Why should I avoid using Properties in C#?

Question

In his excellent book, CLR Via C#, Jeffrey Richter said that he doesn\'t like properties, and recommends not to use them. He gave some reason, but I don\'t really understand

Answer 1

Back in 2009, this advice merely seemed like bellyaching of the Who Moved My Cheese variety. Today, it's almost laughably obsolete.

One very important point that many answers seem to tiptoe around but don't quite address head on is that these purported "dangers" of properties are an intentional part of the framework design!

Yes, properties can:

• Specify different access modifiers for the getter and setter. This is an advantage over fields. A common pattern is to have a public getter and a protected or internal setter, a very useful inheritance technique which isn't achievable by fields alone.

• Throw an exception. To date, this remains one of the most effective methods of validation, especially when working with UI frameworks that involve data-binding concepts. It's much more difficult to ensure that an object remains in a valid state when working with fields.

• Take a long time to execute. The valid comparison here is with methods, which take equally long - not fields. No basis is given for the statement "a method is preferred" other than one author's personal preference.

• Return different values from its getter on subsequent executions. This almost seems like a joke in such close proximity to the point extolling the virtues of ref/out parameters with fields, whose value of a field after a ref/out call is pretty much guaranteed to be different from its previous value, and unpredictably so.

  If we're talking about the specific (and practically academic) case of single-threaded access with no afferent couplings, it's fairly well understood that it's just bad property design to have visible-state-changing side-effects, and maybe my memory is fading, but I just can't seem to recall any examples of folks using DateTime.Now and expecting the same value to come out every time. At least not any instances where they wouldn't have screwed it up just as badly with a hypothetical DateTime.Now().

• Cause observable side effects - which is of course precisely the reason properties were invented as a language feature in the first place. Microsoft's own Property Design guidelines indicate that setter order shouldn't matter, as to do otherwise would imply temporal coupling. Certainly, you can't achieve temporal coupling with fields alone, but that's only because you can't cause any meaningful behaviour at all to happen with fields alone, until some method is executed.

  Property accessors can actually help prevent certain types of temporal coupling by forcing the object into a valid state before any action is taken - for example, if a class has a StartDate and an EndDate, then setting the EndDate before the StartDate could force the StartDate back as well. This is true even in multi-threaded or asynchronous environments, including the obvious example of an event-driven user interface.

Other things that properties can do which fields can't include:

• Lazy loading, one of the most effective ways of preventing initialization-order errors.
• Change Notifications, which are pretty much the entire basis for the MVVM architecture.
• Inheritance, for example defining an abstract Type or Name so derived classes can provide interesting but nevertheless constant metadata about themselves.
• Interception, thanks to the above.
• Indexers, which everyone who has ever had to work with COM interop and the inevitable spew of Item(i) calls will recognize as a wonderful thing.
• Work with PropertyDescriptor which is essential for creating designers and for XAML frameworks in general.

Richter is clearly a prolific author and knows a lot about the CLR and C#, but I have to say, it seems like when he originally wrote this advice (I'm not sure if it's in his more recent revisions - I sincerely hope not) that he just didn't want to let go of old habits and was having trouble accepting the conventions of C# (vs. C++, for example).

What I mean by this is, his "properties considered harmful" argument essentially boils down to a single statement: Properties look like fields, but they might not act like fields. And the problem with the statement is, it isn't true, or at best it's highly misleading. Properties don't look like fields - at least, they aren't supposed to look like fields.

There are two very strong coding conventions in C# with similar conventions shared by other CLR languages, and FXCop will scream at you if you don't follow them:

1. Fields should always be private, never public.
2. Fields should be declared in camelCase. Properties are PascalCase.

Thus, there is no ambiguity over whether Foo.Bar = 42 is a property accessor or a field accessor. It's a property accessor and should be treated like any other method - it might be slow, it might throw an exception, etc. That's the nature of Abstraction - it's entirely up to the discretion of the declaring class how to react. Class designers should apply the principle of least surprise but callers should not assume anything about a property except that it does what it says on the tin. That's on purpose.

The alternative to properties is getter/setter methods everywhere. That's the Java approach, and it's been controversial since the beginning. It's fine if that's your bag, but it's just not how we roll in the .NET camp. We try, at least within the confines of a statically-typed system, to avoid what Fowler calls Syntactic Noise. We don't want extra parentheses, extra get/set warts, or extra method signatures - not if we can avoid them without any loss of clarity.

Say whatever you like, but foo.Bar.Baz = quux.Answers[42] is always going to be a lot easier to read than foo.getBar().setBaz(quux.getAnswers().getItem(42)). And when you're reading thousands of lines of this a day, it makes a difference.

_{(And if your natural response to the above paragraph is to say, "sure it's hard to read, but it would be easier if you split it up in multiple lines", then I'm sorry to say that you have completely missed the point.)}