How to use C# constant at ASP.Net page?

The examples given below could make little sense, but it is because I am focusing on syntax.

Let's say I have such C# code:

public static class Foo
{
  public const string Bar = "hello world.";
}

Now, I would like to use Foo.Bar constant in ASP.Net instead of typing each time "hello world.". So I used this syntax:

<p><%= Foo.Bar %></p>

It works. It works also in such cases:

<p>"<%= Foo.Bar %>"</p>

<p class="<%= Foo.Bar %>">the weird, nonsense example</p>  

So, it works with quotes too. However I have also cases, when quotes get higher priority:

<custom:Header runat='server' Text="<%= Foo.Bar %>"/>

( header is a custom control -- it simply adds some css by default and position ). In such case quotes marks have higher priority and entire text is sucked as-is, in effect I get header with text

<%= Foo.Bar %>

So, my question is -- what is the syntax to get the value of C# constant, no matter what (IOW -- with highest priority)?

Edits:

<custom:Header runat='server' Text="<%# Foo.Bar %>"/>

(note hash instead of equal sign) does not work as well.