How to edit CSS style of a div using C# in .NET

I'm trying to grab a div's ID in the code behind (C#) and set some css on it. Can I grab it from the DOM or do I have to use some kind of control?

<div id="formSpinner">
    <img src="images/spinner.gif" />
    <p>Saving...</p>
</div>

Add the runat="server" attribute to it so you have:

<div id="formSpinner" runat="server">
    <img src="images/spinner.gif">
    <p>Saving...</p>
</div>

That way you can access the class attribute by using:

formSpinner.Attributes["class"] = "classOfYourChoice";

It's also worth mentioning that the asp:Panel control is virtually synonymous (at least as far as rendered markup is concerned) with div, so you could also do:

<asp:Panel id="formSpinner" runat="server">
    <img src="images/spinner.gif">
    <p>Saving...</p>
</asp:Panel>

Which then enables you to write:

formSpinner.CssClass = "classOfYourChoice";

This gives you more defined access to the property and there are others that may, or may not, be of use to you.

Make sure that your div is set to runat="server", then simply reference it in the code-behind and set the "class" attribute.

<div runat="server" id="formSpinner">
   ...content...
</div>

Code-behind

formSpinner.Attributes["class"] = "class-name";

This question makes me nervous. It indicates that maybe you don't understand how using server-side code will impact you're page's DOM state.

Whenever you run server-side code the entire page is rebuilt from scratch. This has several implications:

• A form is submitted from the client to the web server. This is about the slowest action that a web browser can take, especially in ASP.Net where the form might be padded with extra fields (ie: ViewState). Doing it too often for trivial activities will make your app appear to be sluggish, even if everything else is nice and snappy.
• It adds load to your server, in terms of bandwidth (up and down stream) and CPU/memory. Everything involved in rebuilding your page will have to happen again. If there are dynamic controls on the page, don't forget to create them.
• Anything you've done to the DOM since the last request is lost, unless you remember to do it again for this request. Your page's DOM is reset.

If you can get away with it, you might want to push this down to javascript and avoid the postback. Perhaps use an XmlHttpRequest() call to trigger any server-side action you need.

Add the runat="server" attribute to the tag, then you can reference it from the codebehind.