Why does this null check fail?

Question

I am using this simple code to check if an NSString object is null, but for some reason it fails

here is my simple check

NSString *imageUrl = [dict objectForKey:@"imageUrl"];

    NSLog(@"Jonge: %@", imageUrl);

    if(![imageUrl isEqual:[NSNull null]]) {
        NSLog(@"In the loop");
        NSURL *url = [[NSURL alloc]initWithString:imageUrl];
        NSData *data =[NSData dataWithContentsOfURL:url];
        cell.imageView.image = [UIImage imageWithData:data];
    }

The debug clearly shows the object is null as follows

2017-10-23 11:48:22.711228+0800 NWMobileTill[46614:7498779] Jonge: (null)

But I still end up in the loop as below

2017-10-23 11:48:22.711367+0800 NWMobileTill[46614:7498779] In the loop

Why is my null check not working?

Possible duplicate of [What's the difference between \[NSNull null\] and nil?](https://stackoverflow.com/questions/836601/whats-the-difference-between-nsnull-null-and-nil) — Balasubramanian, Oct 23 '17 at 06:16
Closely related: [What's the difference between (null) vs ?](https://stackoverflow.com/q/11287590) — jscs, Oct 23 '17 at 12:35

score 0 · Accepted Answer · answered Oct 23 '17 at 04:20

0

(null) represents nil, not [NSNull null], so the check imageUrl != NSNull succeeds.

You have to write

if (imageUrl) {

which is the same as

if (imageUrl != nil) {

answered Oct 23 '17 at 04:20

vadian

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Is there a document about the different types null nil etc? I googled but could not find a good one – Matt Douhan Oct 23 '17 at 04:23
2

`NSNull` is used only in a few cases. Mattt Thompson wrote an [article about `nil / Nil / NULL / NSNull`](http://nshipster.com/nil/) – vadian Oct 23 '17 at 04:27

Miti · Answer 2 · 2018-07-23T06:37:41.270

0

You can also check for string is blank or not.

if(![imageUrl isEqualToString:@""])

Or

if(![imageUrl isKindOfClass:[NSNull class]])

edited Jul 23 '18 at 06:37

answered Oct 23 '17 at 05:53

Miti

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Sambit Prakash · Answer 3 · 2017-10-23T06:16:28.770

nil represents nothing or in technical term, we can say that '\0' (according to C concepts). When ever we make an instance of any Class, by default Objective C makes it nil.

[NSNull null] will give an Object. In Objective C, use of NSNull is limited. It is used in collections (like NSArray and NSDictionary)

As you are using a NSDictionary, you can check it as below.

if ([dict objectForKey:@"imageUrl"] != [NSNull null]) {
    //your code...
}

Why does this null check fail?

3 Answers3