Related
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.
Consider a class that contains a property Name inside. I would like the class to be implemented as singleton, and I want the Name to be set only once (so it can be assigned once but not changed later).
Should I assign it with a constructor (and give it only get accessor, without set) or create a separate "instance variable" for "Name" and a proper method that will work only once?
The first option would force me to pass a string argument to the GetInstance() method every time I call it, while the second one does not seem too elegant for me (as I wouldn't know if "Name" was already set - so I'd need to call this method every time I try to get an instance anyway.) Am I taking a wrong approach? Is there a good practice for such case?
The problem with passing the value into getInstance() every time is that all calling classes will have to know where the value for Name comes from and how to fetch it. Maybe they do all have this access, but then it makes it redundant storing this data on the singleton object as all callers already know its value.
Assuming that some callers know the value, and others don't, you could use a property similar to the way that you have suggested yourself:
public class MySingleton
{
// Singleton properties omitted
private string name;
public string name
{
get{return this.name;}
set
{
if(String.IsNullOrEmpty(this.name))
name = value;
// The exception could be left out, depending on how critical this is
else
throw new exception("The property 'name' can only be set once");
}
}
}
This assumes that neither null or String.Empty are valid assignations for your property and its still not the most elegant solution, so perhaps there is a different approach altogether.
Perhaps the constructor for the singleton could fetch the required value, rather than being passed the value:
public MySingleton()
{
name = configuration.getName(); // or wherever it is coming from
}
This way the calling classes can always assume that the singleton has a valid value for Name, but without caring where it comes from. If possible, I think this would be my preferred choice
I've been working on creating a class and suddenly a thought came to my mind of what is the difference between the two codes:
public readonly string ProductLocation;
AND
public string ProductLocation
{
get;
private set;
}
Can you guys give me idea when to use the following better. thanks.
The first one is a read-only field, while the second one gets compiled as a pair of methods (and all reads of the property ProductLocation gets compiled into calls to the corresponding get method and writes to it gets compiled into calls to the set method; internally, these methods will read from / write to an internal, automatically generated, non-read-only field). I'd say the most important difference is thread-safety! (how? read on!)
The basic usage of the class will look exactly the same: code in other classes will only be able to read the value, not change it. Also, the code to read the value will look exactly the same (for example, print(myInstace.ProductLocation); here, you cannot tell how it has been declared, cool, eh?)
The first, most trivial difference is that the property with private setter allows for instances of the same class to modify the value, while in the case of the readonly property, not even the object itself will be able to change the value.
Now, for the thread-safety. The readonly attribute on the field will change its memory visibility semantics when you are working with multiple threads (just like Java's final fields).
A readonly field can only be assigned to at declaration or in the constructor. The value assigned to a readonly field cannot be changed (at least not in a normal way) and it is guaranteed that every thread will see the correctly, initialized value after the constructor returns. Therefore, a readonly field is inherently thread-safe.
To achieve the same thread-safety with the property, you'd have to add some synchronization on your code, which is error-prone. It might lead to dead-locks, data races or reduced performance, depending on the case, and especially if you are not experienced.
So, if the value represents something that semantically cannot be changed after the object's construction, you should not declare a private setter (this would imply that the object might change it). Go for the readonly field (and maybe declare it private and declare a public property with only a getter accessing the field! This is actually the preferred form, since it is not good to expose fields, it is better to only expose methods -- there are many reasons explaining why in this answer)
With C# 6.0 auto-property initializer there is less boilerplate way of doing
private readonly string productLocation;
public string ProductLocation { get { return productLocation; } }
Which is
public string ProductLocation { get; }
This is readonly. Only initialized from constructor or inline. It cannot be edited after initialization. (Immutable from anywhere)
However, if you use private set;
public string ProductLocation { get; private set }
This is readonly from outside. But can be initialized anytime anywhere within the class itself. And can be edited within its life cycle by the class itself. (Mutable from class, immutable from outside)
Generally, it is not encouraged in .NET to expose member fields publicly, those should be wrapped by a property. So let's assume you might have
private readonly string productLocation;
public string ProductLocation { get { return productLocation; } }
vs
public string ProductLocation { get; private set; }
In this setup, and ignoring what one might be able to accomplish via reflection, the semantics are that in the first case, the productLocation variable can only be initialized in place and in the class constructor. Other members of the class cannot alter the value. External consumers have no ability to set the value.
In the second version, external consumers continue to have no access towards setting the value. However, the class itself can change the value at any time. If all you have is a DTO (that is, a class that only transports data, it has no logic expressed via methods), then this is essentially not all that different from the readonly version. However, for classes with methods, those methods could alter the value behind ProductLocation.
If you want to enforce the concept of an immutable field post-construction, use readonly. But for a DTO, I might go for the private set; option, mainly because it is less boilerplate code.
The first one (using readonly) will mean that the object can't even modify its own field's value, once the object has been instantiated, and others can never modify it.
The second one (using private set) will mean that object can modify the value of its field after it's been instantiated, but others can never modify it.
I would use the former for something that you know will not change, and use the latter for something where the value may change, but you don't want others to change it.
The first is a field whose value can be set only at instantiation.
The second is a property whose value can be set at any time (but only by its containing object).
Correction: The property can be set at any time by any instance of the same class (and not only by its containing object).
I have the simple class using auto-implemented properies:
Public Class foo
{
public foo() { }
public string BarName {get; set;}
}
I obviously use the variable BarName throughout my class and now need to add logic when the property value is set (it must be all upper case, go figure). Does this mean that I need to now create a private variable for BarName , e.g. _BarName, and change the current BarName variable used throughout my class to _BarName?
Public Class foo
{
public foo() {}
private string _BarName = "";
public string BarName
{
get {return _BarName;}
set {_BarName = Value.ToString().ToUpper();}
}
}
I am trying to make sure I understand the implications of using auto-implemented properties, and what it will entail down the road when/if I need to change something. I am assuming that the refactoring, as shown above, is not a breaking change because the property is basically staying the same; it just took a little work inside the class to keep it that way and add the needed logic.
Another example, which may be more meaningful is that I need to call some method when a setter or getter is used; more then changing the value.
This seems a fair trade off the the lines and lines of code to setup properties.
Does this mean that I need to now
create a private variable for BarName
Yes
and change the current BarName
variable used throughout my class
Do not change the rest of the code in your class to use the new private variable you create. BarName, as a property, is intended to hide the private variable (among other things), for the purpose of avoiding the sweeping changes you contemplate to the rest of your code.
I am assuming that the refactoring, as
shown above, is not a breaking change
because the property is basically
staying the same; it just took a
little work to keep it that way and
add the needed logic.
Correct.
You don't need to change anything. Auto-implemented properties are just syntactic sugar. The compiler is generating the private variable and get/set logic for you, behind the scenes. If you add your own getter/setter logic the compiler will use your code instead of its auto-generated code, but as far as the users of that property are concerned, nothing has changed; any code referencing your property will continue to work.
When using automatic properties you don't get direct access to the underlying "backing" variable and you don't get access to the actual logic that gets implemented in the property getter and setter. You only have access to the property (hence using BarName throughout your code).
If you now need to implement specific logic in the setter, you can no longer use automatic properties and need to implement the property in the "old fashioned" way. In this case, you would need to implement your own private backing variable (the preferred method, at least for me, is to name the private backing variable the same name as the property, but with an initial lowercase (in this case, the backing variable would be named barName). You would then implement the appropriate logic in the getter/setter.
In your example, you are correct that it is not a breaking change. This type of refactoring (moving from automatic properties to "normal" properties should never be a breaking change as you aren't changing the public interface (the name or accessibility of the public property).
Don't use automatic properties if you know that you are going to validate that object. These objects can be domain objects etc. Like if you have a Customer class then use private variables because you might need to validate the name, birthdate etc. But if you are using a Rss class then it will be okay to just use the automatic properties since there is no validation being perform and the class is just used to hold some data.
You are correct about the refactoring and it really shouldn't break anything.
Whether or not you actually need to go through the references within the class to the property name and change those to refer to the private field would depend on whether the internal code needed to access the underlying representation of the data rather than how it was presented to consumers of the class. In most cases you could leave well enough alone.
In your simple example it would be wise to leave well enough alone and ensure that no code internal to the class could subvert the conversion/formatting being performed in the setter.
If on the other hand the getter was doing some magic to change the internal representation of the field into the way consumers needed to view the data then perhaps (in some cases) the internal code within the class would need to access the field.
You would need to look at each occurrence of the access to the auto-property in the class and decide whether it should be touching the field or using the property.
Automatic properties are just syntactic sugar, the compiler in fact creates the private member for it, but since it's generated at compile time, you cannot access it.
And later on, if you want to implement getters and setters for the property, only then you create a explicit private member for it and add the logic.
I see a lot of example code for C# classes that does this:
public class Point {
public int x { get; set; }
public int y { get; set; }
}
Or, in older code, the same with an explicit private backing value and without the new auto-implemented properties:
public class Point {
private int _x;
private int _y;
public int x {
get { return _x; }
set { _x = value; }
}
public int y {
get { return _y; }
set { _y = value; }
}
}
My question is why. Is there any functional difference between doing the above and just making these members public fields, like below?
public class Point {
public int x;
public int y;
}
To be clear, I understand the value of getters and setters when you need to do some translation of the underlying data. But in cases where you're just passing the values through, it seems needlessly verbose.
I tend to agree (that it seems needlessly verbose), although this has been an issue our team hasn't yet resolved and so our coding standards still insist on verbose properties for all classes.
Jeff Atwood dealt with this a few years ago. The most important point he retrospectively noted is that changing from a field to a property is a breaking change in your code; anything that consumes it must be recompiled to work with the new class interface, so if anything outside of your control is consuming your class you might have problems.
It's also much simpler to change it to this later:
public int x { get; private set; }
It encapsulates setting and accessing of those members. If some time from now a developer for the code needs to change logic when a member is accessed or set it can be done without changing the contract of the class.
The idea is that even if the underlying data structure needs to change, the public interface to the class won't have to be changed.
C# can treat properties and variables differently at times. For example, you can't pass properties as ref or out parameters. So if you need to change the data structure for some reason and you were using public variables and now you need to use properties, your interface will have to change and now code that accesses property x may not longer compile like it did when it was variable x:
Point pt = new Point();
if(Int32.TryParse(userInput, out pt.x))
{
Console.WriteLine("x = {0}", pt.x);
Console.WriteLine("x must be a public variable! Otherwise, this won't compile.");
}
Using properties from the start avoids this, and you can feel free to tweak the underlying implementation as much as you need to without breaking client code.
Setter and Getter enables you to add additional abstraction layer and in pure OOP you should always access the objects via the interface they are providing to the outside world ...
Consider this code, which will save you in asp.net and which it would not be possible without the level of abstraction provided by the setters and getters:
class SomeControl
{
private string _SomeProperty ;
public string SomeProperty
{
if ( _SomeProperty == null )
return (string)Session [ "SomeProperty" ] ;
else
return _SomeProperty ;
}
}
Since auto implemented getters takes the same name for the property and the actual private storage variables. How can you change it in the future? I think the point being said is that use the auto implemented instead of field so that you can change it in the future if in case you need to add logic to getter and setter.
For example:
public string x { get; set; }
and for example you already use the x a lot of times and you do not want to break your code.
How do you change the auto getter setter... for example for setter you only allow setting a valid telephone number format... how do you change the code so that only the class is to be change?
My idea is add a new private variable and add the same x getter and setter.
private string _x;
public string x {
get {return _x};
set {
if (Datetime.TryParse(value)) {
_x = value;
}
};
}
Is this what you mean by making it flexible?
Also to be considered is the effect of the change to public members when it comes to binding and serialization. Both of these often rely on public properties to retrieve and set values.
Also, you can put breakpoints on getters and setters, but you can't on fields.
AFAIK the generated CIL interface is different. If you change a public member to a property you are changing it's public interface and need to rebuild every file that uses that class. This is not necessary if you only change the implementation of the getters and setters.
Maybe just making fields public you could leads you to a more Anemic Domain Model.
Kind Regards
It is also worth noting that you can't make Auto Properties Readonly and you cannot initialise them inline. Both of these are things I would like to see in a future release of .NET, but I believe you can do neither in .NET 4.0.
The only times I use a backing field with properties these days is when my class implements INotifyPropertyChanged and I need to fire the OnPropertyChanged event when a property is changed.
Also in these situations I set the backing fields directly when values are passed in from a constructor (no need to try and fire the OnPropertyChangedEvent (which would be NULL at this time anyway), anywhere else I use the property itself.
You never know if you might not need some translation of the data later. You are prepared for that if you hide away your members. Users of your class wont notice if you add the translation since the interface remains the same.
The biggest difrence is that, if ever you change your internal structure, you can still maintain the getters and setters as is, changing their internal logic without hurting the users of your API.
If you have to change how you get x and y in this case, you could just add the properties later. This is what I find most confusing. If you use public member variables, you can easily change that to a property later on, and use private variables called _x and _y if you need to store the value internally.
why we dont just use public fields instead of using properties then
call accessors ( get,set ) when we dont need to make validations ?
A property is a member that provides a flexible mechanism to read only or write only
Properties can be overridden but fields can't be.
Adding getter and setter makes the variable a property as in working in Wpf/C#.
If it's just a public member variable, it's not accessible from XAML because it's not a property (even though its public member variable).
If it has setter and getter, then its accessible from XAML because now its a property.
Setters and getters are bad in principle (they are a bad OO smell--I'll stop short of saying they are an anti-pattern because they really are necessary sometimes).
No, there is technically no difference and when I really want to share access to an object these days, I occasionally make it public final instead of adding a getter.
The way setters and getters were "Sold" is that you might need to know that someone is getting a value or changing one--which only makes sense with primitives.
Property bag objects like DAOs, DTOs and display objects are excluded from this rule because these aren't objects in a real "OO Design" meaning of the word Object. (You don't think of "Passing Messages" to a DTO or bean--those are simply a pile of attribute/value pairs by design).