<p>So here is the simple code:</p>

<pre><code>    [System.ComponentModel.DefaultValue(true)]
    public bool AnyValue { get; set; }
</code></pre>

<p>I am sure I don't set AnyValue to false again (I just created it). This property is a property of a Page class of ASP.NET. And I am cheking the value in a button event handling function. But somehow it is still false. 
I wonder when actually it is set true? On compile time? When class is instantiated?</p>

<p>What do you think about what I am doing wrong?</p>

DefaultValue does
NOT
set the value.
What it does is tell VisualStudio what the default value is.  When a visual element (Button, listbox etc) is selected on a form, and the Property panel is displayed, VS will
bold
the values of properties which are set to something besides the value given in DefaultValue.
Hence in your case, since AnyValue is false, but it's DefaultValue is true, then is will display
false
in bold in the Property panel.  If you were to manually change it to "true", then it will be displayed non-bolded.