In C# there exists a type of member that is called a Property. This allows you to easily and simply define a private field and provide simple or complex getters and setters while saving space by not having to define whole methods. Java does not have anything like this, and from what I can see, the general consensus has been to suck it up and define complete getter and setter methods for private variables.
Currently, I have been toying with the following class:
public class Property<T> {
private T value = null;
public Property(){}
public Property(T initialValue){
value = initialValue;
}
public T get(){
return value;
}
public void set(T newValue){
value = newValue;
}
}
With this implementation, you can define simple properties that only require getters and setters:
final Property<String> name = new Property<>("Dog");
Or more advanced options like the one that MSDN provides for C#:
...
public double seconds;
public final Property<Double> Hours = new Property<Double>(){
#Override
public Double get() {
return seconds/3600;
}
#Override
public void set(Double newValue) {
seconds = newValue * 3600;
}
};
...
What would be the pros and cons of this solution?
The pros are largely obvious. I'll point out some that make it better than C#'s properties:
The backing field is tucked away so that you don't accidentally use it instead of the property. (but the downside is that you can't easily choose to use the backing field if you want)
Unlike C#'s auto-properties, you can choose to override only the get or set method, not both, e.g.
public Property<List<String>> MyList = new Property<List<String>>(){
#Override
public List<String> get() {
if (value == null)
value = new ArrayList<String>();
return value;
}
// set still works
};
There are cons, however:
It is not part of the Java language, or any common libraries, so it can be confusing for people who read your code (including yourself in the future).
You cannot change the visibility of the get and set methods: if the Property<T> can be accessed, you can both get and set the value.
If you don't make your Property field final, anyone that can access it can change it to their own Property implementation. This could be useful, but mostly would be a pain.
(this is a con shared with C#'s properties) You can't change the arguments that are passed to the get and set methods. E.g. you can't have a Property<MyType> with both a set(MyType) and a set(CompatibleType) method (unless you extend Property).
Using generics pervasively means that at run-time, (thanks to type erasure) you're using Object pervasively. This boxing/unboxing might make for a slight performance decrease (not noticeable in most apps) if you use primitives (e.g. using double vs Property<Double>).
By the way, Scala is a language that runs on the JVM that includes properties as a native feature, and interoperates with Java's version of properties (getters/setters). You might want to look into that, since basically someone else already hacked the Java language for you. =)
All in all, I'd say you shouldn't try to make Java have properties. When in Rome, do as the Romans do. If you don't like how the Romans do it, move down the street (Scala) or across the country (C#).
So the complete syntax, say for name, would now be:
theObject.name.set("new name");
The point is, how are you accessing that name object? Is it public / protected Then it could be overridden. Is it private? Then you can't change it outside the class anyways.
The solution you've proposed only works if you already have access to the object, at which point you don't need the solution.
The pros of this solution (your anonymous inner class) is that, if you are not needing to implement this anywhere else, it saves you from writing an entire extra class just for this one situation.
The con of this solution is that later you may want to implement it elsewhere, and then you'd want to refactor your code to extract the implementation of Property<Double> into its own class to avoid repeating yourself.
I'd say, if you're pretty sure you're not going to need this implementation anywhere else (I'm guessing you won't), just go ahead with the later solution of an anonymous inner class. It's a good one.
Related
I've been learning C# for a while now, and I've come across properties in my C# book (Head First C#). I honestly do not understand what they're used for, and why I should use them. I've googled them a few times but still cannot for the life of me understand them.
Can someone please explain to me this foreign concept?
Thanks,
Varmitharen
Properties provide controlled access to data; at the most basic, it can just mean encapsulating a field (public fields are not recommended), which the compiler can make easy for you:
public int Foo {get;set;} // the compiler handles the field for you
You could, however, use the property to enforce logic or handle side effects:
private int foo;
public int Foo {
get { return foo; }
set {
if(value < 0) throw new ArgumentOutOfRangeException();
if(value != foo) {
foo = value;
OnFooChanged(); // fire event notification for UI bindings
}
}
}
Other common options are lazy-loading, calculated members, proxied members, etc.
You can also change the accessibility, for example:
public int Foo { get; protected set; }
which can only be assigned by the type and subclasses, not by unrelated code. It could also only have a get or set.
Basically, properties act as a more formal version of a get/set pair of methods, making it much easier to talk about "Foo", rather than "get_Foo"/"set_Foo" etc (for two-way binding).
Unlike fields, properties can also be advertised on interfaces, which is essential for interface-based code
Though the other answers are pretty good, they are all very much about the mechanism of properties. It is also helpful to understand the philosophy of properties.
In OO programming we spend a lot of time building models of semantic domains. When you say that you have a class "Animal" and a derived class "Tiger", you're modeling in the computer realm a fact about the real world: that of all things in the world, some of them are animals, and of those animals, some of them are tigers.
But you have to understand that the mechanism and the semantics are different. No one says "hey, let's go to the zoo and watch the instances of zookeeper invoke methods on IFeedable on the instances of the tigers".
A field is a mechanism and therefore should be a private implementation detail of a class; it does not describe part of the model. A property is a part of the semantic model. Every tiger has a birthday, so "Birthday" should be a property of the Tiger class. That's part of the "semantic model" of tigers, so expose it as a property. As an implementation detail, the birthday might be stored in a private field accessed by the property.
Does that make sense? In short: use public properties to describe the semantic properties of the things you are modeling; use private fields as implementation mechanisms.
Properties are used to enrich the Encapsulation concept of Object-Oriented Programming.
i.e. They encapsulate a field-member and let you (the developer) control how setting/getting this variable is done. Example?
public class Person
{
private int m_age;
public int Age
{
set
{
if(value < 18)
m_age = 18;
else
m_age = value;
}
get
{
return m_age;
}
}
}
See? using property Age, we guaranteed that the minimum set value of age is 18.
Are you familiar with fields, then? If so, from the point of view of code that consumes your class, properties are exactly like fields. Inside your class though, a property is like a pair of methods, one that deals with returning the value to the consumer, and one method that deals with updating the value. These methods are generally called getters and setters.
A good reason for wanting to use a property instead of a field is that it gives you better control of the values passing in and out of the property.
Just wondering...
Is there any reasons not to use protected properties?
I mean instead of using this:
public abstract class Foo
{
protected Bar { get; private set; }
}
to use this one:
public abstract class Foo
{
private Bar _bar;
protected Foo(Bar bar)
{
_bar = bar;
}
protected GetBar()
{
return _bar;
}
}
I don't see any reason why you would use a GetXXX() method instead of a property regardless of it's modifiers.
Since you tagged this question with C# tag, I strictly recommend using the first approach. That is what properties are for.
Use a method only when value returned by it can be different every time you call it regardless of it's state. For example, DateTime.Now was a mistake and should have been a method.
For more details refer to Property Usage Guidelines on MSDN. On a side note, it's surprising how they haven't had a need to change it since Framework 1.1.
A few more reasons to use properties instead of methods:
Data binding can only see properties
You can use read only or write only semantics.
Only if you have non-writable or non-readable Attributes inside the class
The question is not really about protected it has more with: why should I use properties?
Propeties are logically equivalent with Getter/Setter method pairs, whatever you can do with a with a Name {get{} \ set{}} you can do with a GetName() \ SetName() pair and vice versa, especially as C# has getter and setter accessors, so we can play with the accessibility too.
But, there are some folks that feel that public (or protected) properties are a bit like cheating from an OOP perspective, especially if all that the property does is just expose a backing field. After all person.Name = "SWeko" seems like it changes the state of the object externally, while person.SetName("SWeko") merely tells the object that it needs to change it's state. And from a purely theoretic standpoint I think that those objections have merit.
However, not all of us have the luxury of living in ivory towers, so the concept of properties is really usefull, as it's more readable, less error prone, and IMHO, closer to the real world model. It also provides us with a kind of dichotomy, when i write something like this:
person.Name = "SWeko";
person.Work();
I expect that the first line will a fast assignment, and will not have side-effects, while I expect that the second line will do something more, and probably affect the inner state of the object. When I use Getter and Setter method, no such distiction is immediately obvious:
person.SetName("SWeko");
person.Work();
currently I am thinking about data encapsulation in C# and I am a little bit confused.
Years ago, when I started to learn programing with C++, my professor told me:
- "Create a class and hide it data members, so it can't be manipulated directly from outside"
Example:
You are parsing an XML file and store the parsed data into some data members inside the parser class.
Now, when I am looking at C#. You have there properties. This feature makes the internal state / internal data of a class visible to outside.
There is no encapsulation anymore. Right?
private string _mystring;
public string MyString
{
get {return _mystring;}
set {_mystring = value;}
}
From my point of view there is no difference between making data members public or having public properties, which have getters and setters, where you pass your private data members through.
Can someone explaing me that please?
Thanks
The private data is encapsulated by the property itself. The only way to access the data is through the property.
In your situation above, there is little reason to use the property. However, if you later need to add some validation, you can, without breaking the API, ie::
private string _mystring;
public string MyString
{
get {return _mystring;}
set
{
if (IsAcceptableInput(value))
_mystring = value;
}
}
Remember that a Property, in .NET, is really just a cleaner syntax for 2 methods - one method for the property get section, and one for the property set section. It provides all of the same encapsulation as a pair of methods in C++, but a (arguably) nicer syntax for usage.
Well so properties aren't the wild west you are taking them to be at a first glance. The unfortunate truth of OOP is that a lot of your methods are getters and setters, and properties are just a way to make this easier to write. Also you control what you'd like the property to allow someone to do. You can make a property readable but not writable, like so:
private string _mystring;
public string MyString
{
get {return _mystring;}
}
Or as mentioned by Reed you can have your set method do a transformation or checking of any amount of complexity. For instance something like
private long myPrime;
public long Prime {
get { return myPrime; }
set {
if (prime(value) {
myPrime = prime;
}
else {
//throw an error or do nothing
}
}
}
You generally have all the value you get from encapsulation, with some syntactic sugar to make some common tasks easier. You can do the same thing properties do in other languages, it just looks different.
The benefit of properties is that later on, you could decide to add validation etc to the setter method, or make the getter method do some calculation or caching, and none of the code that already calls your property would need to change - since the class' interface remained stable
There still is data encapsulation, if you need it to be. Encapsulating data is not about hiding it from the client of the class or making it unaccessible, it's about ensuring a consistent interface and internal object state.
Let's say you have an object representing a stick shift car, and a property to set the speed. You probably know that you should shift gears in between speed intervals, so that's where encapsulation comes in.
Instead of simply making the property public, thus allowing public access without any verification, you can use property getters and setters in C#:
class StickShiftCar : Car
{
public int MilesPerHour
{
get {return this._milesPerHour;}
set
{
if (vaule < 20)
this._gearPosition = 1;
else if (value > 30)
this._gearPosition = 2;
...
...
this._milesPerHour = value;
}
}
While this example is not necessarily compilable, I am sure you catch my drift.
You may be missing the fact that you don't have to have properties to correspond to all class member fields. You can decide which properties to add to your class, and whether or not they will be accessible outside of the class.
Looking a bit deeper, why did your professor tell you to encapsulate? Just because it is the proper object-oriented design? Why is that the proper way? Programming languages and paradigms are just a way of coping with the complexity of getting a processor to run our code in an understandable way. There are two readers of code, the machine and humans. The machine will happily load data from, or branch to, any address in the memory space. We humans on the other hand like to think of "things". Our brains deal with "things" that have attributes or that perform actions. A lion will eat you, a spear can defend you, a lion is furry, a spear is pointy. So we can understand programs if they are modeled as "things". Properties are supposed to model the attributes of a thing, and methods are supposed to model the things actions. In practice it can become quite fuzzy and everything cannot be modeled as a real world action, but the effort to do so, when done well, can make a program understandable.
The very first attempt of using property to encapsulate the value is get;set;
But C# provide more advanced feature to enrich functions inside get and set to make the property read-only, write-only or with certain conditions.
For example, to set the value as
private string temp;
public string temp
{
get
{
return temp;
}
}
will be better than using:
public readonly string Temp;
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.
I have a class with various public properties which I allow users to edit through a property grid. For persistence this class is also serialized/deserialized to/from an XML file through DataContractSerializer.
Sometimes I want to user to be able to save (serialize) changes they've made to an instance of the class. Yet at other times I don't want to allow the user to save their changes, and should instead see all the properties in the property grid as read only. I don't want to allow users to make changes that they'll never be able to save later. Similar to how MS Word will allow users to open documents that are currently opened by someone else but only as read only.
My class has a boolean property that determines if the class should be read-only, but is it possible to use this property to somehow dynamically add a read-only attributes to the class properties at run-time? If not what is an alternative solution? Should I wrap my class in a read-only wrapper class?
Immutability is an area where C# still has room to improve. While creating simple immutable types with readonly properties is possible, once you need more sophisticated control over when type are mutable you start running into obstacles.
There are three choices that you have, depending on how strongly you need to "enforce" read-only behavior:
Use a read-only flag in your type (like you're doing) and let the caller be responsible for not attempting to change properties on the type - if a write attempt is made, throw an exception.
Create a read-only interface and have your type implement it. This way you can pass the type via that interface to code that should only perform reads.
Create a wrapper class that aggregates your type and only exposes read operations.
The first option is often the easiest, in that it can require less refactoring of existing code, but offers the least opportunity for the author of a type to inform consumers when an instance is immutable versus when it is not. This option also offers the least support from the compiler in detecting inappropriate use - and relegates error detection to runtime.
The second option is convenient, since implementing an interface is possible without much refactoring effort. Unfortunately, callers can still cast to the underlying type and attempt to write against it. Often, this option is combined with a read-only flag to ensure the immutability is not violated.
The third option is the strongest, as far as enforcement goes, but it can result in duplication of code and is more of a refactoring effort. Often, it's useful to combine option 2 and 3, to make the relationship between the read-only wrapper and the mutable type polymorphic.
Personally, I tend to prefer the third option when writing new code where I expect to need to enforce immutability. I like the fact that it's impossible to "cast-away" the immutable wrapper, and it often allows you to avoid writing messy if-read-only-throw-exception checks into every setter.
If you are creating a library, it is possible to define a public interface with a private/internal class. Any method which needs to return an instance of your read-only class to an external consumer should instead return an instance of the read-only interface instead. Now, down-casting to a concrete type is impossible since the type isn't publicly exposed.
Utility Library
public interface IReadOnlyClass
{
string SomeProperty { get; }
int Foo();
}
public interface IMutableClass
{
string SomeProperty { set; }
void Foo( int arg );
}
Your Library
internal MyReadOnlyClass : IReadOnlyClass, IMutableClass
{
public string SomeProperty { get; set; }
public int Foo()
{
return 4; // chosen by fair dice roll
// guaranteed to be random
}
public void Foo( int arg )
{
this.SomeProperty = arg.ToString();
}
}
public SomeClass
{
private MyThing = new MyReadOnlyClass();
public IReadOnlyClass GetThing
{
get
{
return MyThing as IReadOnlyClass;
}
}
public IMutableClass GetATotallyDifferentThing
{
get
{
return MyThing as IMutableClass
}
}
}
Now, anyone who uses SomeClass will get back what looks like two different objects. Of course, they could use reflection to see the underlying types, which would tell them that this is really the same object with the same type. But the definition of that type is private in an external library. At this point, it is still technically possible to get at the definition, but it requires Heavy Wizardry to pull off.
Depending on your project, you could combine the above libraries into one. There is nothing preventing that; just don't include the above code in whatever DLL you want to restrict the permissions of.
Credit to XKCD for the comments.
Why not something like:
private int someValue;
public int SomeValue
{
get
{
return someValue;
}
set
{
if(ReadOnly)
throw new InvalidOperationException("Object is readonly");
someValue= value;
}
I would use a wrapper class that keeps everything read-only. This is for scalability, reliability and general readability.
I do not foresee any other methods of doing this that will provide the above three mentioned benefits as well as something more. Definitely use a wrapper class here in my opinion.
You can not get compile-time checks (like given with the keyword readonly) by changing a property to readonly at runtime. So there is no other way, as to check manually and throw an exception.
But propably it is better to re-design access to the class. For example create a "writer class", which checks if the underling "data class" can currently be written or not.
You can use PostSharp to create OnFieldAccessAspect that will not pass new value to any field when _readOnly will be set to true. With aspect code repetition is gone and there will be no field forgotten.
Would something like this help:
class Class1
{
private bool _isReadOnly;
private int _property1;
public int Property1
{
get
{
return _property1;
}
set
{
if (_isReadOnly)
throw new Exception("At the moment this is ready only property.");
_property1 = value;
}
}
}
You need to catch exceptions when setting properties.
I hope this is something you are looking for.