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

17 Solutions Collect From Internet About “What is the right way to check for a null string in Objective-C?”

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.

it is just as simple as

if([object length] >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.

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

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]] ) && ( [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.

Whats with all these “works for me answers” ? We’re all coding in the same language and the rules are

  1. Ensure the reference isn’t nil
  2. 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.

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)]
        && [(NSData *) object length] == 0)
        || ([object respondsToSelector:@selector(count)]
        && [(NSArray *) object count] == 0);
}

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.

You just check for nil

if(data[@"Bonds"]==nil){
  NSLog(@"it is nil");
}

or

if ([data[@"Bonds"] isKindOfClass:[NSNull class]]) {
    NSLog(@"it is null");
}
if(textfield.text.length == 0){
   //do your desired work
}

Try this for check null

 if (text == nil)
@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.

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

What works for me is if ( !myobject )

Complete checking of a string for null conditions can be a s follows :<\br>

    if(mystring)
     {
       if([mystring isEqualToString:@""])
        {
          mystring=@"some string";
        }
     }    
    else
     {
        //statements
     }

I only check null string with

if ([myString isEqual:[NSNull null]])

if ([linkedStr isEqual:(id)[NSNull null]])
                {
                    _linkedinLbl.text=@"No";
                }else{
                    _linkedinLbl.text=@"Yes";
                }
if ([strpass isEqual:[NSNull null]] || strpass==nil || [strpass isEqualToString:@"<null>"] || [strpass isEqualToString:@"(null)"] || strpass.length==0 || [strpass isEqualToString:@""])
{
    //string is blank  
}