Check if key exists in NSDictionary is null or not

Question

I did search on how to check if NSDictionary key exists or not and came up with the solution. But still it throws me an error saying adding null value to the key. I am not sure if my code is correct or not. If anyone has any idea about this can help me.

NSDictionary *result;
id myImageURL = [result objectForKey:@"url"];
if ((NSNull *)myImageURL == [NSNull null])
    myImageURL = @"";

id myImage = [result objectForKey:@"image"];
if ((NSNull *)myImage == [NSNull null])
    myImage = @"";

Check if null add nothing and if not add the value. But it still gives me an error dont know why.

/****OUTPUT*****/

2011-08-11 14:56:06.668 Tab_Table_Win[6510:207] RESULTS : {
image = "<UIImage: 0xbc332c0>";
url = "http://a3.twimg.com/profile_images/999228511/normal.jpg";
}

2011-08-11 14:56:06.669 Tab_Table_Win[6510:207] url : http://a3.twimg.com/profile_images/999228511/normal.jpg
2011-08-11 14:56:06.670 Tab_Table_Win[6510:207] IMage : <UIImage: 0xbc332c0>

/*****Breaks Here ***/

2011-08-11 14:56:06.876 Tab_Table_Win[6510:207] RESULTS : {
}
2011-08-11 14:56:06.878 Tab_Table_Win[6510:207] url : (null)
2011-08-11 14:56:06.879 Tab_Table_Win[6510:207] IMage : (null)
2011-08-11 14:56:06.881 Tab_Table_Win[6510:207] *** Terminating app due to uncaught    exception 'NSInvalidArgumentException', reason: '-[__NSCFDictionary setObject:forKey:]: attempt to insert nil key'

Answer 1

Correct answer is :

NSDictionary *result;
NSURL *myImageURL = [result objectForKey:@"url"];
UIImage *myImage = [result objectForKey:@"image"];

/**** Correct way ****/
if (myImageURL != nil && myImage != nil) {
    [images setObject:myImage forKey:myImageURL];
}

Thank you for all the explanation.

Answer 2

If an object doesn't exist for a key, NSDictionary will return nil. An NSNull is an actual object, and therefore a distinct thing. It's like the distinction between being able to record that there was a value and the value as null, and not recording whether there was a value. It also rests a bit on you thinking in C terms of the indirection of a pointer to an object rather than just an object, so it's not completely semantically pleasing from that perspective.

In Objective-C, you may send any message to nil and the result is guaranteed to be nil (or 0). So if your code is designed to ensure that you have a safe object reference, as you might in C++, then what you're doing is unnecessary. Compound statements like:

object = [[Type alloc] init];

Are always explicitly safe, even if alloc fails and returns nil. All that'll happen is that the call to init won't do anything at all, and object will end up with the value nil because the result of sending of init to nil is also nil.

That being said, the answers provided by Bill and Emmanuel should be correct. Compare your result either directly to nil or implicitly to zero. If you're getting a crash later on, I'll guess it's because you're expecting myImageUrl and myImage to be types other than NSString (I notice you've used the typeless id in your original code) and sending them a message they don't respond to.

Answer 3

When you call objectForKeyin nullable dictionary, app gets crashed so I fixed this from this way to avoid from crash.

- (instancetype)initWithDictionary:(NSDictionary*)dictionary {
id object = dictionary;

if (dictionary && (object != [NSNull null])) {
    self.name = [dictionary objectForKey:@"name"];
    self.age = [dictionary objectForKey:@"age"];
}
return self;

}

Answer 4

that was not work for me, i figured it out like this

id myImageURL = [result objectForKey:@"url"];
if ([myImageURL isKindOfClass:[NSNull class]])  
    myImageURL = @"";

Answer 5

Whenever I try to check if an object being returned from a dictionary is null, I do this:

id obj = [myDictionary objectForKey:entityKeyName];
if (obj == [NSNull null]) {
    // do something 
}

Then in your code, it would be:

NSDictionary *result;
NSString *myImageURL = [result objectForKey:@"url"];
if (myImageURL == [NSNull null])
     myImageURL = @"";

That's what I would do in your code.

Also, just making sure, is the NSDictionary result defined? In your code, it doesn't have anything it's being set to. It's just being defined as variable you plan on using called results

Answer 6

this another option:

if (![result objectForKey:@"image"])
    {
        NSLog(@"doesn't exist");
    }
    if ([result objectForKey:@"image"])
    {
        NSLog(@"exist");
    }