How can I check if an object in an NSArray is NSNull? - ios

Question

I am getting an array with null value. Please check the structure of my array below:
(
"< null>"
)
When I'm trying to access index 0 its crashing because of
-[NSNull isEqualToString:]: unrecognized selector sent to instance 0x389cea70
Currently its crashing because of that array with a crash log:
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSNull isEqualToString:]: unrecognized selector sent to instance 0x389cea70'
*** First throw call stack:
(0x2d9fdf53 0x3820a6af 0x2da018e7 0x2da001d3 0x2d94f598 0x1dee57 0x1dfd31 0x302f598d 0x301a03e3 0x3052aeed 0x3016728b 0x301659d3 0x3019ec41 0x3019e5e7 0x30173a25 0x30172221 0x2d9c918b 0x2d9c865b 0x2d9c6e4f 0x2d931ce7 0x2d931acb 0x3262c283 0x301d3a41 0xabb71 0xabaf8)
libc++abi.dylib: terminating with uncaught exception of type NSException

Answer 1

id object = myArray[0];// similar to [myArray objectAtIndex:0]
if(![object isEqual:[NSNull null]])
{
//do something if object is not equals to [NSNull null]
}

Answer 2

if (myArray != (id)[NSNull null])
OR
if(![myArray isKindOfClass:[NSNull class]])

Answer 3

Building off of Toni's answer I made a macro.
#define isNSNull(value) [value isKindOfClass:[NSNull class]]
Then to use it
if (isNSNull(dict[#"key"])) ...

Answer 4

Awww, guys. This is an easy one.
// if no null values have been returned.
if ([myValue class] == [NSNull class]) {
myValue = nil;
}
I'm sure there are better answers, but this one works.

Answer 5

I found the code for working with NSNull has the following problems:
Looks noisy and ugly.
Time consuming.
Error prone.
So I created the following category:
#interface NSObject (NSNullUnwrapping)
/**
* Unwraps NSNull to nil, if the object is NSNull, otherwise returns the object.
*/
- (id)zz_valueOrNil;
#end
With the implementation:
#implementation NSObject (NSNullUnwrapping)
- (id)zz_valueOrNil
{
return self;
}
#end
#implementation NSNull (NSNullUnwrapping)
- (id)zz_valueOrNil
{
return nil;
}
#end
It works by the following rules:
If a category is declared twice for the same Class (ie singleton instance of the Class type) then behavior is undefined. However, a method declared in a subclass is allowed to override a category method in its super-class.
This allows for more terse code:
[site setValue:[resultSet[#"main_contact"] zz_valueOrNil] forKey:#"mainContact"];
. . as opposed to having extra lines to check for NSNull. The zz_ prefix looks a little ugly but is there for safety to avoid namespace collisions.

Answer 6

You can use the following check:
if (myArray[0] != [NSNull null]) {
// Do your thing here
}
The reason for this can be found on Apple's official docs:
Using NSNull
The NSNull class defines a singleton object you use to represent null values in situations where nil is prohibited as a value (typically in a collection object such as an array or a dictionary).
NSNull *nullValue = [NSNull null];
NSArray *arrayWithNull = #[nullValue];
NSLog(#"arrayWithNull: %#", arrayWithNull);
// Output: "arrayWithNull: (<null>)"
It is important to appreciate that the NSNull instance is semantically different from NO or false—these both represent a logical value; the NSNull instance represents the absence of a value. The NSNull instance is semantically equivalent to nil, however it is also important to appreciate that it is not equal to nil. To test for a null object value, you must therefore make a direct object comparison.
id aValue = [arrayWithNull objectAtIndex:0];
if (aValue == nil) {
NSLog(#"equals nil");
}
else if (aValue == [NSNull null]) {
NSLog(#"equals NSNull instance");
if ([aValue isEqual:nil]) {
NSLog(#"isEqual:nil");
}
}
// Output: "equals NSNull instance"
Taken from https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/NumbersandValues/Articles/Null.html

Answer 7

A lot of good and interesting answers have been given already and (nealry) all of them work.
Just for completion (and the fun of it):
[NSNull null] is documented to return a singleton. Therefore
if (ob == [NSNull null]) {...}
works fine too.
However, as this is an exception I don't think that using == for comparing objects is a good idea in general. (If I'd review your code, I'd certainly comment on this).

Answer 8

In Swift (or bridging from Objective-C), it is possible to have NSNull and nil in an array of optionals. NSArrays can only contain objects and will never have nil, but may have NSNull. A Swift array of Any? types may contain nil, however.
let myArray: [Any?] = [nil, NSNull()] // [nil, {{NSObject}}], or [nil, <null>]
To check against NSNull, use is to check an object's type. This process is the same for Swift arrays and NSArray objects:
for obj in myArray {
if obj is NSNull {
// object is of type NSNull
} else {
// object is not of type NSNull
}
}
You can also use an if let or guard to check if your object can be casted to NSNull:
guard let _ = obj as? NSNull else {
// obj is not NSNull
continue;
}
or
if let _ = obj as? NSNull {
// obj is NSNull
}

Answer 9

Consider this approach:
Option 1:
NSString *str = array[0];
if ( str != (id)[NSNull null] && str.length > 0 {
// you have a valid string.
}
Option 2:
NSString *str = array[0];
str = str == (id)[NSNull null]? nil : str;
if (str.length > 0) {
// you have a valid string.
}