I started learning C#, but I don't understand how properties provide encapsulation when the set method does not do any manipulation, validation, etc. My code:
class SitePage {
private string _specialString;
public string SpecialString
{
get { return _specialString; }
set { _specialString = value; }
}}
Here, instead of using the field _specialString, we are using the property SpecialString. Basically, instead of exposing the field, we are exposing the property. Why hide the field but expose the property to the client class?
When you use the c# property, the compiler is automatically creating the the field in the background. Thus, whenever you get the property, it is using a method to retrieve the value stored in the underlying field. When you set the property, it is setting that underlying field.
Using the {get;set;} syntax is essentially a shortcut when you need a simple implementation.
The real benefit is when you need to exert control over how it gets or sets those values.
for example, you can make use of this syntax:
public SpecialString{get;private set;}
This will make it so that you can retrieve the value for the property from outside the class, but you can only set the value internally, using the class's own internal logic.
Or you could do something like this:
private string _specialString;
public string SpecialString{
get;
set{
if(value.Length < 5)
{
throw new Exception();
}
else
{
this._specialstring = value;
}
}
Or you could trigger other methods to fire when you get or set a certain property. Think of properties as a gatekeeper. You can set whatever rules you want in order to let data pass in or out.
Pretty much the idea is that you usually want to return a copy. This becomes really important when you are handling sensitive data like tax information. You don't want just anyone or anything being able to access someone's tax information directly. A lot of programs won't NEED a system like this, but is still considered good practice just because it keeps you in the habit of it.
Going through an intermediary like this ensures that everything is done deliberately, and allows you to do some error checking only changing it, but not forcing it to run locally in the file. For instance, if you had a number that should never be negative, you could include an error check for that inside of your set method, instead of having to do that everywhere you use the variable.
If you need more detail or examples, just let me know. I don't feel like I explained that very well, but I hopefully explained it well enough to give you an rough idea as to why you would code this way.
Typically, properties make up the public interface and fields are the backing data. You can also have an auto-implemented property by writing it like this:
public string SpecialString { get; set; }
This way, the compiler will build the backing field automatically.
There are other advantages to having a public property be the interface for a private backing field, like when you want to see if the value has changed and raise an event.
private string _specialString;
public string SpecialString
{
get
{
return _specialString;
}
set
{
if (_specialString != value)
{
_specialString = value;
OnValueChanged();
}
}
}
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.
In C#, we can do something like:
private string _myField;
public string MyProperty
{
get { return _myField; }
private set { _myField = value; }
}
What is the advantage of using a private setter in the property here while we can set _myField inside the class as we want? Why would we want to use the setter of MyProperty?
A setter can implement other behaviour/logic when a property is updated, so that you don't have to manually implement it at every location where a property might be updated.
It can:
automatically update other fields
Validate the new value (eg. ensure an email address matches a regular expression)
To keep code that needs to be run every time a field is updated in one place
For example:
private string _myField;
private int _myField_num_updated;
private DateTime _myField_updated_at;
public string MyProperty
{
get { return _myField; }
private set {
_myField = value;
_myField_num_updated++;
_myField_updated_at = DateTime.Now;
}
}
The goal of using property accessors (get and set) is to hide the internal implementation on how particular values are queried or modified. In your case, the setter is simple, but in the future, it might become more complicated. By hiding the implementation, you minimize the impact of potential changes and keep the interface simple.
Now concerning your question: why using a private setter? The reason for using a private setter is exactly the same as the reason for using a setter in general. In the case of a private setter, the user of that setter is the code of the class itself. In case of a public setter, the user could be anybody using the class. The advantage of using the setter remains the same.
Because while properties more often than not are wrapping backing fields, there is no reason that they must. Properties can also implement custom business logic (usually some kind of validation or conversion) and call other functions. Using a private setter makes your code more idiomatic (meaning others will more readily understand what you're doing) and more expressive (meaning the code will, or should, more closely reflect the semantics of what you are trying to accomplish. Private setters also allow this custom logic to be encapsulated, so that there's only one place that you need to change it in the event that logic changes or the code needs refactoring.
With a setter you can control the value when you try to set it, for example if you have an int that you want to be sure does not have a value greater than ten.
public int MyProp
{
get { return _my_prop;}
private set {
if value > 10 {
_my_prop = 10;
}
}
}
Property mainly serves for 2 purpose:
avoid unexpected access to the underlying field from outside
hide the implementation detail of the underlying field, so you can change the implementation freely as long as you keep the interface.
As to a private setting, while the first doesn't make much sense, the second item is still valid
need a private setter in a property is to use your field wrapped in property only for being modify directly by other function, properties of your class. So in one place (property) you set value of your field but all other elements of your class have not to access directly to your private field but through property that wrapped it.
I'm a bit confused on the point of Automatic properties in C# e.g
public string Forename{ get; set; }
I get that you are saving code by not having to declare a private variable, but what's the point of a property when you are not using any get or set logic? Why not just use
public string Forename;
I'm not sure what the difference between these 2 statements is, I always thought you used properties if you wanted additional get/set logic?
Properties can have code put into them without breaking contract, fields can't have code put into them without changing them to properties (and breaking the interface). Properties can be read only or write only, fields can't. Properties can be data bound, fields can't.
You can write
public string Forename{ get; private set; }
to get read-only properties... Still not nearly as versatile as real properties, but it's a compromise that for some works.
I'm not sure what the difference between these 2 statements is, I always thought you used properties if you wanted additional get/set logic?
In the first case, the compiler will automatically add a field for you, and wrap the property. It's basically the equivalent to doing:
private string forename;
public string Forename
{
get
{
return this.forename;
}
set
{
this.forename = value;
}
}
There are many advantages to using properties over fields. Even if you don't need some of the specific reasons, such as databinding, this helps to future-proof your API.
The main problem is that, if you make a field, but in v2 of your application, need a property, you'll break the API. By using an automatic property up front, you have the potential to change your API at any time, with no worry about source or binary compatibility issues.
It is meant that you expect to add the logic later.
If you do so and have it as property from the beginning, you will not have to rebuild the dependent code. If you change it from a variable to a property, then you will have to.
Consider looking at some related threads about Difference Between Automatic Properties and Public Fields, Fields vs Properties, Automatic Properties - Useful or Not?, Why Not to Use Public Fields.
Public data members are evil (in that the object doesn't control modification of it's own state - It becomes a global variable). Breaks encapsulation - a tenet of OOP.
Automatic properties are there to provide encapsulation and avoid drudgery of writing boiler plate code for simple properties.
public string ID { get; set;}
You can change automatic properties to non-automatic properties in the future (e.g. you have some validation in a setter for example)... and not break existing clients.
string m_ID;
public string ID
{
get { return m_ID; }
set
{
//validate value conforms to a certain pattern via a regex match
m_ID = value;
}
}
You cannot do the same with public data attributes. Changing a data attribute to a property will force existing clients to recompile before they can interact again.
When adding auto properties the compiler will add get set logic into the application, this means that if you later add to this logic, and references to your property from external libraries will still work.
If you migrated from a public variable to a property, this would be a breaking change for other libraries that reference yours - hence, why not start with an auto property? :)
For one, you can set the property to virtual and implement logic in an inheriting class.
You can also implement logic in the same class afterwards and there won't be side-effects on any code relying on the class.
Not all properties need get/set logic. If they do, you use a private variable.
For example, in a MV-something pattern, your model would not have much logic. But you can mix and match as needed.
If you were to use a field like you suggested in place of a property, you can't for example define an interface to describe your class correctly, since interfaces cannot contain data fields.
A property is like a contract, and you can change the implemenation of a property without affecting the clients using your classes and properties. You may not have any logic today, but as business requirements change and if you want to introduce any code, properties are your safest bet. The following 2 links are excellent c# video tutorials. The first one explains the need of properties over just using fields and the second video explains different types of properties. I found them very useful.
Need for the Properties in C#
Poperties in C#, Read Only, Write Only, Read/Write, Auto Implemented
Take a look at the following code and explanation.
The most common implementation for a property is getter or a setter that simply reads and writes to a private field of the same type as a property. An automatic property declaration instructs the compiler to provide this implementation. The compiler automatically generates a private backing field.
Look into the following code:-
public class Stock
{
decimal currentPrice ; // private backing field.
public decimal CurrentPrice
{
get { return currentPrice ; }
set { currentPrice = value ; }
}
}
The same code can be rewritten as :-
public class Stock
{
public decimal CurrentPrice { get ; set ; } // The compiler will auto generate a backing field.
}
SOURCE:- C# in a Nutshell
Let's say I have a class that exposes one property. Is it considered to be a good aproach to use the private "holder variable" for internal use in the class? Or should I use the property for internal use also.
To explain, should I use:
public class foo
{
String _statusHolder;
public String myStaus
{
get { return _statusHolder; }
set{ _statusHolder = value; }
}
public void DisplayMyStatus()
{
Console.WriteLine(_statusHolder);
}
}
Or:
public class foo
{
String _statusHolder;
public String myStaus
{
get { return _statusHolder; }
set{ _statusHolder = value; }
}
public void DisplayMyStatus()
{
Console.WriteLine(myStaus);
}
}
I could see it as beeing more consistent and more readable to use the second approach. It would also be more effective if I would later do some modificatins in the set-statement. But are there any performance issues or is it considered bad-practise for some reason?
EDIT:
It seems that everybody is leaning towards using the property internally. My initial thoughts was the same, but as a novice programmer, you can never know.
Thanks everyone for the quick feedback!
Performance issues should be negligable, as the JITer or compiler will happily work out that your function call (the getter of the property) doesn't do anything exciting, and can be inlined.
The benefit is future changes to business logic that might be put in the getter, which your class will then automatically take advantage of, without refactoring too much.
Of course, the downside is, you might want to avoid that new business logic in some circumstances, so it is something that needs to be considered based on how likely a) logic will change, and b) that logic might need to be circumvented.
The other (potential) advantage of using the property internally is that you can easily move to, or from, automatic properties.
I tend to go with calling the properties cause once stuff gets tricky you can put in locking and business logic in the getter
For C# 3.0 I would go with something along these lines (and only explicitly create the backing field when its really needed)
public class foo
{
public String Status
{
get;
set;
}
public void DisplayMyStatus()
{
Console.WriteLine(Status);
}
}
Use the property if there is one. A property can have side effects like lazy initializing that you want to have regardless from where you access the variable.
Even if the property has no side effects now, other developers could add them later, and the places where the "raw" variable is used may be error-prone because the new code is not called.
And lastly, the property makes refactoring easier, for example, when the value later is no longer stored in a variable but is calculated inside the property accessor or comes from some other source variable.
Programming in Java, I prefer using the getter method because I can put a breakpoint there and/or see changes to it in logging output.
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).