how to display none through code behind

I believe this should work:

login_div.Attributes.Add("style","display:none");

if(displayit){
  login_div.Style["display"]="inline"; //the default display mode
}else{
  login_div.Style["display"]="none";
}

Adding this code into Page_Load should work. (if doing it at Page_Init you'll have to contend with viewstate changing what you put in it)

Since this is a login div, shouldn't the default be to NOT display it. I am going to go ahead and assume then you want to display it then via javascript.

<div id="login" style="display:none;">Content</div>

Then using jQuery:

<script type="javascript">$('#login').show();</script>

Another method you might consider is something like this:

<div id="login" style="display:<%=SetDisplay() %>">Content</div>

And the SetDisplay() method output "none" or "block"

div_id.Attributes.Add("style","display:none");

div_id: id which you want to hide.

Attributes: that will use value.

Add: keyword will add the attribute.

Style: is the attribute.

and last one is the name and value of the attribute