Tuesday, April 13, 2010

3 Reasons to Avoid Automatic Properties

That’s dramatic overstatement of course, because automatic properties are great in many cases (though are public fields really so wrong?) But now that VB.Net has joined the party too [1], it’s worth remembering that they are not all good news:

1/ They Can’t be Made ReadOnly

Sure you can make them have a private setter, but that’s not the same as a readonly field, which is a great check against whole classes of screw-ups. If a field shouldn’t change during an instance lifetime, make it readonly, and save yourself some pain.

2/ No Field Initializers (in C#)

The nice thing about initializing fields in the field initializers is you can’t forget to do so in one of the constructor overloads, and (in conjunction with readonly fields) you can ensure it can never be null. Since this is all on one line it’s easy to inspect visually, without having to chase down code paths / constructor chains by eye.

(You can vote for this, for all the good it will do [2])

3/ Poor Debugging Experience

Properties are methods, right, even auto-generated ones, and need to be executed for the debugger to ‘see’ the value. But that’s not always possible. If the managed thread is suspended (either via a threading / async wait, or by entering unmanaged code) then the debugger can’t execute the property at all, and you’ll just see errors the below:

Cannot evaluate expression because the current thread is in a sleep, wait or join

Here you can only determine the value of ‘AutoProperty’ through inference and guesswork, whereas ‘ManualProperty’ can always be determined from the backing field. This can be a real pain in the arse, so it’s worth avoiding automatic properties for code around thread synchronisation regions.

As an aside remember that there are backing fields, it’s just you didn’t create them, the compiler did, and it used it’s own naming convention (to avoid collisions) which is a bit odd. So if you write any ‘looping over fields’ diagnostic code you will see some strange names, which might take some getting used to. You’ll also see those in WinDBG and CDB when you’re looking at crash dumps and the like.


[1] ...but I bet the VB community spat chips over the curly brackets in Collection Initializers
[2] And yet whilst VB.Net 4 has this, they don’t have mixed accessibility for auto properties yet. Go figure.

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