ASP.Net Calander Control Visibility problem

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C#

ASP.NET

Visual-Studio

C#4.0

I have added a calender control to myasp.net form and I disabled it using the visible property. But Iwant to visible true it when a textbox_textchanged and I did it as the below!

protected void txtDtApp_TextChanged(object sender, EventArgs e)
        {
            clnDates.Visible = true;
        }

But it didn't displayed and I don't know what to do!

Updated Calander Control HTML Code

<asp:Calendar ID="clnDates" runat="server" BackColor="White" 
                        BorderColor="#999999" CellPadding="4" DayNameFormat="Shortest" 
                        Font-Names="Verdana" Font-Size="8pt" ForeColor="Black" Height="180px" 
                        onselectionchanged="clnDates_SelectionChanged" Visible="False" Width="200px">
                        <DayHeaderStyle BackColor="#CCCCCC" Font-Bold="True" Font-Size="7pt" />
                        <NextPrevStyle VerticalAlign="Bottom" />
                        <OtherMonthDayStyle ForeColor="#808080" />
                        <SelectedDayStyle BackColor="#666666" Font-Bold="True" ForeColor="White" />
                        <SelectorStyle BackColor="#CCCCCC" />
                        <TitleStyle BackColor="#999999" BorderColor="Black" Font-Bold="True" />
                        <TodayDayStyle BackColor="#CCCCCC" ForeColor="Black" />
                        <WeekendDayStyle BackColor="#FFFFCC" />
                    </asp:Calendar>