Explicit assignment of null
<pre><code>string s1;
string s2 = null;

if (s1 == null) // compile error
if (s2 == null) // ok
</code></pre>

<p>I don't really understand why the explicit assignment is needed. Whats the difference between a null variable and an unassigned variable? I always assumed that unassigned variables were simply assigned as null by the runtime/compiler anyway. If they're not null, then what are they?</p>
