Why do automatic properties require both getters AND setters?

Question

In C#, if I declare an auto-implemented property, why do I have to declare BOTH the get and set part?

i.e.

public string ThisWorks { get; set; }

public string ThisDoesnt { get; }

Isn't this just syntactic sugar - i.e. the compiler inserts a private field for the property? So why the problem?

Curious.

But remember to add a type to the property. Your example won't work as it is now. — Rune Grimstad, Dec 03 '08 at 14:17
Oops, cheers! But I'll have to take you to task on your claims - I get a compiler error, straight up. — Duncan, Dec 03 '08 at 14:18
@ALL: Thanks - that's one for the Book of Blindingly Obvious! Because I was setting it in the constructor I was thinking along the lines of Jon Skeet, and missed the other cases. — Duncan, Dec 03 '08 at 14:25

score 29 · Accepted Answer · answered Dec 03 '08 at 14:20

29

If you didn't have a setter - then how would you ever set the property?

Incidentally, you can specify the accessibility, eg:

public string Foo
{
  get;
  private set;
}

answered Dec 03 '08 at 14:20

stusmith

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score 12 · Answer 2 · answered Dec 03 '08 at 14:20

12

Without a setter, you would never be able to provide a value - as you don't have any way of specifying the backing variable's name.

I've requested a readonly automatic property, declared like this:

public string ReadonlyProperty { get; readonly set; }

which would create a readonly backing variable, a property with only a getter, and translate all calls to the setter into direct access to the variable. You could only call the setter within the constructor - just like for normal readonly variables.

We'll see whether this request does any good... it's a real shame it's not in there at the moment, as it makes it harder to implement immutable types than mutable types :(

answered Dec 03 '08 at 14:20

Jon Skeet

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What would this provide that: "public readonly string ReadonlyProperty" would not other than the ability to break point on accessing the property? – Jeff Yates Dec 03 '08 at 14:30
@ffpf - public readonly string Blah; <-- that's not a property, that's a field. You need a get/set to be a "property". Fields aren't picked up on things like databinding, property-grids, etc. They have a different semantic meaning. – Timothy Khouri Dec 03 '08 at 14:36
See http://csharpindepth.com/Articles/Chapter8/PropertiesMatter.aspx for why I don't like exposing public fields other than *sometimes* constants. – Jon Skeet Dec 03 '08 at 14:37
VB 10 might get this: "ReadOnly Property MaxItems As Integer = 100" – Jonathan Allen Dec 04 '08 at 01:00
@jon: Thanks, that makes a lot of sense. I hadn't considered the serialization or ref points. @Timothy: I understand the difference. That wasn't the point I was making in this instance. Thanks. – Jeff Yates Dec 04 '08 at 22:02

score 11 · Answer 3 · answered Dec 03 '08 at 14:20

11

An auto-implemented property has no accessible private store, so you would have no way to set the value without a setter, making it totally useless.

answered Dec 03 '08 at 14:20

GalacticCowboy

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score 3 · Answer 4 · answered Dec 03 '08 at 14:20

3

You need a set - otherwise, how does your auto-implemented property get its value? When auto-implementing the property, you have to have a set accessor to at least give it a value during construction.

answered Dec 03 '08 at 14:20

Jeff Yates

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score 0 · Answer 5 · answered Nov 04 '15 at 14:53

0

Interestingly, the new Roslyn compiler in Visual Studio 2015 now allows this, even if the project is configured to use C# version 5.

answered Nov 04 '15 at 14:53

Wim Coenen

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Why do automatic properties require both getters AND setters?

5 Answers5