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

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])

要么

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

Answer 3

基于托尼的回答,我做了一个宏.

#define isNSNull(value) [value isKindOfClass:[NSNull class]]

然后使用它

if (isNSNull(dict[@"key"])) ...

Answer 4

Awww,伙计们.这是个简单的.

// if no null values have been returned.
if ([myValue class] == [NSNull class]) {
    myValue = nil;
}

我确信有更好的答案,但这个有效.

Answer 5

我发现使用的代码NSNull有以下问题:

• 看起来很吵和丑陋.
• 耗时的.
• 容易出错.

所以我创建了以下类别:

@interface NSObject (NSNullUnwrapping)

/**
* Unwraps NSNull to nil, if the object is NSNull, otherwise returns the object.
*/
- (id)zz_valueOrNil;

@end

随着实施:

@implementation NSObject (NSNullUnwrapping)

- (id)zz_valueOrNil
{
    return self;
}

@end

@implementation NSNull (NSNullUnwrapping)

- (id)zz_valueOrNil
{
    return nil;
}

@end

它的工作原理如下:

• 如果类别被声明两次相同Class(即该Class类型的单例实例),则行为未定义.但是,允许在子类中声明的方法覆盖其超类中的类别方法.

这允许更简洁的代码:

[site setValue:[resultSet[@"main_contact"] zz_valueOrNil] forKey:@"mainContact"];

..而不是有额外的行来检查NSNull.该zz_前缀看起来有点丑,但那里的安全,以避免命名空间冲突.