As String, ToString and (string)
((object)1).ToString() equals "1"
((object)1) as string equals null
(string)((object)1) throws an exception
Basically, "ToString()" calls a method on "object" or in the closest derived class that overrides the ToString() method.
"As string" performs a cast, but returns null rather than an exception is the cast is not implicitly possible.
(string) peroforms the same cast, but throws an exception on failure.