swift 3 - ios : convert anyObject to string

Here's three options for you:

Option 1 - if let

if let b = a as? String {
    print(b) // Was a string
} else {
    print("Error") // Was not a string
}

Option 2 - guard let

guard let b = a as? String
else {
    print("Error") // Was not a string
    return // needs a return or break here
}
print(b) // Was a string

Option 3 - let with ?? (null coalescing operator)

let b = a as? String ?? ""
print(b) // Print a blank string if a was not a string

Here's a simple function (repl.it) that will mash any value into a string, with nil becoming an empty string. I found it useful for dealing with JSON that inconsistently uses null, blank, numbers, and numeric strings for IDs.

import Foundation

func toString(_ value: Any?) -> String {
  return String(describing: value ?? "")
}

let d: NSDictionary = [
    "i" : 42,
    "s" : "Hello, World!"
]

dump(toString(d["i"]))
dump(toString(d["s"]))
dump(toString(d["x"]))

Prints:

- "42"
- "Hello, World!"
- ""

The compiler suggests that you replace your code with:

let s = String(describing: str)

One other option is available if you have a situation where you want to silently fail with an empty string rather than store something that might not originally be a string as a string.

let s =  str as? String ?? ""

else you have the ways of identifying and throwing an error in the answers above/below.

Try

let a = "Test" as AnyObject
guard let b = a as? String else { // Something went wrong handle it here }
print(b) // Test