What is the right way to check for a null string in Objective-C

Question

I was using this in my iPhone app  if  title    nil           do something     but it throws some exception  and the console shows that the title is   null     So I m using this now   if  title    nil     title isKindOfClass  NSNull class            do something     What is the difference  and what is the best way to determine whether a string is null

User · Answer

There are two situations   It is possible that an object is  NSNull null   or it is impossible   Your application usually shouldn t use  NSNull null   you only use it if you want to put a  null  object into an array  or use it as a dictionary value  And then you should know which arrays or dictionaries might contain null values  and which might not   If you think that an array never contains  NSNull null  values  then don t check for it  If there is an  NSNull null   you might get an exception but that is fine  Objective-C exceptions indicate programming errors  And you have a programming error that needs fixing by changing some code    If an object could be  NSNull null   then you check for this quite simply by testing   object     NSNull null    Calling isEqual or checking the class of the object is nonsense  There is only one  NSNull null  object  and the plain old C operator checks for it just fine in the most straightforward and most efficient way    If you check an NSString object that cannot be  NSNull null   because you know it cannot be  NSNull null  or because you just checked that it is different from  NSNull null   then you need to ask yourself how you want to treat an empty string  that is one with length 0  If you treat it is a null string like nil  then test  object length    0   object length will return 0 if object    nil  so this test covers nil objects and strings with length 0  If you treat a string of length 0 different from a nil string  just check if object    nil    Finally  if you want to add a string to an array or a dictionary  and the string could be nil  you have the choice of not adding it  replacing it with      or replacing it with  NSNull null   Replacing it with     means you lose the ability to distinguish between  no string  and  string of length 0   Replacing it with  NSNull null  means you have to write code when you access the array or dictionary that checks for  NSNull null  objects

User · Answer

if   strpass isEqual  NSNull null      strpass  nil     strpass isEqualToString    lt null gt        strpass isEqualToString    null       strpass length  0     strpass isEqualToString               string is blank

User · Answer

If you want to test against all nil empty objects  like empty strings or empty arrays sets  you can use the following   static inline BOOL IsEmpty id object        return object    nil              object respondsToSelector  selector length            amp  amp    NSData    object length     0               object respondsToSelector  selector count            amp  amp    NSArray    object count     0

User · Answer

For string      BOOL  checkStringIsNotEmpty  NSString  string   if  string    nil    string length    0  return NO  return YES

User · Answer

For string      BOOL  checkStringIsNotEmpty  NSString  string   if  string    nil    string length    0  return NO  return YES       Refer the picture below

User · Answer

Whats with all these  works for me answers    We re all coding in the same language and the rules are   Ensure the reference isn t nil Check and make sure the length of the string isn t 0   That is what will work for all  If a given solution only  works for you   its only because your application flow won t allow for a scenario where the reference may be null or the string length to be 0  The proper way to do this is the method that will handle what you want in all cases

User · Answer

As others have pointed out  there are many kinds of  null  under Cocoa Objective C   But one further thing to note is that  title isKindOfClass  NSNull class   is pointlessly complex since  NSNull null  is documented to be a singleton so you can just check for pointer equality  See Topics for Cocoa  Using Null   So a good test might be   if  title     id  NSNull null     title length    0   title     Something     Note how you can use the fact that even if title is nil  title length will return 0 nil false  ie 0 in this case  so you do not have to special case it   This is something that people who are new to Objective C have trouble getting used to  especially coming form other languages where messages method calls to nil crash

User · Answer

MACRO Solution  2020   Here is the macro that I use for safe string instead of getting   null   string on a UILabel for example    define SafeString STRING    STRING length     0         STRING    let say you have an member class and name property  and name is nil   NSLog        member name      prints  null  on UILabel   with macro   NSLog        SafeString member name       prints empty string on UILabel   nice and clean   Extension Solution  2020   If you prefer checking nil Null and empty string in your project you can use my extension line below   NSString Extension h          Checks if giving String is an empty string or a nil object or a Null       param string string value to check         BOOL isNullOrEmpty  NSString  string    NSString Extension m     BOOL isNullOrEmpty  NSString  string       if  string       is not Nil         NSRange range    string rangeOfString string           BOOL isEmpty    range length  lt   0     string isEqualToString                 BOOL isNull   string     id  NSNull null            return  isNull    isEmpty              return YES      Example Usage  if    NSString isNullOrEmpty someTitle            You can safely use on a Label or even add in an Array for example  Remember  Arrays don t like the nil values

User · Answer

Complete checking of a string for null conditions can be a s follows   lt  br        if mystring                if  mystring isEqualToString                           mystring   some string                            else                  statements

User · Answer

Try this for check null   if  text    nil

User · Answer

interface NSString  StringFunctions  -  BOOL  hasCharacters   end   implementation NSString  StringFunctions  -  BOOL  hasCharacters       if self     id  NSNull null             return NO       else           if  self length     0                return NO                      return YES     end  NSString  strOne   nil  if  strOne hasCharacters         NSLog       strOne    else       NSLog   String is Empty        This would work with the following cases  NSString  strOne       OR NSString  strOne     StackOverflow  OR NSString  strOne    NSNull null  OR NSString  strOne

User · Answer

What works for me is if    myobject

User · Answer

I only check null string with     if   myString isEqual  NSNull null

User · Answer

it is just as simple as  if  object length   gt 0         do something     remember that in objective C if object is null it returns 0 as the value   This will get you both a null string and a 0 length string

User · Answer

if   linkedStr isEqual  id  NSNull null                                           linkedinLbl text   No                    else                       linkedinLbl text   Yes

User · Answer

You just check for nil  if data   Bonds    nil     NSLog   it is nil        or  if   data   Bonds   isKindOfClass  NSNull class          NSLog   it is null

User · Answer

if textfield text length    0        do your desired work

User · Answer

If that kind of thing does not already exist  you can make an NSString category    interface NSString  TrucBiduleChoseAdditions   -  BOOL isEmpty    end   implementation NSString  TrucBiduleChoseAdditions   -  BOOL isEmpty       return self    nil         isEqualToString self       end

User · Answer

I have found that in order to really do it right you end up having to do something similar to                if       myString isEqual  NSNull null      amp  amp     myString length     0           Otherwise you get weird situations where control will still bypass your check  I haven t come across one that makes it past the isEqual and length checks

User · Answer

Refer to the following related articles on this site    Is if  variable  the same as if  variable    nil  in Objective-C h   I think your error is related to something else as you shouldn t need to do the extra checking   Also see this related question  Proper checking of nil sqlite text column

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