Dynamically set properties of textbox based on the value selected from dropdown

Try this,

In .aspx page,

<asp:dropdownlist id="ddlTxt" runat="server" onclick="ddlfn()" autopostback="true">
<asp:listitem value="0" text="1">

<asp:listitem value="1" text="2">

<asp:listitem value="2" text="3">

<asp:listitem value="3" text="4">


<asp:textbox id="txtDynamic" runat="server" xmlns:asp="#unknown">

<script language="javascript" type="text/javascript">
function ddlfn()
{
var keyValue =document.getElementById('<%=ddlTxt.ClientID %>').options[document.getElementById('<%=ddlTxt.ClientID%>').selectedIndex].value;
if(keyValue==0)
document.getElementById('<%=txtDynamic.ClientID %>').value="1";
else
document.getElementById('<%=txtDynamic.ClientID %>').value="2,3,4";
}
</script>

........
@Nidhish

try this,
In .aspx page,

function CheckMandatory()
{
var keyValue =document.getElementById('&lt;%=ddlDynamic.ClientID %&gt;').options[document.getElementById('&lt;%=ddlDynamic.ClientID%&gt;').selectedIndex].value;
if(keyValue==0)
{
if(document.getElementById('&lt;%=txtDynamic.ClientID %&gt;').value=="")
alert('Please enter a value');
}
}


<asp:dropdownlist id="ddlDynamic" runat="server" onselectedindexchanged="ddlDynamic_OnSelectedIndexChanged" autopostback="true" xmlns:asp="#unknown">
<asp:listitem value="0" text="1">

<asp:listitem value="1" text="2">

<asp:listitem value="2" text="3">

<asp:listitem value="3" text="4">


<asp:textbox id="txtDynamic" runat="server" cssclass="style1" xmlns:asp="#unknown">
<asp:button id="btnDynamic" runat="server" text="Submit" onclientclick="return CheckMandatory()" xmlns:asp="#unknown">
--------------------------------------------------------------------------
In .aspx.cs page,
protected void ddlDynamic_OnSelectedIndexChanged(object sender, EventArgs e)
{
if (ddlDynamic.SelectedIndex == 0)
txtDynamic.CssClass="style1";
else
txtDynamic.CssClass = "style2";
}