Hi,
During this postback, the current state of the page and its controls are sent to the server for processing. This process causes the page to refresh and is costly because of the amount of data sent back and forth to the server.
Solution: using Ajax
In AJAX word, A stands for Asynchronous. Asynchronous means not occurring at the same time.
AJAX uses asynchronous data transfer (HTTP requests) between the browser and the web server, allowing web pages to request small bits of information from the server instead of whole pages.
Here is the sample code:
CODE
<form id="form1" runat="server">
<div>
<asp:ScriptManager ID="ScriptManager1" EnablePartialRendering="true" runat="server">
</asp:ScriptManager>
</div>
<asp:DropDownList ID="DropDownList1" runat="server"
AutoPostBack="True">
<asp:ListItem Value="1">Jan</asp:ListItem>
<asp:ListItem Value="2">Feb</asp:ListItem>
<asp:ListItem Value="3">Mar</asp:ListItem>
<asp:ListItem Value="4">Apr</asp:ListItem>
<asp:ListItem Value="5">May</asp:ListItem>
<asp:ListItem Value="6">Jun</asp:ListItem>
<asp:ListItem Value="7">Jul</asp:ListItem>
<asp:ListItem Value="8">Aug</asp:ListItem>
<asp:ListItem Value="9">Sep</asp:ListItem>
<asp:ListItem Value="10">Oct</asp:ListItem>
<asp:ListItem Value="11">Nov</asp:ListItem>
<asp:ListItem Value="12">Dec</asp:ListItem>
</asp:DropDownList>
Year
<asp:DropDownList ID="DropDownList2" runat="server"
AutoPostBack="True">
<asp:ListItem>2005</asp:ListItem>
<asp:ListItem>2006</asp:ListItem>
<asp:ListItem>2007</asp:ListItem>
</asp:DropDownList><br />
<br />
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:Calendar ID="calendar1" runat="server" BackColor="AliceBlue">
</asp:Calendar>
</ContentTemplate>
</asp:UpdatePanel>
</form>
more about
server-centric and client-centric programming in Ajax
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