What are the differences between nil, NULL and [NSNULL nil]?

I got an error like this -

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: -[NSCFDictionary initWithObjects:forKeys:count:]: attempt to insert nil value at objects[0] (key: searched)'

Then I tried the below code

if (Obj == (id)[NSNull null])  //Obj is an id 
{
    NSLog(@" Obj is (id)[NSNull null]");    
}
else if (Obj == nil)
{
    NSLog(@"Obj is nil");   
} 
else if (Obj == NULL)
{
    NSLog(@"Obj is NULL");  
} 
else
{
    NSLog(@"Obj is something else");
}

searchedDict = [NSDictionary dictionaryWithObject:Obj forKey:@"searched"];

And I got the NSLog message as 'Obj is nil'.

But when I used the same above code when Obj was NSString and my app got terminated due to nil value, I got the NSLog message as 'Obj is (id)[NSNull null]'.

What are the differences between nil, NULL and [NSNULL nil] ?

Are there different kinds of Null values? if yes, How to check commonly for these null values?

Thanks :-)

NULL is the original C-style value given to a pointer which is deemed to point to no explicit address. It's generally just used to mean "unset" or as a sentinel value.

char * oldstring = NULL;

nil is the Objective-C version of the same thing as applied to Obj-C object references:

NSString * str = nil;
id foo = nil;

They are both defined to be equal to the value zero, and although you could use them interchangeably, the former is seen mainly within Obj-C for traditional pointers (like setting void * context pointers to NULL) and nil is used for all Obj-C object references.

The difference exists because although Obj-C's syntax happens to ride on top of C's, the design of the language is higher-level, and its object IDs aren't conceptually the same as just "pointers". Hence the creation of a special sentinel for them.

[NSNull null] is an object that is intended to be uses in very certain cases (like inserting into collections) where an "non-value" value is desired, but actual nil is not allowed, and an object must be used.

Note that nil and NSNull stringify very similarly: nil stringifies as (null), and NSNull as <null>.