NSUserDefaults - How to tell if a key exists

Question

objectForKey: will return nil if it doesn't exist.

As mentioned above it wont work for primitive types where 0/NO could be a valid value. I am using this code.

NSUserDefaults *defaults= [NSUserDefaults standardUserDefaults];
if([[[defaults dictionaryRepresentation] allKeys] containsObject:@"mykey"]){

    NSLog(@"mykey found");
}

The objectForKey: method will return nil if the value does not exist. Here's a simple IF / THEN test that will tell you if the value is nil:

if([[NSUserDefaults standardUserDefaults] objectForKey:@"YOUR_KEY"] != nil) {
    ...
}

Swift 3 / 4:

Here is a simple extension for Int/Double/Float/Bool key-value types that mimic the Optional-return behavior of the other types accessed through UserDefaults.

(Edit Aug 30 2018: Updated with more efficient syntax from Leo's suggestion.)

extension UserDefaults {
    /// Convenience method to wrap the built-in .integer(forKey:) method in an optional returning nil if the key doesn't exist.
    func integerOptional(forKey: String) -> Int? {
        return self.object(forKey: forKey) as? Int
    }
    /// Convenience method to wrap the built-in .double(forKey:) method in an optional returning nil if the key doesn't exist.
    func doubleOptional(forKey: String) -> Double? {
        return self.object(forKey: forKey) as? Double
    }
    /// Convenience method to wrap the built-in .float(forKey:) method in an optional returning nil if the key doesn't exist.
    func floatOptional(forKey: String) -> Float? {
        return self.object(forKey: forKey) as? Float
    }
    /// Convenience method to wrap the built-in .bool(forKey:) method in an optional returning nil if the key doesn't exist.
    func boolOptional(forKey: String) -> Bool? {
        return self.object(forKey: forKey) as? Bool
    }
}

They are now more consistent alongside the other built-in get methods (string, data, etc.). Just use the get methods in place of the old ones.

let AppDefaults = UserDefaults.standard

// assuming the key "Test" does not exist...

// old:
print(AppDefaults.integer(forKey: "Test")) // == 0
// new:
print(AppDefaults.integerOptional(forKey: "Test")) // == nil

"objectForKey will return nil if it doesn't exist." It will also return nil if it does exist and it is either an integer or a boolean with a value of zero (i.e. FALSE or NO for the boolean).

I've tested this in the simulator for both 5.1 and 6.1. This means that you cannot really test for either integers or booleans having been set by asking for "the object". You can get away with this for integers if you don't mind treating "not set" as if it were "set to zero".

The people who already tested this appear to have been fooled by the false negative aspect, i.e. testing this by seeing if objectForKey returns nil when you know the key hasn't been set but failing to notice that it also returns nil if the key has been set but has been set to NO.

For my own problem, that sent me here, I just ended up changing the semantics of my boolean so that my desired default was in congruence with the value being set to NO. If that's not an option, you'll need to store as something other than a boolean and make sure that you can tell the difference between YES, NO, and "not set."

Extend UserDefaults once to don't copy-paste this solution:

extension UserDefaults {

    func hasValue(forKey key: String) -> Bool {
        return nil != object(forKey: key)
    }
}

// Example
UserDefaults.standard.hasValue(forKey: "username")