c# using directive depth
<p>I'm trying to understand the c# using directive...Why does this work...</p>

<pre><code>using System;
using System.ComponentModel.DataAnnotations;
namespace BusinessRuleDemo
{
    class MyBusinessClass
    {
        [Required]
        public string SomeRequiredProperty { get; set; }
    }
}
</code></pre>

<p>but this does not?</p>

<pre><code>using System;
using System.ComponentModel;
namespace BusinessRuleDemo
{
    class MyBusinessClass
    {
        [DataAnnotations.Required]
        public string SomeRequiredProperty { get; set; }
    }
}
</code></pre>

<p>The second results in a compile error "The type or namespace DataAnnotations cannot be found.  Are you missing are missing a using directive or an assembly reference?</p>
