Question

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="WebUserControl.ascx.cs" Inherits="WebUserControl" %>

<script type="text/javascript">
    function Incrementer() {
        var txtBox = document.getElementById('MainContent_TextBox1').value;

        var i = parseInt(txtBox);
        i = i + 1;
        var v = i + "";
        document.getElementById('MainContent_TextBox1').value = v;
        return false;
    }
    function Decrementer() {
        var txtBox = document.getElementById('MainContent_TextBox1').value;

        var i = parseInt(txtBox);
        i = i - 1;
        var v = i + "";
        document.getElementById('MainContent_TextBox1').value = v;
        return false;
    }

</script>
<asp:TextBox ID="TextBox1" runat="server" Text="0"></asp:TextBox><br />
<asp:Button ID="Button1"  runat="server" Text="Up"  OnClientClick="return Incrementer();"/>
<asp:Button ID="Button2" Text="Down" runat="server" OnClientClick="return Decrementer();"/>

Default.aspx as follows:-

<%@ Page Title="Home Page" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true"
    CodeFile="Default.aspx.cs" Inherits="_Default" %>
    <%@ Register Src="~/WebUserControl.ascx" TagName="IncrementerUserControl" TagPrefix="uc1" %>
<asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent">
</asp:Content>
<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">

<uc1:IncrementerUserControl ID="Incrementer1" runat="server"/>
    </asp:Content>

The incrementer function now doesn’t work when I have it included in the usercontrol. Why? If I use the incrementer and decrementer without the user control it works fine.

Answer 1

MainContent_TextBox1 is the problem. When you include the TextBox in the user control, its name gets mangled due to runat=server

Try using:
document.getElementById('%=TextBox1.ClientID %')

See:

http://jagregory.com/writings/how-to-use-clientids-in-javascript-without-the-ugliness/