asp.net textbox ajaxcontroltoolkit extender

ASP.NET Textbox DropDownExtender

How to get the value from the dropdown to return to the TextBox? The following does not work. You can select the item from list though.

<body>
<form id="form1" runat="server">
<script type="text/javascript">
    function pageLoad() {
        //Same Width
        $get('ListBox1').style.width = $get('TextBox1').clientWidth;
    }
</script>
<ajax:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
</ajax:ToolkitScriptManager>
<div>
    <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
    <ajax:DropDownExtender ID="TextBox1_DropDownExtender" DropDownControlID="ListBox1"
        runat="server" DynamicServicePath="" Enabled="True" TargetControlID="TextBox1"
        HighlightBackColor="WhiteSmoke">
    </ajax:DropDownExtender>
</div>
<asp:ListBox ID="ListBox1" runat="server" AutoPostBack="True">
    <asp:ListItem>Item 1</asp:ListItem>
    <asp:ListItem>Item 2</asp:ListItem>
    <asp:ListItem>Item 3</asp:ListItem>
</asp:ListBox>
</form>

Solution

You can put the selected item's text into the TextBox if you add a handler for the ListBox's SelectedIndexChanged event:

Markup:

<div>
    <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>    
    <ajaxtoolkit:dropdownextender id="TextBox1_DropDownExtender" dropdowncontrolid="ListBox1"
        runat="server" enabled="True" targetcontrolid="TextBox1"
        highlightbackcolor="WhiteSmoke" />
</div>
<asp:ListBox ID="ListBox1" runat="server" AutoPostBack="True" OnSelectedIndexChanged="ListBox_SelectedIndexChanged">
    <asp:ListItem>Item 1</asp:ListItem>
    <asp:ListItem>Item 2</asp:ListItem>
    <asp:ListItem>Item 3</asp:ListItem>
</asp:ListBox>

Code:

protected void ListBox_SelectedIndexChanged(object sender, EventArgs e)
{
    TextBox1.Text = ListBox1.SelectedItem.Text;
}

However, this could get really problematic to manage if you have a number of TextBoxes, Extenders and ListBoxes on the page so you might want to consider wrapping the TextBox, Extender and ListBox all up together in a UserControl.