Friday, May 28, 2010

Is public field still so evil in C#?

In Java, there is a rule: "never make your field public". This rule is borrowed by C# when a lot of Java programmers jumped to .NET. I'm one of them and I loyally kept that habit, you will find no public field defined in classes from my hand. While, not exactly. That changed recently.

One of the main reason of "keep fields private" in Java was, field accessors and method calls are very different. If you expose your field as public, then all dependent applications will use obj.fieldName to access it; while if you encapsulate it using a access method, it's like obj.getField(). So if you have make the field public, and then you need to introduce some business logic on the field, then you've got a headache: you need to change all the depending source codes. With IDEs like IDEA and Eclipse, this kind of change can be much easier and robust than before, but still it's going to take effort, which is a waste.

Since the first day of C#'s birth, it has a special language feature, Property, which I believe was borrowed by the success of Delphi. By this feature, a field can be encapsulated by a property and accessed in the same syntax as field, except under the hood, it's really a method call, and if you disassembly the IL code, there is really setField and getField methods generated. So in this case, is it still so evil to mark your field private and provide a property for it? In most of my applications (and I believe in most of most programmers' applications) the property is nothing but just a dummy get/set pair. And to make this pain less painful, MS invented the "auto-property". But the question is, is this encapsulate really necessary?

I don't think so! For C# applications, the "private-field-rule" is not applicable any more. Since if you want to change a field to a property, none of the dependent codes should change (unless if you use reflection). The only thing a field can not do is, it can not be marked as virtual while properties can; and fields can not be put into interfaces while properties can be (you can make a property writeonly and not field, might be useful for setting injection by IoC). So if you need to make a NHibernate friendly entity, or if you need to define interfaces to other module, use property. For most of rest, try to use field, until you find the needs to introduce property. This will help you get code clean.

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