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How do I display an image within the HiddenField asp.NET field?

Note: I'm just an lowly intern here, so be nice :) I've spent some time researching before asking but didn't find the answer I'm looking for.
Suppose I have a line in my .aspx file like so:
<asp:HiddenField runat="server" ID="hiddenfield" Value=??? >
What should I put for the value if I would like this hidden field to display a .png image when shown?
Sorry I can't give much more detail, I don't want to violate my company's policies. If you need any more information, let me know and I'll try to give an example.

If you want to display an image, you should use the Image control.
If you want to hide it, wrap it in a div and set the visibility to hidden. For example:
<div id="divHidden" style="visibility: hidden">
<asp:Image runat="server" AlternateText="this is an image" ImageUrl="/img/myimage.gif"/>
</div>

May be you can save image path only in hidden field using this.
<asp:HiddenField runat="server" ID="hiddenfield" Value="/img/myimage.png" >
And later retrieve that using javscript document.getElementById("hiddenfield").value , and set that path to actual img element if needed.

You probably want to use:
<asp:Image ID="myImage" ImageUrl="myimage.png" Visible="false" AlternateText="foo" />
And when you need to show it from your codeBehind:
myImage.Visible=true;

Aligning html inputs on same line 2

I have the same behavior from many similar questions here.
But I tried anything and nothing happens to me.
I have 2 html controls. One anchor tag and a input button.
I applied vertical-align:top; float:right; display:inline-block; independently and together and nothing happens.
When I click on Close, I see a shadow like a button! I want them inline both controls
At the end of table I incluided a Div like this.
<div style="float:right;">
<asp:Button ID="btnInput" runat="server" Text="Add selected"></asp:Button>Close
</div>

You need to set a width for each element when you make them display:inline-block, otherwise they default to 100%.
Here is a jsFiddle with the closest equivalent markup I could make. (asp buttons don't work)

As Dolchio said, every element must have display: inline-block for this to work. Note that your float:right will not achieve anything helpful in this scenario.
Try adding the CssClass attribute to the asp button and styling that class.
So it would look like <asp:Button ID="btnInput" runat="server" CssClass="myButton" Text="Add selected"></asp> and in your css myButton{display:inline-block,vertical-align: top, width: 200px substitute width for whatever the width of the button is (not entirely familiar with asp buttons and their implied widths).

Access div element from code

I have a div element which I would like to change it'e dir attribute from the code.
The problem is that I can't set the runat=server attribute on the div, so it's not accessible from the code.
Using asp.net, c#, framework 4.0.
Any idea?
Thanks.

If you don't want to set the div to runat="server" so you can do the following:
<div dir='<%= GetDir() %>'>
Your text
</div>
and in the code behind you can set the direction using the following code:
if(You Condition)
{
return "ltr";
}
else
{
return "rtl";
}

Yes, you can't access to the client elements on the page, only to the server-ones (asp.net is a server-oriented framework). The client controls are compiled to the Literal controls with html in them.
If you don't want to set div for the runat="server" attribute, you can register client script to edit your divs content.
If you set the runat="server" attribute, and set the ID="YOUR_DIV_ID_HERE" for it, it will be accessible from code under YOUR_DIV_ID_HERE name.

If you want you can do this also
<div><%= your server variable %> </div>

Use Jquery and call back function to call server side function.

You can set the runat="server" attribute on a HTML element. Just did a local demo, works just fine. It will be represented as an HtmlGenericControl (http://msdn.microsoft.com/en-us/library/7512d0d0(v=VS.100).aspx).
EDIT: You say you cannot use runat="server" to access the element from code. I am not sure why that would be a problem, as the html rendering should be no different. Maybe it has to do with the elements client side id beeing changed? If you depend on this, you could try setting ClientIdMode=Static for your div.

Creating clickable links in runtime of c# aspx webform

I'm creating asp links using response.write in c#, the same HyperLink code works smoothly when inserted directly in the asp code, but when i copy/paste it to the response.write("...") it appears as an unclickable black text.
Am i forgetting something?
<asp:HyperLink ID='HyperLink1' runat='server' NavigateUrl='Exibe.aspx'> CLICK HERE </asp:HyperLink>
this exact code above thrown in the aspx source works greatly
response.write("<asp:HyperLink ID='HyperLink1' runat='server' NavigateUrl='Exibe.aspx'> CLICK HERE </asp:HyperLink>");
and this turns into a black text

You cannot insert an asp:Hyperlink tag directly into the response stream like that, as the hyperlink is actually a control that needs to "render" itself (if you replaced that with a normal "a" anchor/hyperlink tag it would work fine).
Instead you need to either create the control and add it to the page programatically, or maybe use a repeater control to render the anchors.

You are trying to do totally different things:
the markup (asp:HyperLink) will be compiled.
the Response.Write("asp:HyperLink") will NOT. It will render text as is, and of course you wont't see any link, in fact you should see the text inside the tag asp:HyperLink (inluding the tag itself in the HTML source).
If you want to create a link dunamically you can do it using code snippets below:
<asp:HyperLink ID='HyperLink1' runat='server' NavigateUrl='<%= GetDynamicUrl() %>'> CLICK HERE </asp:HyperLink>
/// Or plain HTML
<%= GetTheLinkText() %>

If you want to generate a hyperlink dynamically on the server-side like this, you can either use Response.Write with an <a> tag like slugster says, or alternatively consider the ASP:Literal control which renders exactly what you give it even if it contains markup e.g.
In your markup:
<asp:literal runat="server" id="MyLiteral" />
In your code:
string myHTMLFragment;
myHTMLFragment = "Hello. I am a link pointing to StackOverflow";
MyLiteral.Text = myHTMLFragment;

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.

Add runat to the element in the markup
<div id="formSpinner" runat="server">
<img src="images/spinner.gif">
<p>Saving...</p>
</div
Then you can get to the control's class attributes by using
formSpinner.Attributes("class")
It will only be a string, but you should be able to edit it.

How do you do this without runat="server"? For example, if you have a
<body runat="server" id="body1">
...and try to update it from within an Updatepanel it will never get updated.
However, if you keep it as an ordinary non-server HTML control you can. Here's the Jquery to update it:
$("#body1").addClass('modalBackground');
How do you do this in codebehind though?

If you do not want to make your control runat server in case you need the ID or simply don't want to add it to the viewstate,
<div id="formSpinner" class="<%= _css %>">
</div>
in the back-end:
protected string _css = "modalBackground";

If all you want to do is conditionally show or hide a <div>, then you could declare it as an <asp:panel > (renders to html as a div tag) and set it's .Visible property.

To expand on Peri's post & why we may not want to use viewstate the following code:
style="<%= _myCSS %>"
Protected _myCSS As String = "display: none"
Is the approach to look at if you're using AJAX, it allows for manipulating the display via asp.net back end code rather than jquery/jscript.