Why we use "this" in Extension Methods?
<p>I want to ask why we use "this" keyword before the parameter in an extension method (C# Language)...........
like this function :</p>

<pre><code>    public static int ToInt(this string number)
    {
        return Int32.Parse(number);
    }
</code></pre>

<p>I know that we have to use it but I don't know why.</p>
