How to hide a div from code (c#)

one fast and simple way is to make the div as

<div runat="server" id="MyDiv"></div>

and on code behind you set MyDiv.Visible=false

if your div has the runat set to server, you surely can do a myDiv.Visible = false in your Page_PreRender event for example.

if you need help on using the session, have a look in msdn, it's very easy.

Give the div "runat="server" and an id and you can reference it in your code behind.

<div runat="server" id="theDiv">

In code behind:

{
    theDiv.Visible = false;
}