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Troubleshooting Question

Populate ASP.Net TextBox with value from DropDownList using JQuery

Last Modified: 2020-10-13
I am trying to populate a TextBox within an ASPX page (C#) with a dropdownlist value using JQuery. I have no idea if this is right. I'm simply trying things I am readying on the internet. The Javascript says that DDL_ID (which I assume would be the dropdowlist) and TextBoxID (which i assume is the textbox) CANT BE FOUND. I don't know if this will work or not but I am trying anything I can at this point to prevent users from having to type in a freeformtextbox.

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm3.aspx.cs" Inherits="echodevelopment.WebForm3" %>

<!DOCTYPE html>

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
 <script src="jquery-3.5.1.min.js"></script>
<script type="text/javascript">
    $('#<%= DDL_ID.ClientID %>').change(function() {
    $('#<%= TextBoxID.ClientID %>').val($(this).val());

    <form id="form1" runat="server">
            <asp:FormView ID="FormView1" runat="server" AllowPaging="True" DataKeyNames="id" DataSourceID="SqlDataSource1">
                    <asp:Label ID="idLabel1" runat="server" Text='<%# Eval("id") %>' />
                    <br />
                    <asp:TextBox ID="CityTextBox" runat="server" Text='<%# Bind("City") %>' />
                    <br />
                      <asp:DropDownList ID="DDL_ID" runat="server"   >
                        <asp:ListItem Value="0" Selected="True">Select</asp:ListItem>
                    <asp:TextBox ID="TextBoxID" runat="server" Text='<%# Bind("Citycode") %>'/>
                    <br />
                    <asp:LinkButton ID="UpdateButton" runat="server" CausesValidation="True" CommandName="Update" Text="Update" />
                    &nbsp;<asp:LinkButton ID="UpdateCancelButton" runat="server" CausesValidation="False" CommandName="Cancel" Text="Cancel" />
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