Downcasting optionals in Swift: as? Type, or as! Type?

Question

Given the following in Swift:

var optionalString: String?
let dict = NSDictionary()

What is the practical difference between the following

Answer 1

It may be easiest to remember the pattern for these operators in Swift as: ! implies "this might trap," while ? indicates "this might be nil."

refer to: https://developer.apple.com/swift/blog/?id=23