C# compiler can't find interface Extension Method?
<p>I have an interface:</p>

<pre><code>public interface: IA { ... }
</code></pre>

<p>and I try to extend it into </p>

<pre><code>class public A : IA {
    private static void foo(this IA a) {
        a.foo();
    }
}
</code></pre>

<p>but compiler says, that it can't find foo(), with first parameter of type IA.
How can I fix it?</p>
