How to check the NULL value in NSString in iOS?

问题:

I have an NSString and I want to check if it has a NULL value. If it does, then the if condition should execute. Else it should execute the else condition.

Below is the code which I am using:

if ([appDelegate.categoryName isEqual:[NSNull null]])
{
    select = [[NSString alloc] initWithFormat:@"select * FROM ContentMaster LEFT JOIN Category ON ContentMaster.CategoryID=Category.CategoryID where ContentMaster.ContentTagText='%@'", appDelegate.tagInput];
}
else
{
    select = [[NSString alloc] initWithFormat:@"select * FROM ContentMaster LEFT JOIN Category ON ContentMaster.CategoryID=Category.CategoryID LEFT JOIN Topic ON ContentMaster.TopicID=Topic.TopicID where ContentMaster.ContentTagText='%@' && Category.CategoryName='%@' && Topic.TopicName='%@'", appDelegate.tagInput, appDelegate.categoryName, appDelegate.topicName];
}    

It always executes the else condition, and never the if condition, even when the value is NULL.

回答1:

In Objective-C and Cocoa, the property may not be set—that is, it's nil—or it may be set to the object representation of nil, which is an instance of NSNull. You probably want to check for either of these conditions, like this:

NSString* categoryName = appDelegate.categoryName;
if (categoryName == nil || categoryName == (id)[NSNull null]) {
  // nil branch
} else {
  // category name is set
}

This will execute the nil branch if the categoryName property is set to nil (the default for all properties), or if it's been explicitly set to the NSNull singleton.

回答2:

The NULL value for Objective-C objects (type id) is nil.

While NULL is used for C pointers (type void *).

(In the end both end up holding the same value (0x0). They differ in type however.)

In Objective-C:

nil (all lower-case) is a null pointer to an Objective-C object.
Nil (capitalized) is a null pointer to an Objective-C class.
NULL (all caps) is a null pointer to anything else (C pointers, that is).
[NSNull null] (singleton) for situations where use of nil is not possible (adding/receiving nil to/from NSArrays e.g.)

So to check against NSNull one can either use:

if ((NSNull *)myString == [NSNull null])

or if one wants to omit the need of casting to NSNull:

if ([myString isKindOfClass:[NSNull class]])

回答3:

Try to use this. Check your value is kind of NULL class or not rather than comparing Pointers value.

if ([appDelegate.categoryName isKindOfClass:[NSNull class]]){

        select = [[NSString alloc] initWithFormat:@"select * FROM ContentMaster LEFT JOIN  Category  ON  ContentMaster.CategoryID= Category.CategoryID where ContentMaster.ContentTagText='%@'",appDelegate.tagInput];

        }

    else {


      select = [[NSString alloc] initWithFormat:@"select * FROM ContentMaster LEFT JOIN  Category  ON  ContentMaster.CategoryID= Category.CategoryID  LEFT JOIN Topic ON ContentMaster.TopicID=Topic.TopicID where ContentMaster.ContentTagText='%@' && Category.CategoryName='%@' && Topic.TopicName='%@'",appDelegate.tagInput,appDelegate.categoryName,appDelegate.topicName];


    }

回答4:

#define SAFESTRING(str) ISVALIDSTRING(str) ? str : @""
#define ISVALIDSTRING(str) (str != nil && [str isKindOfClass:[NSNull class]] == NO)
#define VALIDSTRING_PREDICATE [NSPredicate predicateWithBlock:^(id evaluatedObject, NSDictionary *bindings) {return (BOOL)ISVALIDSTRING(evaluatedObject);}]

SAFESTRING("PASS_OBJECT_HERE");

回答5:

Its better to be on safer side in checking null values , as it can lead to crash.

if (![string isKindOfClass:[NSNull class]] && string && string != NULL) 

回答6:

Use the following code:

-(void)viewDidLoad {
  [super viewDidLoad];
  //Example - 1
    NSString *myString;
    if([[self checkForNull:myString] isEqualToString:@""]){

        NSLog(@"myString is Null or Nil");

   }
   else{

     NSLog(@"myString contains %@",myString);

   }

 //Example - 2
   NSString *sampleString = @"iOS Programming";
    if([[self checkForNull:sampleString] isEqualToString:@""]){

        NSLog(@"sampleString is Null or Nil");

   }
   else{

     NSLog(@"sampleString contains %@",sampleString);

   }


}

-(id)checkForNull:(id)value{
    if ([value isEqual:[NSNull null]]) {

            return @"";
   }

    else if (value == nil)

            return @"";
    return value;

}

In Example -1, myString contains nothing. So the output is:

         myString is Null or Nil

In Example -2, sampleString contains some value. So the output is:

    sampleString contains iOS Programming

回答7:

+(BOOL)isEmpty:(NSString *)str{
    if (str == nil || str == (id)[NSNull null] || [[NSString stringWithFormat:@"%@",str] length] == 0 || [[[NSString stringWithFormat:@"%@",str] stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] length] == 0){
        return YES;
    }
    return NO;
}

Just pass your string in Method :)

回答8:

You can do it by using - [NSNull class]

if ([appDelegate.categoryName isEqual:[NSNull class]])
    {
        select = [[NSString alloc] initWithFormat:@"select * FROM ContentMaster LEFT JOIN Category ON ContentMaster.CategoryID=Category.CategoryID where ContentMaster.ContentTagText='%@'", appDelegate.tagInput];
    }
    else
    {
        select = [[NSString alloc] initWithFormat:@"select * FROM ContentMaster LEFT JOIN Category ON ContentMaster.CategoryID=Category.CategoryID LEFT JOIN Topic ON ContentMaster.TopicID=Topic.TopicID where ContentMaster.ContentTagText='%@' && Category.CategoryName='%@' && Topic.TopicName='%@'", appDelegate.tagInput, appDelegate.categoryName, appDelegate.topicName];
    }    

回答9:

Add an additional check for length also. This will definitely work.

if ([appDelegate.categoryName isEqual:[NSNull null]] && appDelegate.categoryName.length>0){


        select = [[NSString alloc] initWithFormat:@"select * FROM ContentMaster LEFT JOIN  Category  ON  ContentMaster.CategoryID= Category.CategoryID where ContentMaster.ContentTagText='%@'",appDelegate.tagInput];

        }        else {
          select = [[NSString alloc] initWithFormat:@"select * FROM ContentMaster LEFT JOIN  Category  ON  ContentMaster.CategoryID= Category.CategoryID  LEFT JOIN Topic ON ContentMaster.TopicID=Topic.TopicID where ContentMaster.ContentTagText='%@' && Category.CategoryName='%@' && Topic.TopicName='%@'",appDelegate.tagInput,appDelegate.categoryName,appDelegate.topicName];

    }

回答10:

  NSString *str;

  if ([[str stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]]          isEqualToString:@""] || str==nil) 
  {

  }