What will execute faster?

104.

void Method(object obj)
   {
   if (obj is IList)
   {
((IList)obj).Clear();
   }
   }

void Method(object obj)
   {
   IList list = obj as IList;
   if (list != null)
   {
list.Clear();
   }
   }

void Method(object obj)
   {
   if (obj is IList)
   {
(obj as IList).Clear();
   }
   }

void Method(object obj)
{ IList list = obj as IList;
if (list != null) {
list.Clear();
}
}

Using the 'as' does the casting and if it cannot perform a cast the resulting value becomes a NULL