Should I prefer the 'is' or 'as' operator? [duplicate]

Question

Possible Duplicate:
casting vs using the 'as' keyword in the CLR

foreach (MyClass i in x)
{
        if (i is IMy)
        {
                IMy a = (IMy)i;
                a.M1();
        }
}

or

foreach (MyClass i in x)
{
        IMy a = i as IMy;
        if (a != null)
        {
                a.M1();
        }

}

Answer 1

Second is more preferable as you cast 1 time