Assign NSNull Object to NSString

Question

I'm writing an iOS App where i need to get data from a SQL-Database over mobile Services from Azure.

After downloading the data I get a NSDictionary with all attributes from the SQL-Table. If an attribute is empty, the value is NSNull.

Is there a way to pass NSNull to NSString without an IF-Statement (I don't want to have 20 if statements..)?

Write a simple help method or function that returns what you want based on passing in either a string or `NSNull`. Then you don't need lots of `if` statements. Just a simple method or function call. — rmaddy, Feb 20 '14 at 23:36
"Pass NSNull to NSString" makes no sense. You can certainly assign an NSNull to a pointer that's typed `NSString*`, though, without any real difficulty. Maybe you need to give us an example of your problem. — Hot Licks, Feb 20 '14 at 23:47

score 7 · Accepted Answer · answered Feb 20 '14 at 23:47

7

I wrote a category just for dealing with this issue. I used it with Core Data but it should help you, too.

@interface NSDictionary (Extensions)

- (id)NSNullToNilForKey:(NSString *)key;

@end

@implementation NSDictionary (Extensions)

- (id)NSNullToNilForKey:(NSString *)key
{
    id value = [self valueForKey:key];

    return value != [NSNull null] ? value : nil;
}

@end

Sample use:

NSString *value = [dictionary NSNullToNilForKey:@"key"];

answered Feb 20 '14 at 23:47

NathanAldenSr

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You're welcome. :) I use this category extensively when converting JSON structures to Core Data entities, since the JSON deserializer I use deserializes null values to `[NSNull null]`. – NathanAldenSr Feb 21 '14 at 15:02
exactly the thing i want to use it! – fahu Feb 21 '14 at 18:10
Could you list steps on how to implement this? – John Mar 01 '15 at 19:05
@John Sorry, I haven't done iOS programming in years, but it should be as simple as just creating a new class file and header file, no? – NathanAldenSr Sep 03 '16 at 22:18

score 2 · Answer 2 · answered Feb 20 '14 at 23:49

You can't just assign it, but you can filter out all of the NSNull instances using something like this:

NSDictionary *dictionary = // data from server
NSDictionary *filteredDictionary = [dictionary mutableCopy];

NSSet *keysToRemove = [orig keysOfEntriesPassingTest:^BOOL(id key, id obj, BOOL *stop) {
    if (obj == [NSNull null]) {
        return YES;
    } else {
        return NO;
    }
}];
[filteredDictionary removeObjectsForKeys:[keysToRemove allObjects]];

Now you have the same dictionary except that every key with an NSNull has been removed.

Assign NSNull Object to NSString

2 Answers2

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