Why are C# auto-implemented properties public?

Question

In all the examples I see, C# auto-implemented properties are made public, even in the MSDN documentation examples. Coming from a C++ background, I've always been taught that it is a good idea to make member data private, unless there is a good reason not to.

Why is the following never used (at least I've never seen it):

private Name { get; set; }

I've looked through the MSDN documentation and read several tutorials regarding auto-implemented properties but there does not seem to be any advice on their pros and cons and when they should be avoided. Do auto-implemented properties compromise program security? Are there situations where they should be avoided? In which situations are they the ideal choice?

Thanks.

Answer 1

You are correct that auto-implemented properties that simply expose a backing field are not much of a gain over a public field.

As Alan Kay said:

But most people who use setters simply use them to simulate direct assignments to interior variables, and this violates the spirit and intent of real OOP.

However, there is an advantage to an auto-implemented property over a public field, and that is that it's a non-breaking change to later revise the implementation. If you have a public field, and code outside your class manipulates that public field, you can't change it to a private field in a future version of the class, or else any other code that touches that field will have to be recompiled. By contrast, once you have a public property, you can revise the implementation of that property in a future version, and client classes can continue using it with zero changes.

So it's useful to use auto-implemented properties for properties that right now would have trivial getter and setter implementations, but that may have more complex implementations in the future.

Answer 2

Have you asked yourself why you've always been taught that it's a good idea to make members private?

It's because (among other reasons) fields are an implementation detail. The detail of "storing data in memory", and it is an unimportant detail to any object which wishes to retrieve or set the data. Another class doesn't need to care whether he can access some memory slot somewhere - he just wants an interface for which he can pass or retrieve a value - there are the getters and setters, or properties.

Having decoupled the property from the detail of "memory based storage", we're given a large number of advantages. Primarily - we can override the behaviour of getting and setting without upsetting any code which uses the property. We can also use the property as an abstraction for retrieving data over a number of different implementations. That becomes extremely useful for testing/mocking behaviour, and providing alternative storage. If other classes depend on the implementation detail of "memory storage", you are not going to be able to change the behaviour of your class without breaking all those.

Before auto properties came along, we would typically store a field and create a getter and setter to encapsulate it for the reasons described above. An auto property automates that for us. We might write code that commonly uses fields everywhere in code, but we do so holding the idea of "I'll do this as a field for now, but that may be subject to change later if the criteria change".

Since a class knows about it's own implementation, it's usually a pointless endeavour to create private auto properties, you're not hiding the detail that's already known. Protected auto properties can be useful if you need to expose to subclasses.

As for situations where they should be avoided: When you want readonly data. (data which will not change after the object is constructed). Auto-properties lack the syntax to allow you to create an automated property that's backed by readonly data.