C# properties. "if (value is double)" is always true : <p>So, I'm supposed to check for erroneous value but I get the message that "if (value is double)" is always true, what am I doing wrong?</p>

<pre><code> public double _ProductCost
    {
        get
        { return Cost;}
        set
        {
            if (value is double)
            {
                Cost = value;
            }
            else
            {
                NumberServed = 0;
                Console.WriteLine("erroneous value!");
            }
        }
    }
</code></pre>
