This question came up in the comments of this answer. The inability to have readonly properties was proposed as a potential reason to use fields instead of properties.
For example:
class Rectangle
{
private readonly int _width;
private readonly int _height;
public Rectangle(int width, int height)
{
_width = width;
_height = height;
}
public int Width { get { return _width; } }
public int Height { get { return _height; } }
}
But why can't you just do this?
public int Width { get; readonly set; }
Edit (clarification): You can achieve this functionality in the first example. But why can't you use the auto-implemented property shorthand to do the same thing? It would also be less messy, since you wouldn't have to directly access the fields in your constructor; all access would be through the property.
Edit (update): As of C# 6.0, readonly properties are supported! object MyProp { get; } This property can be set inline (object MyProp { get; } = ...) or in the constructor, but nowhere else (just like readonly fields).
Because the language doesn't allow it.
This may seem like a frivolous answer: after all, the language designers could have declared that if you used readonly on an automatic property then it would mean "the property is settable but only in the constructor".
But features don't come for free. (Eric Gunnerson expresses it as "Every feature starts with minus 100 points.") To implement read-only automatic properties would have required additional compiler effort to support the readonly modifier on a property (it currently applies only to fields), to generate the appropriate backing field and to transform sets of the property to assignments to the backing field. That's quite a bit of work to support something that the user could do reasonably easily by declaring a readonly backing field and writing a one-line property getter, and that work would have a cost in terms of not implementing other features.
So, quite seriously, the answer is that either the language designers and implementers either never thought of the idea, or -- more likely -- they thought it would be nice to have, but decided there were better places to spend their finite resources. There's no technical constraint that prevents the language designers and implementers providing the feature you suggest: the reasons are more about the economics of software development.
If you want to make a property "read only" as far as functionality is concerned, you do so by only supplying the get method, as you indicated in your post.
public int Width { get { return _width; } }
public int Height { get { return _height; } }
The compiler will even reference these as "read only" if you try to write to them.
Having an additional term of readonly for a property would clash with also providing the set method. It seems to be poor syntax to me, i.e. how does the person reading it (or the compiler, for that matter) know what takes precedence: readonly or set?
Furthermore, as was explained in the answer you referenced, readonly applies only to fields and limits writing to those fields to the instantiation of the class. With properties, you can't write to them (I don't think) even within the constructor if they only have a get method.
You can make an automatic property read only by specifying the private access modifier for set like so
public bool Property {get; private set;}
The setter is still defined but it is no longer visible outside the class where the property is defined. As an aside, it is sometimes useful to define the setter as internal so that properties can be easily set from within the same assembly, but not by external callers.
Properties can be read-only, just not automatic properties.
Both get and set are required for automatic properties, and it makes no sense for a read-only property to have a set.
You can define a regular property as a read-only property by just defining the get - however, even if the requirement for both get and set for automatic properties didn't exist - the read-only property couldn't be automatically defined because you have to know the backing field to be able to set it's value internally (i.e. through the constructor).
I suppose there could be a template/macro or something defined in VS to generate the code this, but it couldn't be a part of the language itself.
I think fundamentally the problem is that properties are merely syntactic sugar for a field with optional getter/setter methods. Automatic properties generate the backing field so they require the "setter" or there would be no way to set the value of the backing field. Since properties really map onto methods, not fields, it doesn't make any sense to make them readonly.
Even if allowed, readonly could only apply to automatic properties. For traditional properties, you can put arbitrary code in both the getter and the setter. Even if the setter were able to be invoked only in the constructor of the class, the getter could still mutate the value based on whatever logic you decided to put in it. This would wholly inconsistent with the concept of readonly, thus necessitating different syntax rules and support for automatic/traditional properties. Since there is a mechanism -- using traditional properties with only a getter defined AND a readonly backing field as in the referenced question -- I see no point in mucking up the property syntax and potentially introducing confusion for something with a fairly easy and straightforward implementation using the current language constructs.
If the propert has a private set, then it is readonly from the outside world, i.e:
string _name;
public string Name
{
get{ return _name; }
private set { _name = value; }
}
Or, it can be made readonly if it doesnt have the setter at all, i.e.:
string _name;
public string Name
{
get{ return _name; }
}
In C# 6, auto properties can be readonly
https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-6#read-only-auto-properties
Related
I know that when using auto-properties, the compiler creates its own backing field behind the screen. However, in many programs I read to learn from, I see people explicitly write
private int _backingField;
public int Property { get { return _backingField; } }
What is the difference between above, and below?
public int Property { get; private set; }
I understand that its obvious to use the property when you actually have side-effects in the getter or setter, but that's often not the case. Also, I understand that you have to explicitly use the backing field in the case of structs, you can't access their members via properties.
The only difference I have been able to find is that the way of calling the value is different inside the class it is defined in. Is it then simple preference, or is there something more to calling a value through its property or by directly accessing the field?
Simple conventions?
There's not much difference between those two snippets - you can't pass a property by reference, for example, but that's rarely an issue. However, if you want the field to be readonly, like this:
private readonly int _backingField;
public int Property { get { return _backingField; } }
then there's a difference. The code I've written above prevents the value from being changed elsewhere within the class, making it clear that this is really meant to be immutable. I'd really like to be able to declare a read-only field with a read-only automatically implement property, settable only within the constructor - but that's not available at the moment.
This is rather confusing, by the way:
Also, I understand that you have to explicitly use the backing field in the case of structs, you can't access their members via properties.
What do you mean? You can definitely use properties within structs. Are you talking about backing fields which are mutable structs, i.e. the difference between:
foo.someField.X = 10;
and
foo.SomeProperty.X = 10;
? If so, I normally avoid that being an issue by making my structs immutable to start with :)
I know that automatic properties must define a get and set accessor method, I also know that it is possible for either of these accessors to be made invisible by means of an access modifier.
Is there a technical reason why the compiler is happy with
public object Property { get; set; }
but not
public object Property { get; }
My (possibly wrong) understanding of this code is that the compiler generates a backing field that is hidden from the calling code like so:
private object hiddenField; //hidden by compiler.
public object Property
{
get { return hiddenField; }
set { hiddenField = value;}
}
If the compiler can generate that, is there a reason that it can't omit the set accessor function based on the presence (or lack thereof) of a setter in the property declaration.
I understand that this may be an issue of feature scope rather than a technical limitation, I also freely admit that I have not yet consulted the C# language specification as yet.
[UPDATE 2]
Forgive me...I'm an idiot :P, I see now, thank you everyone for tollerating my senior moment/
Without the set accessor, there is no way to set the value, since you don't have a way to access "hiddenField".
Similarly, without a get accessor, there would be no way to get back a value you set.
Since it really becomes useless, it's not allowed.
However, you can have different accessibility on the two methods:
public object Property { get; private set; }
This provides you the ability to hide the set from outside, but still have a usable property.
public object Property { get; private set; }
will work, and it will have the semantics you expect it to.
How could you use a property such the following?
public object Property { get; }
Theoritically if you could write something like that it always returns null as it lacks to the set accessor. I think it is useless unless you set the hidden field in some way to have a static value to always return it.
From the C# spec:
Because the backing field is
inaccessible, it can be read and
written only through the property
accessors, even within the containing
type.
Leaving one of the accessors out would mean that the property would either be read-only or write-only, even within the constructor of the class/struct. Not very useful.
I just realized that the C# property construct can also be used with a private access modifier:
private string Password { get; set; }
Although this is technically interesting, I can't imagine when I would use it since a private field involves even less ceremony:
private string _password;
and I can't imagine when I would ever need to be able to internally get but not set or set but not get a private field:
private string Password { get; }
or
private string Password { set; }
but perhaps there is a use case with nested / inherited classes or perhaps where a get/set might contain logic instead of just giving back the value of the property, although I would tend to keep properties strictly simple and let explicit methods do any logic, e.g. GetEncodedPassword().
Does anyone use private properties in C# for any reason or is it just one of those technically-possible-yet-rarely-used-in-actual-code constructs?
Addendum
Nice answers, reading through them I culled these uses for private properties:
when private fields need to be lazily loaded
when private fields need extra logic or are calculated values
since private fields can be difficult to debug
in order to "present a contract to yourself"
to internally convert/simplify an exposed property as part of serialization
wrapping global variables to be used inside your class
I use them if I need to cache a value and want to lazy load it.
private string _password;
private string Password
{
get
{
if (_password == null)
{
_password = CallExpensiveOperation();
}
return _password;
}
}
The primary usage of this in my code is lazy initialization, as others have mentioned.
Another reason for private properties over fields is that private properties are much, much easier to debug than private fields. I frequently want to know things like "this field is getting set unexpectedly; who is the first caller that sets this field?" and it is way easier if you can just put a breakpoint on the setter and hit go. You can put logging in there. You can put performance metrics in there. You can put in consistency checks that run in the debug build.
Basically, it comes down to : code is far more powerful than data. Any technique that lets me write the code I need is a good one. Fields don't let you write code in them, properties do.
perhaps there is a use case with nested / inherited classes or perhaps where a get/set might contain logic instead of just giving back the value of the property
I personally use this even when I don't need logic on the getter or setter of a property. Using a property, even a private one, does help future-proof your code so that you can add the logic to a getter later, if required.
If I feel that a property may eventually require extra logic, I will sometimes wrap it into a private property instead of using a field, just so I don't have to change my code later.
In a semi-related case (though different than your question), I very frequently use the private setters on public properties:
public string Password
{
get;
private set;
}
This gives you a public getter, but keeps the setter private.
One good usage for private get only properties are calculated values. Several times I've had properties which are private readonly and just do a calculation over other fields in my type. It's not worthy of a method and not interesting to other classes so private property it is.
Lazy initialization is one place where they can be neat, e.g.
private Lazy<MyType> mytype = new Lazy<MyType>(/* expensive factory function */);
private MyType MyType { get { return this.mytype.Value; } }
// In C#6, you replace the last line with: private MyType MyType => myType.Value;
Then you can write: this.MyType everywhere rather than this.mytype.Value and encapsulate the fact that it is lazily instantiated in a single place.
One thing that's a shame is that C# doesn't support scoping the backing field to the property (i.e. declaring it inside the property definition) to hide it completely and ensure that it can only ever be accessed via the property.
The only one usage that I can think of
private bool IsPasswordSet
{
get
{
return !String.IsNullOrEmpty(_password);
}
}
Properties and fields are not one to one. A property is about the interface of a class (whether talking about its public or internal interface), while a field is about the class's implementation. Properties should not be seen as a way to just expose fields, they should be seen as a way to expose the intent and purpose of the class.
Just like you use properties to present a contract to your consumers on what constitutes your class, you can also present a contract to yourself for very similar reasons. So yes, I do use private properties when it makes sense. Sometimes a private property can hide away implementation details like lazy loading, the fact that a property is really a conglomeration of several fields and aspects, or that a property needs to be virtually instantiated with each call (think DateTime.Now). There are definitely times when it makes sense to enforce this even on yourself in the backend of the class.
I use them in serialization, with things like DataContractSerializer or protobuf-net which support this usage (XmlSerializer doesn't). It is useful if you need to simplify an object as part of serialization:
public SomeComplexType SomeProp { get;set;}
[DataMember(Order=1)]
private int SomePropProxy {
get { return SomeProp.ToInt32(); }
set { SomeProp = SomeComplexType.FromInt32(value); }
}
I use private properties to reduce code for accessing sub properties which often to use.
private double MonitorResolution
{
get { return this.Computer.Accesories.Monitor.Settings.Resolution; }
}
It is useful if there are many sub properties.
One thing I do all the time is store "global" variables/cache into HttpContext.Current
private static string SomeValue{
get{
if(HttpContext.Current.Items["MyClass:SomeValue"]==null){
HttpContext.Current.Items["MyClass:SomeValue"]="";
}
return HttpContext.Current.Items["MyClass:SomeValue"];
}
set{
HttpContext.Current.Items["MyClass:SomeValue"]=value;
}
}
I use them every now and then. They can make it easier to debug things when you can easily put in a breakpoint in the property or you can add a logging statement etc.
Can be also be useful if you later need to change the type of your data in some way or if you need to use reflection.
I know this question is very old but the information below was not in any of the current answers.
I can't imagine when I would ever need to be able to internally get but not set
If you are injecting your dependencies you may well want to have a Getter on a Property and not a setter as this would denote a readonly Property. In other words the Property can only be set in the constructor and cannot be changed by any other code within the class.
Also Visual Studio Professional will give information about a Property and not a field making it easier to see what your field is being used.
It is a common practice to only modify members with get/set methods, even private ones. Now, the logic behind this is so you know your get/set always behave in a particular way (for instance, firing off events) which doesn't seem to make sense since those won't be included in the property scheme... but old habits die hard.
It makes perfect sense when there is logic associated with the property set or get (think lazy initialization) and the property is used in a few places in the class.
If it's just a straight backing field? Nothing comes to mind as a good reason.
Well, as no one mentioned you can use it to validate data or to lock variables.
Validation
string _password;
string Password
{
get { return _password; }
set
{
// Validation logic.
if (value.Length < 8)
{
throw new Exception("Password too short!");
}
_password = value;
}
}
Locking
object _lock = new object();
object _lockedReference;
object LockedReference
{
get
{
lock (_lock)
{
return _lockedReference;
}
}
set
{
lock (_lock)
{
_lockedReference = value;
}
}
}
Note: When locking a reference you do not lock access to members of the referenced object.
Lazy reference: When lazy loading you may end up needing to do it async for which nowadays there is AsyncLazy. If you are on older versions than of the Visual Studio SDK 2015 or not using it you can also use AsyncEx's AsyncLazy.
One more usage would be to do some extra operations when setting value.
It happens in WPF in my case, when I display some info based on private object (which doesn't implement INotifyPropertyChanged):
private MyAggregateClass _mac;
private MyAggregateClass Mac
{
get => _mac;
set
{
if(value == _mac) return;
_mac = value;
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(DisplayInfo)));
}
}
public string DisplayInfo => _mac.SomeStringInformationToDisplayOnUI;
One could also have some private method, such as
private void SetMac(MyAggregateClass newValue)
to do that.
Some more exotic uses of explicit fields include:
you need to use ref or out with the value - perhaps because it is an Interlocked counter
it is intended to represent fundamental layout for example on a struct with explicit layout (perhaps to map to a C++ dump, or unsafe code)
historically the type has been used with BinaryFormatter with automatic field handling (changing to auto-props changes the names and thus breaks the serializer)
Various answers have mentioned using properties to implement a lazy member. And this answer discussed using properties to make live aliases. I just wanted to point out that those two concepts sometimes go together.
When using a property to make an alias of another object's public property, the laziness of that property is preserved:
[DebuggerBrowsable(DebuggerBrowsableState.Never)]
private IDbConnection Conn => foo.bar.LazyDbConnection;
On the other hand, retrieving that property in the constructor would negate the lazy aspect:
Conn = foo.bar.LazyDbConnection;
Looking into the guideline (Properties (C# Programming Guide)) it seems no one expects to use properties as private members.
Properties enable a class to expose a public way of getting and setting values, while hiding implementation or verification code.
In any case it can be interchanged by one or two methods and vice versa.
So the reason can be to spare parentheses on getting and get field syntax on setting.
could someone explain me what's the idea behind using Auto-Implemented Properties c#?
public class Customer
{
public int ID { get; set; }
public string Name { get; set; }
}
I get the motivation to use properties for private field, so we can determine how one can access a private field. But here - it's just like defining the field to be public from the first place. no?
Is there a difference between defining a field to be "public const" or define it to have a get-only property ?
A public automatic property is not the same as a public field, they are not binary compatible. If you implement a public field and later on want to add some logic, you will have to change it into a property and thereby introduce a breaking change (because of the binary incompatibility). This is the reason why many conventions state that you should never expose public fields but rather use properties.
So, automatic properties are just a convenient starting point for any simple non-private class value member, allowing one to add logic later on while keeping binary compatibility.
Properties can be databound, whereas fields can not.
Automatically implemented properties are essentially syntactic sugar. Once compiled, the backing store exists. It just isn't available from the source code.
As others have stated, properties and fields are not equivalent. Fields and properties aren't compatible so changing between them is a breaking change. In addition, you cannot use data binding with fields.
Final point. Though in your case there's little functional difference between the example and a public field, you can change the visibility of one of the accessors. So, to create a read-only property using an automatic property, you may do something like:
public int ID { get; private set; }
In this case, the get accessor is public, as per the entire signature, but the set accessor is private.
I will let MSDN do the talking here....
"In C# 3.0 and later, auto-implemented properties make property-declaration more concise when no additional logic is required in the property accessors. They also enable client code to create objects. When you declare a property as shown in the following example (see MSDN article for example), the compiler creates a private, anonymous backing field that can only be accessed through the property's get and set accessors"
Probably the most advantageous difference is you can do pre/post validation, raise PropertyChanged events etc
Is there a difference between defining a field to be "public const" or define it to have a get-only property?
Yes, a get-only field must have a private field declaration. This field can be changed by the class internally, marking a field as const means it cannot be modified.
2: a public const has to be defined at compiletime, you cannot use reference objects for that. Only classes that inherit from System.ValueType (string, int, double, ...)
A const is also static whereas a property with only a getter is not (every class has it's own instance.)
Add logic to getter function. Can access values of another property
public string Status
{
get { return DeactivateDate != null ? "InActive" : "Active"; }
private set { }
}
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.
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;
}
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 :(
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.
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.
Interestingly, the new Roslyn compiler in Visual Studio 2015 now allows this, even if the project is configured to use C# version 5.