protected void Page_Load(object sender, EventArgs e)
{
    Response.Cache.SetNoStore();
    Response.Cache.SetCacheability(System.Web.HttpCacheability.NoCache);
}

Remember to remove the line doing the DropDown.SelectedIndex = 0; before trying this solution.

Answer 2

There are couple of reasons:

1. First need to remove:

   if (IsPostBack) {
   DropDown.SelectedIndex = 0;
   }

As you have AutoPostback="true" enabled, page_load will be called along with the OnSelectedIndexChanged event which is changing the SelectedIndex to 0.

2. Also, need to ensure Enable.ViewState property is set to true.

Answer 3

Please use the following code in page load

if (!IsPostBack)
{                
    DropDown.SelectedIndex = 0;
}