How to add nil value to Swift Dictionary?

How to add nil value to Swift Dictionary?

Basically the same way you add any other value to a dictionary. You first need a dictionary which has a value type that can hold your value. The type AnyObject cannot have a value nil. So a dictionary of type [String : AnyObject] cannot have a value nil.

If you had a dictionary with a value type that was an optional type, like [String : AnyObject?], then it can hold nil values. For example,

let x : [String : AnyObject?] = ["foo" : nil]

If you want to use the subscript syntax to assign an element, it is a little tricky. Note that a subscript of type [K:V] has type V?. The optional is for, when you get it out, indicating whether there is an entry for that key or not, and if so, the value; and when you put it in, it allows you to either set a value or remove the entry (by assigning nil).

That means for our dictionary of type [String : AnyObject?], the subscript has type AnyObject??. Again, when you put a value into the subscript, the "outer" optional allows you to set a value or remove the entry. If we simply wrote

x["foo"] = nil

the compiler infers that to be nil of type AnyObject??, the outer optional, which would mean remove the entry for key "foo".

In order to set the value for key "foo" to the AnyObject? value nil, we need to pass in a non-nil outer optional, containing an inner optional (of type AnyObject?) of value nil. In order to do this, we can do

let v : AnyObject? = nil
x["foo"] = v

or

x["foo"] = nil as AnyObject?

Anything that indicates that we have a nil of AnyObject?, and not AnyObject??.

As documented in here, setting nil for a key in dictionary means removing the element itself.

If you want null when converting to JSON for example, you can use NSNull()

var postDict = Dictionary<String,AnyObject>()
postDict["pass"]=123
postDict["name"]="ali"
postDict["surname"]=NSNull()

let jsonData = NSJSONSerialization.dataWithJSONObject(postDict, options: NSJSONWritingOptions.allZeros, error: nil)!
let jsonString = NSString(data: jsonData, encoding: NSUTF8StringEncoding)!
// -> {"pass":123,"surname":null,"name":"ali"}

You can use the updateValue method:

postDict.updateValue(nil, forKey: surname)

postDict[surname] = Optional<AnyObject>(nil)