<form id="form1" runat="server">
<div>
<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
<asp:updatePanel id="UPD1" runat="Server" UpdateMode="Conditional">
<ContentTemplate>
<br />
<asp:CheckBox ID="chk1" runat="server" AutoPostBack="true" OnCheckedChanged="chkAllView_CheckedChanged" />
<asp:DropDownList ID="ddl1" runat="server">
</asp:DropDownList>
</ContentTemplate>
</asp:updatePanel>
</div>
</form>
protected void Page_Load(object sender, EventArgs e)
{
if(!IsPostBack)
{
DataSet ds ;
ddl1.DataSource=ds;
ddl1.DataBind();
}
}
protected void chkAllView_CheckedChanged(object sender, EventArgs e)
{
if(chk1.Checked)
ddl1.Items.Clear();
}
Modify as above.