Show JavaScript Alert Message when using AJAX UpdatePanel in ASP.Net

This way

HTML

<asp:ScriptManager ID = "ScriptManager1" runat = "server"></asp:ScriptManager>
<asp:UpdatePanel ID = "UpdatePanel1" runat="server">
    <ContentTemplate>
        <asp:Button Text="Show Message" runat="server" OnClick = "ShowMessage" />
    </ContentTemplate>
</asp:UpdatePanel>
<asp:UpdateProgress runat="server" AssociatedUpdatePanelID = "UpdatePanel1">
    <ProgressTemplate>
        <img alt = "" src="103.gif" />
    </ProgressTemplate>
</asp:UpdateProgress>

Code

protected void ShowMessage(object sender, EventArgs e)
{
    System.Threading.Thread.Sleep(1000);
    string message = "alert('Hello! Mudassar.')";
    ScriptManager.RegisterClientScriptBlock((sender as Control), this.GetType(), "alert", message, true);
}