-[NSNull length]: unrecognized selector sent to JSON objects - ios

Question

I'm developing an iOS 5.0+ app with latest SDK.
I get a very strange error with this code:
- (NSMutableURLRequest*)setupRequestWithService:(NSString*)service andMethod:(NSString*)method
{
NSString* url = [NSString stringWithFormat:#"%#%#.svc/%#", serverUrl, service, method];
NSMutableURLRequest* request = [[NSMutableURLRequest alloc] initWithURL:[NSURL URLWithString:url]];
// Set authentication token.
NSLog(#"???????????? %#", authenticationToken);
if (authenticationToken == nil)
NSLog(#"NULL AUTHTOKEN");
if ([authenticationToken isEqual:[NSNull null]])
NSLog(#"NSNULL AUTHTOKEN");
if (request == nil)
NSLog(#"NULL REQUEST");
[request addValue:authenticationToken forHTTPHeaderField:REQUEST_HEADER_AUTH_TOKEN];
return request;
}
This is my log:
???????????? <null>
NSNULL AUTHTOKEN
-[NSNull length]: unrecognized selector sent to instance 0x3b5a5090
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSNull length]: unrecognized selector sent to instance 0x3b5a5090'
It seems that authenticationToken is NULL. But I don't understand that, if authenticationToken is NULL why I don't see NULL AUTHTOKEN on the log.
I get this error the second time I run that method, the first time, I don't get any error. This is my log:
???????????? (null)
NULL AUTHTOKEN
By the way:
NSString* authenticationToken;
Any advice?
Maybe there is a Memory Leak somewhere...

Answer 1

My solution to this maddening use of NSNull by JSON interpreters is to create a category on NSNull, where I define integerValue, floatValue, length, etc - return 0 for all. Everytime you get another crash add a new category. I think I had 6 or 7 when I had this issue.
The problem with NOT doing this is you have to look for the NULL everywhere in your converted objects - a PITA in my opinion.
EDIT: the code I'm using, all in a NSNull+JSON.m file:
#interface NSNull (JSON)
#end
#implementation NSNull (JSON)
- (NSUInteger)length { return 0; }
- (NSInteger)integerValue { return 0; };
- (float)floatValue { return 0; };
- (NSString *)description { return #"0(NSNull)"; }
- (NSArray *)componentsSeparatedByString:(NSString *)separator { return #[]; }
- (id)objectForKey:(id)key { return nil; }
- (BOOL)boolValue { return NO; }
#end
EDIT2: Now in Swift 3:
extension NSNull {
func length() -> Int { return 0 }
func integerValue() -> Int { return 0 }
func floatValue() -> Float { return 0 };
open override var description: String { return "0(NSNull)" }
func componentsSeparatedByString(separator: String) -> [AnyObject] { return [AnyObject]() }
func objectForKey(key: AnyObject) -> AnyObject? { return nil }
func boolValue() -> Bool { return false }
}

Answer 2

The error message is pretty clear. NSNull and nil are different things:
The NSNull class defines a singleton object used to represent null values in
collection objects (which don’t allow nil values).
If you want to check if authenticationToken is NSNull try: [authenticationToken isEqual: [NSNull null]]

Answer 3

In line with David H's answer, how about a category on NSNull that just uses ObjC's message forwarding to "do nothing", to emulate the runtime's behavior when sending messages to nil?
Like this:
#interface NSNull (ForwardInvocation)
#end
#implementation NSNull (ForwardInvocation)
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
return [NSNull methodSignatureForSelector:#selector(description)];
}
- (void)forwardInvocation:(NSInvocation *)anInvocation {
// do nothing; prevent 'unrecognized selector' crashes
}
#end
The [NSNull methodSignatureForSelector:#selector(description)]; takes advantage of the fact that NSNull inherits from NSObject, which provides the description method. This satisfies the forwarding mechanism requirement for implementing -methodSignatureForSelector:.

Answer 4

The problem comes because your method return an NSNull object.
You can't check [authenticationToken isEqual:[NSNull null]]) because [NSNull null] give an instance of an object. So it's different from your object itself.
If you want to check if you received an NSNull object you need to check like this: [authenticationToken isKindOfClass:[NSNull class]] instead.