iOS - How to access object properties in an array properly?

You can do this

PersonClass *tmp = (PersonClass *) [MyArray objectAtIndex:0];
NSLog(@"%@",tmp.name);

Or we can put this on just one line:

NSLog(@"%@",((PersonClass *)[MyArray objectatIndex:0]).name);

The reason that NSLog(@"%@", [self.contactsArray objectAtIndex:0].name); doesn't work is you haven't told the compiler what type of object is at [self.contactsArray objectAtIndex:0]. Because it doesn't know what type of object is in your array, there is no way for it to know that it has a name property

((PersonClass *)[MyArray objectAtIndex:0]).name

or just [[MyArray objectAtIndex:0] name]

Do I have to temporarily create an instance of PersonClass like this?

PersonClass *tmp = [PersonClass alloc] init];
tmp = [MyArray objectAtIndex:0];

NSLog(@"%@", tmp.name);

No. Just use

PersonClass *tmp = (PersonClass *)[myArray objectAtIndex:0];
NSLog(@"%@", tmp.name);

As the object you are getting from myArray is (or should be!) already alloc'd and init'd before being added to the array. The things in the array are pointers to objects. That's what you are passing around. You have to tell it which type of object you have in there though (the cast is (PersonClass *)) because NSArray just stores NSObjects.

You could also do it in one line as thus

((PersonClass *)[myArray objectAtIndex:0]).name;

if you want or need the type to be present, then introduce the type like so:

PersonClass * person = [array objectAtIndex:0];

i do this quite regularly for readability, and because included files' declarations can otherwise break things.

note that the cast is not required when assigning an objc variable from an id value.

Do I have to temporarily create an instance of PersonClass?

No. The program you wrote creates a temporary which is never used.