Related
So I was creating a Monopoly console app to explore some design concepts as well as to try to understand Test-Driven Development better.
That being the case, I created unit tests to make sure that all of my classes can be serialized and deserialized properly (because I'm going to need that later and if they can't be I'd rather know that sooner than later so I can avoid rework; doing so has, in fact, helped me catch several flaws in my initial design).
This property is part of a larger Property class. Owner is the player that owns the property.
What I'm trying to achieve here is that when a player mortgages a particular property, the property should be marked as "Mortgaged" and the player should receive half of the purchase price in cash. When they unmortgage the property, they pay the money back. There will be a few other checks that I haven't written yet as well; for example, game rules specify that you're not allowed to mortgage a property while it still has houses on it. I also don't yet check to see if the player has enough money to unmortgage the property, but I plan to add that shortly as that's a trivial rule to implement. That's why I'm using a setter here instead of an auto-generated property.
Here's the problem: this serializes and deserializes without throwing an exception, but if I serialize a mortgaged property it'll add the money a second time when I deserialize it (i.e. deserializing basically has the effect of re-mortgaging the property), so needless to say the unit test fails (which it should given that there's an obvious bug). I suppose that that's not surprising given that that's exactly how I wrote it - when you change its value it'll either mortgage or unmortgage the property.
Here's the code:
[JsonProperty(Order = 1)]
public Player Owner
{
get;
set;
}
private bool _isMortaged = false;
[JsonProperty(Order = 2)]
public bool IsMortgaged
{
get { return _isMortaged; }
set
{
if (!IsOwned)
{
throw new InvalidOperationException("You can't mortgage a property that no one owns");
}
// If we're setting this property to the same thing that it
// already was, log it. You could argue that that would indicate
// a bug (it probably would), but throwing an exception here
// breaks deserialization.
Debug.WriteIf(value == _isMortaged, $"Setting IsMortgaged to the same thing as it was before. Stack trace:{Environment.NewLine}{(new StackTrace()).ToString()}");
// If we're setting this to the same thing that it already was,
// then the setter should have no effect at all.
if (value != _isMortaged)
{
// The player is mortgaging the property
if (value)
{
_isMortaged = true;
Owner.Money += (PurchasePrice / 2);
}
// The player is unmortgaging the property
else
{
_isMortaged = false;
Owner.Money -= (PurchasePrice / 2);
}
}
}
}
Is there an easy way to have a different setter for deserialization, or to tell whether or not I'm deserializing in the setter? (The latter seems like a "hack"). I suppose that there's also the possibility of writing a custom deserializer, which would solve the problem I guess, but I'd much rather avoid that if possible (it doesn't seem like a particularly "clean" way of doing this).
Alternatively, is this just a design flaw?
This is a design flaw; setters shouldn't have side effects beyond the obvious of updating a value (and those that are expected eg. INotifyPropertyChanged)
If you need special behaviour, then either introduce a special setting method, or think of it the other way around:
You're mortgaging/un-mortgaging a property, so you want the owner of that property's balance to update
Instead of having the property update the owner's balance (which it shouldn't concern itself with: Separation of Concerns; you could instead have the owner's balance property be dynamically computed:
public class Property {
public bool IsMortgaged {get;set;}
}
public class Player
{
private List<Property> _properties = new List<Property>();
private double _cash = 0;
public int AvailableMoney =>
_cash + _properties.Where(p => p.IsMortgaged).Select(p => p.MortgageValue).Sum();
}
Now, this will obviously need to be updated to properly match your model, but ideally you want your data to be as dumb as possible; whether a property is mortgaged or not is a simple true/false. It's the balance that is special, and there's no reason not to re-compute it.
This is a violation of the Single Responsibility or Separation of concerns.
The way I would go about this would be to have a private setter on Property.IsMortgaged and use some JSON.NET functionality that exists already to deserialize it.
This would need some new methods on Property and Owner
For Property you would need to add:
public class Property
{
public double Value {get;} = 5000.00
private bool _isMortgaged
public bool SetMortgaged()
{
//Validation for if the property can be mortgaged goes here, return false if fails
if(_isMortgaged == true)
{
return false;
}
_isMortgaged = true;
return true;
}
}
and on the Owner class
public class Owner //or player
{
private double _cash;
public double AvailableCash
{
get {return _cash;}
}
public void MortgageProperty(Property item)
{
if(item.SetMortgaged())
{
_cash += item.Value / 2
}
else
{
//Throw an error or something else to let the user know it failed
}
}
}
This will encapsulate the logic in the respective classes, the Property will know if it can be mortgaged and the Owner takes care of the money.
In C#, what makes a field different from a property, and when should a field be used instead of a property?
Properties expose fields. Fields should (almost always) be kept private to a class and accessed via get and set properties. Properties provide a level of abstraction allowing you to change the fields while not affecting the external way they are accessed by the things that use your class.
public class MyClass
{
// this is a field. It is private to your class and stores the actual data.
private string _myField;
// this is a property. When accessed it uses the underlying field,
// but only exposes the contract, which will not be affected by the underlying field
public string MyProperty
{
get
{
return _myField;
}
set
{
_myField = value;
}
}
// This is an AutoProperty (C# 3.0 and higher) - which is a shorthand syntax
// used to generate a private field for you
public int AnotherProperty { get; set; }
}
#Kent points out that Properties are not required to encapsulate fields, they could do a calculation on other fields, or serve other purposes.
#GSS points out that you can also do other logic, such as validation, when a property is accessed, another useful feature.
Object orientated programming principles say that, the internal workings of a class should be hidden from the outside world. If you expose a field you're in essence exposing the internal implementation of the class. Therefore we wrap fields with Properties (or methods in Java's case) to give us the ability to change the implementation without breaking code depending on us. Seeing as we can put logic in the Property also allows us to perform validation logic etc if we need it.
C# 3 has the possibly confusing notion of autoproperties. This allows us to simply define the Property and the C#3 compiler will generate the private field for us.
public class Person
{
private string _name;
public string Name
{
get
{
return _name;
}
set
{
_name = value;
}
}
public int Age{get;set;} //AutoProperty generates private field for us
}
An important difference is that interfaces can have properties but not fields. This, to me, underlines that properties should be used to define a class's public interface while fields are meant to be used in the private, internal workings of a class. As a rule I rarely create public fields and similarly I rarely create non-public properties.
I'll give you a couple examples of using properties that might get the gears turning:
Lazy Initialization: If you have a property of an object that's expensive to load, but isn't accessed all that much in normal runs of the code, you can delay its loading via the property. That way, it's just sitting there, but the first time another module tries to call that property, it checks if the underlying field is null - if it is, it goes ahead and loads it, unknown to the calling module. This can greatly speed up object initialization.
Dirty Tracking: Which I actually learned about from my own question here on StackOverflow. When I have a lot of objects which values might have changed during a run, I can use the property to track if they need to be saved back to the database or not. If not a single property of an object has changed, the IsDirty flag won't get tripped, and therefore the saving functionality will skip over it when deciding what needs to get back to the database.
Using Properties, you can raise an event, when the value of the property is changed (aka. PropertyChangedEvent) or before the value is changed to support cancellation.
This is not possible with (direct access to) fields.
public class Person {
private string _name;
public event EventHandler NameChanging;
public event EventHandler NameChanged;
public string Name{
get
{
return _name;
}
set
{
OnNameChanging();
_name = value;
OnNameChanged();
}
}
private void OnNameChanging(){
NameChanging?.Invoke(this,EventArgs.Empty);
}
private void OnNameChanged(){
NameChanged?.Invoke(this,EventArgs.Empty);
}
}
Since many of them have explained with technical pros and cons of Properties and Field, it's time to get into real time examples.
1. Properties allows you to set the read-only access level
Consider the case of dataTable.Rows.Count and dataTable.Columns[i].Caption. They come from the class DataTable and both are public to us. The difference in the access-level to them is that we cannot set value to dataTable.Rows.Count but we can read and write to dataTable.Columns[i].Caption. Is that possible through Field? No!!! This can be done with Properties only.
public class DataTable
{
public class Rows
{
private string _count;
// This Count will be accessable to us but have used only "get" ie, readonly
public int Count
{
get
{
return _count;
}
}
}
public class Columns
{
private string _caption;
// Used both "get" and "set" ie, readable and writable
public string Caption
{
get
{
return _caption;
}
set
{
_caption = value;
}
}
}
}
2. Properties in PropertyGrid
You might have worked with Button in Visual Studio. Its properties are shown in the PropertyGrid like Text,Name etc. When we drag and drop a button, and when we click the properties, it will automatically find the class Button and filters Properties and show that in PropertyGrid (where PropertyGrid won't show Field even though they are public).
public class Button
{
private string _text;
private string _name;
private string _someProperty;
public string Text
{
get
{
return _text;
}
set
{
_text = value;
}
}
public string Name
{
get
{
return _name;
}
set
{
_name = value;
}
}
[Browsable(false)]
public string SomeProperty
{
get
{
return _someProperty;
}
set
{
_someProperty= value;
}
}
In PropertyGrid, the properties Name and Text will be shown, but not SomeProperty. Why??? Because Properties can accept Attributes. It does not show in case where [Browsable(false)] is false.
3. Can execute statements inside Properties
public class Rows
{
private string _count;
public int Count
{
get
{
return CalculateNoOfRows();
}
}
public int CalculateNoOfRows()
{
// Calculation here and finally set the value to _count
return _count;
}
}
4. Only Properties can be used in Binding Source
Binding Source helps us to decrease the number of lines of code. Fields are not accepted by BindingSource. We should use Properties for that.
5. Debugging mode
Consider we are using Field to hold a value. At some point we need to debug and check where the value is getting null for that field. It will be difficult to do where the number of lines of code are more than 1000. In such situations we can use Property and can set debug mode inside Property.
public string Name
{
// Can set debug mode inside get or set
get
{
return _name;
}
set
{
_name = value;
}
}
DIFFERENCES - USES (when and why)
A field is a variable that is declared directly in a class or struct. A class or struct may have instance fields or static fields or both. Generally, you should use fields only for variables that have private or protected accessibility. Data that your class exposes to client code should be provided through methods, properties and indexers. By using these constructs for indirect access to internal fields, you can guard against invalid input values.
A property is a member that provides a flexible mechanism to read, write, or compute the value of a private field. Properties can be used as if they are public data members, but they are actually special methods called accessors. This enables data to be accessed easily and still helps promote the safety and flexibility of methods.
Properties enable a class to expose a public way of getting and setting values, while hiding implementation or verification code. A get property accessor is used to return the property value, and a set accessor is used to assign a new value.
Though fields and properties look to be similar to each other, they are 2 completely different language elements.
Fields are the only mechanism how to store data on class level. Fields are conceptually variables at class scope. If you want to store some data to instances of your classes (objects) you need to use fields. There is no other choice. Properties can't store any data even though, it may look they are able to do so. See bellow.
Properties on the other hand never store data. They are just the pairs of methods (get and set) that can be syntactically called in a similar way as fields and in most cases they access (for read or write) fields, which is the source of some confusion. But because property methods are (with some limitations like fixed prototype) regular C# methods they can do whatever regular methods can do. It means they can have 1000 lines of code, they can throw exceptions, call another methods, can be even virtual, abstract or overridden. What makes properties special, is the fact that C# compiler stores some extra metadata into assemblies that can be used to search for specific properties - widely used feature.
Get and set property methods has the following prototypes.
PROPERTY_TYPE get();
void set(PROPERTY_TYPE value);
So it means that properties can be 'emulated' by defining a field and 2 corresponding methods.
class PropertyEmulation
{
private string MSomeValue;
public string GetSomeValue()
{
return(MSomeValue);
}
public void SetSomeValue(string value)
{
MSomeValue=value;
}
}
Such property emulation is typical for programming languages that don't support properties - like standard C++. In C# there you should always prefer properties as the way how to access to your fields.
Because only the fields can store a data, it means that more fields class contains, more memory objects of such class will consume. On the other hand, adding new properties into a class doesn't make objects of such class bigger. Here is the example.
class OneHundredFields
{
public int Field1;
public int Field2;
...
public int Field100;
}
OneHundredFields Instance=new OneHundredFields() // Variable 'Instance' consumes 100*sizeof(int) bytes of memory.
class OneHundredProperties
{
public int Property1
{
get
{
return(1000);
}
set
{
// Empty.
}
}
public int Property2
{
get
{
return(1000);
}
set
{
// Empty.
}
}
...
public int Property100
{
get
{
return(1000);
}
set
{
// Empty.
}
}
}
OneHundredProperties Instance=new OneHundredProperties() // !!!!! Variable 'Instance' consumes 0 bytes of memory. (In fact a some bytes are consumed becasue every object contais some auxiliarity data, but size doesn't depend on number of properties).
Though property methods can do anything, in most cases they serve as a way how to access objects' fields. If you want to make a field accessible to other classes you can do by 2 ways.
Making fields as public - not advisable.
Using properties.
Here is a class using public fields.
class Name
{
public string FullName;
public int YearOfBirth;
public int Age;
}
Name name=new Name();
name.FullName="Tim Anderson";
name.YearOfBirth=1979;
name.Age=40;
While the code is perfectly valid, from design point of view, it has several drawbacks. Because fields can be both read and written, you can't prevent user from writing to fields. You can apply readonly keyword, but in this way, you have to initialize readonly fields only in constructor. What's more, nothing prevents you to store invalid values into your fields.
name.FullName=null;
name.YearOfBirth=2200;
name.Age=-140;
The code is valid, all assignments will be executed though they are illogical. Age has a negative value, YearOfBirth is far in future and doesn't correspond to Age and FullName is null. With fields you can't prevent users of class Name to make such mistakes.
Here is a code with properties that fixes these issues.
class Name
{
private string MFullName="";
private int MYearOfBirth;
public string FullName
{
get
{
return(MFullName);
}
set
{
if (value==null)
{
throw(new InvalidOperationException("Error !"));
}
MFullName=value;
}
}
public int YearOfBirth
{
get
{
return(MYearOfBirth);
}
set
{
if (MYearOfBirth<1900 || MYearOfBirth>DateTime.Now.Year)
{
throw(new InvalidOperationException("Error !"));
}
MYearOfBirth=value;
}
}
public int Age
{
get
{
return(DateTime.Now.Year-MYearOfBirth);
}
}
public string FullNameInUppercase
{
get
{
return(MFullName.ToUpper());
}
}
}
The updated version of class has the following advantages.
FullName and YearOfBirth are checked for invalid values.
Age is not writtable. It's callculated from YearOfBirth and current year.
A new property FullNameInUppercase converts FullName to UPPER CASE. This is a little contrived example of property usage, where properties are commonly used to present field values in the format that is more appropriate for user - for instance using current locale on specific numeric of DateTime format.
Beside this, properties can be defined as virtual or overridden - simply because they are regular .NET methods. The same rules applies for such property methods as for regular methods.
C# also supports indexers which are the properties that have an index parameter in property methods. Here is the example.
class MyList
{
private string[] MBuffer;
public MyList()
{
MBuffer=new string[100];
}
public string this[int Index]
{
get
{
return(MBuffer[Index]);
}
set
{
MBuffer[Index]=value;
}
}
}
MyList List=new MyList();
List[10]="ABC";
Console.WriteLine(List[10]);
Since C# 3.0 allows you to define automatic properties. Here is the example.
class AutoProps
{
public int Value1
{
get;
set;
}
public int Value2
{
get;
set;
}
}
Even though class AutoProps contains only properties (or it looks like), it can store 2 values and size of objects of this class is equal to sizeof(Value1)+sizeof(Value2)=4+4=8 bytes.
The reason for this is simple. When you define an automatic property, C# compiler generates automatic code that contains hidden field and a property with property methods accessing this hidden field. Here is the code compiler produces.
Here is a code generated by the ILSpy from compiled assembly. Class contains generated hidden fields and properties.
internal class AutoProps
{
[CompilerGenerated]
[DebuggerBrowsable(DebuggerBrowsableState.Never)]
private int <Value1>k__BackingField;
[CompilerGenerated]
[DebuggerBrowsable(DebuggerBrowsableState.Never)]
private int <Value2>k__BackingField;
public int Value1
{
[CompilerGenerated]
get
{
return <Value1>k__BackingField;
}
[CompilerGenerated]
set
{
<Value1>k__BackingField = value;
}
}
public int Value2
{
[CompilerGenerated]
get
{
return <Value2>k__BackingField;
}
[CompilerGenerated]
set
{
<Value2>k__BackingField = value;
}
}
}
So, as you can see, the compiler still uses the fields to store the values - since fields are the only way how to store values into objects.
So as you can see, though properties and fields have similar usage syntax they are very different concepts. Even if you use automatic properties or events - hidden fields are generated by compiler where the real data are stored.
If you need to make a field value accessible to the outside world (users of your class) don't use public or protected fields. Fields always should be marked as private. Properties allow you to make value checks, formatting, conversions etc. and generally make your code safer, more readable and more extensible for future modifications.
Properties have the primary advantage of allowing you to change the way data on an object is accessed without breaking it's public interface. For example, if you need to add extra validation, or to change a stored field into a calculated you can do so easily if you initially exposed the field as a property. If you just exposed a field directly, then you would have to change the public interface of your class to add the new functionality. That change would break existing clients, requiring them to be recompiled before they could use the new version of your code.
If you write a class library designed for wide consumption (like the .NET Framework, which is used by millions of people), that can be a problem. However, if you are writing a class used internally inside a small code base (say <= 50 K lines), it's really not a big deal, because no one would be adversely affected by your changes. In that case it really just comes down to personal preference.
Properties support asymmetric access, i.e. you can have either a getter and a setter or just one of the two. Similarly properties support individual accessibility for getter/setter. Fields are always symmetric, i.e. you can always both get and set the value. Exception to this is readonly fields which obviously cannot be set after initialization.
Properties may run for a very long time, have side effects, and may even throw exceptions. Fields are fast, with no side effects, and will never throw exceptions. Due to side effects a property may return a different value for each call (as may be the case for DateTime.Now, i.e. DateTime.Now is not always equal to DateTime.Now). Fields always return the same value.
Fields may be used for out / ref parameters, properties may not.
Properties support additional logic – this could be used to implement lazy loading among other things.
Properties support a level of abstraction by encapsulating whatever it means to get/set the value.
Use properties in most / all cases, but try to avoid side effects.
In the background a property is compiled into methods. So a Name property is compiled into get_Name() and set_Name(string value). You can see this if you study the compiled code.
So there is a (very) small performance overhead when using them. Normally you will always use a Property if you expose a field to the outside, and you will often use it internally if you need to do validation of the value.
When you want your private variable(field) to be accessible to object of your class from other classes you need to create properties for those variables.
for example if I have variables named as "id" and "name" which is private
but there might be situation where this variable needed for read/write operation outside of the class. At that situation , property can help me to get that variable to read/write depending upon the get/set defined for the property. A property can be a readonly / writeonly / readwrite both.
here is the demo
class Employee
{
// Private Fields for Employee
private int id;
private string name;
//Property for id variable/field
public int EmployeeId
{
get
{
return id;
}
set
{
id = value;
}
}
//Property for name variable/field
public string EmployeeName
{
get
{
return name;
}
set
{
name = value;
}
}
}
class MyMain
{
public static void Main(string [] args)
{
Employee aEmployee = new Employee();
aEmployee.EmployeeId = 101;
aEmployee.EmployeeName = "Sundaran S";
}
}
The second question here, "when should a field be used instead of a property?", is only briefly touched on in this other answer and kinda this one too, but not really much detail.
In general, all the other answers are spot-on about good design: prefer exposing properties over exposing fields. While you probably won't regularly find yourself saying "wow, imagine how much worse things would be if I had made this a field instead of a property", it's so much more rare to think of a situation where you would say "wow, thank God I used a field here instead of a property."
But there's one advantage that fields have over properties, and that's their ability to be used as "ref" / "out" parameters. Suppose you have a method with the following signature:
public void TransformPoint(ref double x, ref double y);
and suppose that you want to use that method to transform an array created like this:
System.Windows.Point[] points = new Point[1000000];
Initialize(points);
Here's I think the fastest way to do it, since X and Y are properties:
for (int i = 0; i < points.Length; i++)
{
double x = points[i].X;
double y = points[i].Y;
TransformPoint(ref x, ref y);
points[i].X = x;
points[i].Y = y;
}
And that's going to be pretty good! Unless you have measurements that prove otherwise, there's no reason to throw a stink. But I believe it's not technically guaranteed to be as fast as this:
internal struct MyPoint
{
internal double X;
internal double Y;
}
// ...
MyPoint[] points = new MyPoint[1000000];
Initialize(points);
// ...
for (int i = 0; i < points.Length; i++)
{
TransformPoint(ref points[i].X, ref points[i].Y);
}
Doing some measurements myself, the version with fields takes about 61% of the time as the version with properties (.NET 4.6, Windows 7, x64, release mode, no debugger attached). The more expensive the TransformPoint method gets, the less pronounced that the difference becomes. To repeat this yourself, run with the first line commented-out and with it not commented-out.
Even if there were no performance benefits for the above, there are other places where being able to use ref and out parameters might be beneficial, such as when calling the Interlocked or Volatile family of methods. Note: In case this is new to you, Volatile is basically a way to get at the same behavior provided by the volatile keyword. As such, like volatile, it doesn't magically solve all thread-safety woes like its name suggests that it might.
I definitely don't want to seem like I'm advocating that you go "oh, I should start exposing fields instead of properties." The point is that if you need to regularly use these members in calls that take "ref" or "out" parameters, especially on something that might be a simple value type that's unlikely to ever need any of the value-added elements of properties, an argument can be made.
Also, properties allow you to use logic when setting values.
So you can say you only want to set a value to an integer field, if the value is greater than x, otherwise throw an exception.
Really useful feature.
(This should really be a comment, but I can't post a comment, so please excuse if it is not appropriate as a post).
I once worked at a place where the recommended practice was to use public fields instead of properties when the equivalent property def would just have been accessing a field, as in :
get { return _afield; }
set { _afield = value; }
Their reasoning was that the public field could be converted into a property later in future if required. It seemed a little strange to me at the time. Judging by these posts, it looks like not many here would agree either. What might you have said to try to change things ?
Edit : I should add that all of the code base at this place was compiled at the same time, so they might have thought that changing the public interface of classes (by changing a public field to a property) was not a problem.
Technically, i don't think that there is a difference, because properties are just wrappers around fields created by the user or automatically created by the compiler.The purpose of properties is to enforce encapsuation and to offer a lightweight method-like feature.
It's just a bad practice to declare fields as public, but it does not have any issues.
Fields are ordinary member variables or member instances of a class. Properties are an abstraction to get and set their values. Properties are also called accessors because they offer a way to change and retrieve a field if you expose a field in the class as private. Generally, you should declare your member variables private, then declare or define properties for them.
class SomeClass
{
int numbera; //Field
//Property
public static int numbera { get; set;}
}
If you are going to use thread primitives you are forced to use fields. Properties can break your threaded code. Apart from that, what cory said is correct.
My design of a field is that a field needs to be modified only by its parent, hence the class. Result the variable becomes private, then to be able to give the right to read the classes / methods outside I go through the system of property with only the Get. The field is then retrieved by the property and read-only! If you want to modify it you have to go through methods (for example the constructor) and I find that thanks to this way of making you secure, we have better control over our code because we "flange". One could very well always put everything in public so every possible case, the notion of variables / methods / classes etc ... in my opinion is just an aid to the development, maintenance of the code. For example, if a person resumes a code with public fields, he can do anything and therefore things "illogical" in relation to the objective, the logic of why the code was written. It's my point of view.
When i use a classic model private field / public readonly properties,for 10 privates fields i should write 10 publics properties! The code can be really big faster. I discover the private setter and now i only use public properties with a private setter.
The setter create in background a private field.
That why my old classic programming style was:
public class MyClass
{
private int _id;
public int ID { get { return _id; } }
public MyClass(int id)
{
_id = id;
}
}
My new programming style:
public class MyClass
{
public int ID { get; private set; }
public MyClass(int id)
{
ID = id;
}
}
Basic and general difference is:
Fields
ALWAYS give both get and set access
CAN NOT cause side effects (throwing exceptions, calling methods, changing fields except the one being got/set, etc)
Properties
NOT ALWAYS give both get and set access
CAN cause side effects
Properties encapsulate fields, thus enabling you to perform additional processing on the value to be set or retrieved. It is typically overkill to use properties if you will not be doing any pre- or postprocessing on the field value.
IMO, Properties are just the "SetXXX()" "GetXXX()" functions/methods/interfaces pairs we used before, but they are more concise and elegant.
Traditionally private fields are set via getter and setter methods. For the sake of less code you can use properties to set fields instead.
when you have a class which is "Car". The properties are color,shape..
Where as fields are variables defined within the scope of a class.
From Wikipedia -- Object-oriented programming:
Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which are data structures that contain data, in the form of fields, often known as attributes; and code, in the form of procedures, often known as methods. (emphasis added)
Properties are actually part of an object's behavior, but are designed to give consumers of the object the illusion/abstraction of working with the object's data.
Properties are special kind of class member, In properties we use a predefined Set or Get method.They use accessors through which we can read, written or change the values of the private fields.
For example, let us take a class named Employee, with private fields for name, age and Employee_Id. We cannot access these fields from outside the class , but we can access these private fields through properties.
Why do we use properties?
Making the class field public & exposing it is risky, as you will not have control what gets assigned & returned.
To understand this clearly with an example lets take a student class who have ID, passmark, name. Now in this example some problem with public field
ID should not be -ve.
Name can not be set to null
Pass mark should be read only.
If student name is missing No Name should be return.
To remove this problem We use Get and set method.
// A simple example
public class student
{
public int ID;
public int passmark;
public string name;
}
public class Program
{
public static void Main(string[] args)
{
student s1 = new student();
s1.ID = -101; // here ID can't be -ve
s1.Name = null ; // here Name can't be null
}
}
Now we take an example of get and set method
public class student
{
private int _ID;
private int _passmark;
private string_name ;
// for id property
public void SetID(int ID)
{
if(ID<=0)
{
throw new exception("student ID should be greater then 0");
}
this._ID = ID;
}
public int getID()
{
return_ID;
}
}
public class programme
{
public static void main()
{
student s1 = new student ();
s1.SetID(101);
}
// Like this we also can use for Name property
public void SetName(string Name)
{
if(string.IsNullOrEmpty(Name))
{
throw new exeception("name can not be null");
}
this._Name = Name;
}
public string GetName()
{
if( string.IsNullOrEmpty(This.Name))
{
return "No Name";
}
else
{
return this._name;
}
}
// Like this we also can use for Passmark property
public int Getpassmark()
{
return this._passmark;
}
}
Additional info:
By default, get and set accessors are as accessible as the property itself.
You can control/restrict accessor accessibility individually (for get and set) by applying more restrictive access modifiers on them.
Example:
public string Name
{
get
{
return name;
}
protected set
{
name = value;
}
}
Here get is still publicly accessed (as the property is public), but set is protected (a more restricted access specifier).
Think about it : You have a room and a door to enter this room. If you want to check how who is coming in and secure your room, then you should use properties otherwise they won't be any door and every one easily come in w/o any regulation
class Room {
public string sectionOne;
public string sectionTwo;
}
Room r = new Room();
r.sectionOne = "enter";
People is getting in to sectionOne pretty easily, there wasn't any checking
class Room
{
private string sectionOne;
private string sectionTwo;
public string SectionOne
{
get
{
return sectionOne;
}
set
{
sectionOne = Check(value);
}
}
}
Room r = new Room();
r.SectionOne = "enter";
Now you checked the person and know about whether he has something evil with him
Fields are the variables in classes. Fields are the data which you can encapsulate through the use of access modifiers.
Properties are similar to Fields in that they define states and the data associated with an object.
Unlike a field a property has a special syntax that controls how a person reads the data and writes the data, these are known as the get and set operators. The set logic can often be used to do validation.
Properties are used to expose field. They use accessors(set, get) through which the values of the private fields can be read, written or manipulated.
Properties do not name the storage locations. Instead, they have accessors that read, write, or compute their values.
Using properties we can set validation on the type of data that is set on a field.
For example we have private integer field age on that we should allow positive values since age cannot be negative.
We can do this in two ways using getter and setters and using property.
Using Getter and Setter
// field
private int _age;
// setter
public void set(int age){
if (age <=0)
throw new Exception();
this._age = age;
}
// getter
public int get (){
return this._age;
}
Now using property we can do the same thing. In the value is a key word
private int _age;
public int Age{
get{
return this._age;
}
set{
if (value <= 0)
throw new Exception()
}
}
Auto Implemented property if we don't logic in get and set accessors we can use auto implemented property.
When use auto-implemented property compiles creates a private, anonymous field that can only be accessed through get and set accessors.
public int Age{get;set;}
Abstract Properties
An abstract class may have an abstract property, which should be implemented in the derived class
public abstract class Person
{
public abstract string Name
{
get;
set;
}
public abstract int Age
{
get;
set;
}
}
// overriden something like this
// Declare a Name property of type string:
public override string Name
{
get
{
return name;
}
set
{
name = value;
}
}
We can privately set a property
In this we can privately set the auto property(set with in the class)
public int MyProperty
{
get; private set;
}
You can achieve same with this code. In this property set feature is not available as we have to set value to field directly.
private int myProperty;
public int MyProperty
{
get { return myProperty; }
}
In C#, what makes a field different from a property, and when should a field be used instead of a property?
Properties expose fields. Fields should (almost always) be kept private to a class and accessed via get and set properties. Properties provide a level of abstraction allowing you to change the fields while not affecting the external way they are accessed by the things that use your class.
public class MyClass
{
// this is a field. It is private to your class and stores the actual data.
private string _myField;
// this is a property. When accessed it uses the underlying field,
// but only exposes the contract, which will not be affected by the underlying field
public string MyProperty
{
get
{
return _myField;
}
set
{
_myField = value;
}
}
// This is an AutoProperty (C# 3.0 and higher) - which is a shorthand syntax
// used to generate a private field for you
public int AnotherProperty { get; set; }
}
#Kent points out that Properties are not required to encapsulate fields, they could do a calculation on other fields, or serve other purposes.
#GSS points out that you can also do other logic, such as validation, when a property is accessed, another useful feature.
Object orientated programming principles say that, the internal workings of a class should be hidden from the outside world. If you expose a field you're in essence exposing the internal implementation of the class. Therefore we wrap fields with Properties (or methods in Java's case) to give us the ability to change the implementation without breaking code depending on us. Seeing as we can put logic in the Property also allows us to perform validation logic etc if we need it.
C# 3 has the possibly confusing notion of autoproperties. This allows us to simply define the Property and the C#3 compiler will generate the private field for us.
public class Person
{
private string _name;
public string Name
{
get
{
return _name;
}
set
{
_name = value;
}
}
public int Age{get;set;} //AutoProperty generates private field for us
}
An important difference is that interfaces can have properties but not fields. This, to me, underlines that properties should be used to define a class's public interface while fields are meant to be used in the private, internal workings of a class. As a rule I rarely create public fields and similarly I rarely create non-public properties.
I'll give you a couple examples of using properties that might get the gears turning:
Lazy Initialization: If you have a property of an object that's expensive to load, but isn't accessed all that much in normal runs of the code, you can delay its loading via the property. That way, it's just sitting there, but the first time another module tries to call that property, it checks if the underlying field is null - if it is, it goes ahead and loads it, unknown to the calling module. This can greatly speed up object initialization.
Dirty Tracking: Which I actually learned about from my own question here on StackOverflow. When I have a lot of objects which values might have changed during a run, I can use the property to track if they need to be saved back to the database or not. If not a single property of an object has changed, the IsDirty flag won't get tripped, and therefore the saving functionality will skip over it when deciding what needs to get back to the database.
Using Properties, you can raise an event, when the value of the property is changed (aka. PropertyChangedEvent) or before the value is changed to support cancellation.
This is not possible with (direct access to) fields.
public class Person {
private string _name;
public event EventHandler NameChanging;
public event EventHandler NameChanged;
public string Name{
get
{
return _name;
}
set
{
OnNameChanging();
_name = value;
OnNameChanged();
}
}
private void OnNameChanging(){
NameChanging?.Invoke(this,EventArgs.Empty);
}
private void OnNameChanged(){
NameChanged?.Invoke(this,EventArgs.Empty);
}
}
Since many of them have explained with technical pros and cons of Properties and Field, it's time to get into real time examples.
1. Properties allows you to set the read-only access level
Consider the case of dataTable.Rows.Count and dataTable.Columns[i].Caption. They come from the class DataTable and both are public to us. The difference in the access-level to them is that we cannot set value to dataTable.Rows.Count but we can read and write to dataTable.Columns[i].Caption. Is that possible through Field? No!!! This can be done with Properties only.
public class DataTable
{
public class Rows
{
private string _count;
// This Count will be accessable to us but have used only "get" ie, readonly
public int Count
{
get
{
return _count;
}
}
}
public class Columns
{
private string _caption;
// Used both "get" and "set" ie, readable and writable
public string Caption
{
get
{
return _caption;
}
set
{
_caption = value;
}
}
}
}
2. Properties in PropertyGrid
You might have worked with Button in Visual Studio. Its properties are shown in the PropertyGrid like Text,Name etc. When we drag and drop a button, and when we click the properties, it will automatically find the class Button and filters Properties and show that in PropertyGrid (where PropertyGrid won't show Field even though they are public).
public class Button
{
private string _text;
private string _name;
private string _someProperty;
public string Text
{
get
{
return _text;
}
set
{
_text = value;
}
}
public string Name
{
get
{
return _name;
}
set
{
_name = value;
}
}
[Browsable(false)]
public string SomeProperty
{
get
{
return _someProperty;
}
set
{
_someProperty= value;
}
}
In PropertyGrid, the properties Name and Text will be shown, but not SomeProperty. Why??? Because Properties can accept Attributes. It does not show in case where [Browsable(false)] is false.
3. Can execute statements inside Properties
public class Rows
{
private string _count;
public int Count
{
get
{
return CalculateNoOfRows();
}
}
public int CalculateNoOfRows()
{
// Calculation here and finally set the value to _count
return _count;
}
}
4. Only Properties can be used in Binding Source
Binding Source helps us to decrease the number of lines of code. Fields are not accepted by BindingSource. We should use Properties for that.
5. Debugging mode
Consider we are using Field to hold a value. At some point we need to debug and check where the value is getting null for that field. It will be difficult to do where the number of lines of code are more than 1000. In such situations we can use Property and can set debug mode inside Property.
public string Name
{
// Can set debug mode inside get or set
get
{
return _name;
}
set
{
_name = value;
}
}
DIFFERENCES - USES (when and why)
A field is a variable that is declared directly in a class or struct. A class or struct may have instance fields or static fields or both. Generally, you should use fields only for variables that have private or protected accessibility. Data that your class exposes to client code should be provided through methods, properties and indexers. By using these constructs for indirect access to internal fields, you can guard against invalid input values.
A property is a member that provides a flexible mechanism to read, write, or compute the value of a private field. Properties can be used as if they are public data members, but they are actually special methods called accessors. This enables data to be accessed easily and still helps promote the safety and flexibility of methods.
Properties enable a class to expose a public way of getting and setting values, while hiding implementation or verification code. A get property accessor is used to return the property value, and a set accessor is used to assign a new value.
Though fields and properties look to be similar to each other, they are 2 completely different language elements.
Fields are the only mechanism how to store data on class level. Fields are conceptually variables at class scope. If you want to store some data to instances of your classes (objects) you need to use fields. There is no other choice. Properties can't store any data even though, it may look they are able to do so. See bellow.
Properties on the other hand never store data. They are just the pairs of methods (get and set) that can be syntactically called in a similar way as fields and in most cases they access (for read or write) fields, which is the source of some confusion. But because property methods are (with some limitations like fixed prototype) regular C# methods they can do whatever regular methods can do. It means they can have 1000 lines of code, they can throw exceptions, call another methods, can be even virtual, abstract or overridden. What makes properties special, is the fact that C# compiler stores some extra metadata into assemblies that can be used to search for specific properties - widely used feature.
Get and set property methods has the following prototypes.
PROPERTY_TYPE get();
void set(PROPERTY_TYPE value);
So it means that properties can be 'emulated' by defining a field and 2 corresponding methods.
class PropertyEmulation
{
private string MSomeValue;
public string GetSomeValue()
{
return(MSomeValue);
}
public void SetSomeValue(string value)
{
MSomeValue=value;
}
}
Such property emulation is typical for programming languages that don't support properties - like standard C++. In C# there you should always prefer properties as the way how to access to your fields.
Because only the fields can store a data, it means that more fields class contains, more memory objects of such class will consume. On the other hand, adding new properties into a class doesn't make objects of such class bigger. Here is the example.
class OneHundredFields
{
public int Field1;
public int Field2;
...
public int Field100;
}
OneHundredFields Instance=new OneHundredFields() // Variable 'Instance' consumes 100*sizeof(int) bytes of memory.
class OneHundredProperties
{
public int Property1
{
get
{
return(1000);
}
set
{
// Empty.
}
}
public int Property2
{
get
{
return(1000);
}
set
{
// Empty.
}
}
...
public int Property100
{
get
{
return(1000);
}
set
{
// Empty.
}
}
}
OneHundredProperties Instance=new OneHundredProperties() // !!!!! Variable 'Instance' consumes 0 bytes of memory. (In fact a some bytes are consumed becasue every object contais some auxiliarity data, but size doesn't depend on number of properties).
Though property methods can do anything, in most cases they serve as a way how to access objects' fields. If you want to make a field accessible to other classes you can do by 2 ways.
Making fields as public - not advisable.
Using properties.
Here is a class using public fields.
class Name
{
public string FullName;
public int YearOfBirth;
public int Age;
}
Name name=new Name();
name.FullName="Tim Anderson";
name.YearOfBirth=1979;
name.Age=40;
While the code is perfectly valid, from design point of view, it has several drawbacks. Because fields can be both read and written, you can't prevent user from writing to fields. You can apply readonly keyword, but in this way, you have to initialize readonly fields only in constructor. What's more, nothing prevents you to store invalid values into your fields.
name.FullName=null;
name.YearOfBirth=2200;
name.Age=-140;
The code is valid, all assignments will be executed though they are illogical. Age has a negative value, YearOfBirth is far in future and doesn't correspond to Age and FullName is null. With fields you can't prevent users of class Name to make such mistakes.
Here is a code with properties that fixes these issues.
class Name
{
private string MFullName="";
private int MYearOfBirth;
public string FullName
{
get
{
return(MFullName);
}
set
{
if (value==null)
{
throw(new InvalidOperationException("Error !"));
}
MFullName=value;
}
}
public int YearOfBirth
{
get
{
return(MYearOfBirth);
}
set
{
if (MYearOfBirth<1900 || MYearOfBirth>DateTime.Now.Year)
{
throw(new InvalidOperationException("Error !"));
}
MYearOfBirth=value;
}
}
public int Age
{
get
{
return(DateTime.Now.Year-MYearOfBirth);
}
}
public string FullNameInUppercase
{
get
{
return(MFullName.ToUpper());
}
}
}
The updated version of class has the following advantages.
FullName and YearOfBirth are checked for invalid values.
Age is not writtable. It's callculated from YearOfBirth and current year.
A new property FullNameInUppercase converts FullName to UPPER CASE. This is a little contrived example of property usage, where properties are commonly used to present field values in the format that is more appropriate for user - for instance using current locale on specific numeric of DateTime format.
Beside this, properties can be defined as virtual or overridden - simply because they are regular .NET methods. The same rules applies for such property methods as for regular methods.
C# also supports indexers which are the properties that have an index parameter in property methods. Here is the example.
class MyList
{
private string[] MBuffer;
public MyList()
{
MBuffer=new string[100];
}
public string this[int Index]
{
get
{
return(MBuffer[Index]);
}
set
{
MBuffer[Index]=value;
}
}
}
MyList List=new MyList();
List[10]="ABC";
Console.WriteLine(List[10]);
Since C# 3.0 allows you to define automatic properties. Here is the example.
class AutoProps
{
public int Value1
{
get;
set;
}
public int Value2
{
get;
set;
}
}
Even though class AutoProps contains only properties (or it looks like), it can store 2 values and size of objects of this class is equal to sizeof(Value1)+sizeof(Value2)=4+4=8 bytes.
The reason for this is simple. When you define an automatic property, C# compiler generates automatic code that contains hidden field and a property with property methods accessing this hidden field. Here is the code compiler produces.
Here is a code generated by the ILSpy from compiled assembly. Class contains generated hidden fields and properties.
internal class AutoProps
{
[CompilerGenerated]
[DebuggerBrowsable(DebuggerBrowsableState.Never)]
private int <Value1>k__BackingField;
[CompilerGenerated]
[DebuggerBrowsable(DebuggerBrowsableState.Never)]
private int <Value2>k__BackingField;
public int Value1
{
[CompilerGenerated]
get
{
return <Value1>k__BackingField;
}
[CompilerGenerated]
set
{
<Value1>k__BackingField = value;
}
}
public int Value2
{
[CompilerGenerated]
get
{
return <Value2>k__BackingField;
}
[CompilerGenerated]
set
{
<Value2>k__BackingField = value;
}
}
}
So, as you can see, the compiler still uses the fields to store the values - since fields are the only way how to store values into objects.
So as you can see, though properties and fields have similar usage syntax they are very different concepts. Even if you use automatic properties or events - hidden fields are generated by compiler where the real data are stored.
If you need to make a field value accessible to the outside world (users of your class) don't use public or protected fields. Fields always should be marked as private. Properties allow you to make value checks, formatting, conversions etc. and generally make your code safer, more readable and more extensible for future modifications.
Properties have the primary advantage of allowing you to change the way data on an object is accessed without breaking it's public interface. For example, if you need to add extra validation, or to change a stored field into a calculated you can do so easily if you initially exposed the field as a property. If you just exposed a field directly, then you would have to change the public interface of your class to add the new functionality. That change would break existing clients, requiring them to be recompiled before they could use the new version of your code.
If you write a class library designed for wide consumption (like the .NET Framework, which is used by millions of people), that can be a problem. However, if you are writing a class used internally inside a small code base (say <= 50 K lines), it's really not a big deal, because no one would be adversely affected by your changes. In that case it really just comes down to personal preference.
Properties support asymmetric access, i.e. you can have either a getter and a setter or just one of the two. Similarly properties support individual accessibility for getter/setter. Fields are always symmetric, i.e. you can always both get and set the value. Exception to this is readonly fields which obviously cannot be set after initialization.
Properties may run for a very long time, have side effects, and may even throw exceptions. Fields are fast, with no side effects, and will never throw exceptions. Due to side effects a property may return a different value for each call (as may be the case for DateTime.Now, i.e. DateTime.Now is not always equal to DateTime.Now). Fields always return the same value.
Fields may be used for out / ref parameters, properties may not.
Properties support additional logic – this could be used to implement lazy loading among other things.
Properties support a level of abstraction by encapsulating whatever it means to get/set the value.
Use properties in most / all cases, but try to avoid side effects.
In the background a property is compiled into methods. So a Name property is compiled into get_Name() and set_Name(string value). You can see this if you study the compiled code.
So there is a (very) small performance overhead when using them. Normally you will always use a Property if you expose a field to the outside, and you will often use it internally if you need to do validation of the value.
When you want your private variable(field) to be accessible to object of your class from other classes you need to create properties for those variables.
for example if I have variables named as "id" and "name" which is private
but there might be situation where this variable needed for read/write operation outside of the class. At that situation , property can help me to get that variable to read/write depending upon the get/set defined for the property. A property can be a readonly / writeonly / readwrite both.
here is the demo
class Employee
{
// Private Fields for Employee
private int id;
private string name;
//Property for id variable/field
public int EmployeeId
{
get
{
return id;
}
set
{
id = value;
}
}
//Property for name variable/field
public string EmployeeName
{
get
{
return name;
}
set
{
name = value;
}
}
}
class MyMain
{
public static void Main(string [] args)
{
Employee aEmployee = new Employee();
aEmployee.EmployeeId = 101;
aEmployee.EmployeeName = "Sundaran S";
}
}
The second question here, "when should a field be used instead of a property?", is only briefly touched on in this other answer and kinda this one too, but not really much detail.
In general, all the other answers are spot-on about good design: prefer exposing properties over exposing fields. While you probably won't regularly find yourself saying "wow, imagine how much worse things would be if I had made this a field instead of a property", it's so much more rare to think of a situation where you would say "wow, thank God I used a field here instead of a property."
But there's one advantage that fields have over properties, and that's their ability to be used as "ref" / "out" parameters. Suppose you have a method with the following signature:
public void TransformPoint(ref double x, ref double y);
and suppose that you want to use that method to transform an array created like this:
System.Windows.Point[] points = new Point[1000000];
Initialize(points);
Here's I think the fastest way to do it, since X and Y are properties:
for (int i = 0; i < points.Length; i++)
{
double x = points[i].X;
double y = points[i].Y;
TransformPoint(ref x, ref y);
points[i].X = x;
points[i].Y = y;
}
And that's going to be pretty good! Unless you have measurements that prove otherwise, there's no reason to throw a stink. But I believe it's not technically guaranteed to be as fast as this:
internal struct MyPoint
{
internal double X;
internal double Y;
}
// ...
MyPoint[] points = new MyPoint[1000000];
Initialize(points);
// ...
for (int i = 0; i < points.Length; i++)
{
TransformPoint(ref points[i].X, ref points[i].Y);
}
Doing some measurements myself, the version with fields takes about 61% of the time as the version with properties (.NET 4.6, Windows 7, x64, release mode, no debugger attached). The more expensive the TransformPoint method gets, the less pronounced that the difference becomes. To repeat this yourself, run with the first line commented-out and with it not commented-out.
Even if there were no performance benefits for the above, there are other places where being able to use ref and out parameters might be beneficial, such as when calling the Interlocked or Volatile family of methods. Note: In case this is new to you, Volatile is basically a way to get at the same behavior provided by the volatile keyword. As such, like volatile, it doesn't magically solve all thread-safety woes like its name suggests that it might.
I definitely don't want to seem like I'm advocating that you go "oh, I should start exposing fields instead of properties." The point is that if you need to regularly use these members in calls that take "ref" or "out" parameters, especially on something that might be a simple value type that's unlikely to ever need any of the value-added elements of properties, an argument can be made.
Also, properties allow you to use logic when setting values.
So you can say you only want to set a value to an integer field, if the value is greater than x, otherwise throw an exception.
Really useful feature.
(This should really be a comment, but I can't post a comment, so please excuse if it is not appropriate as a post).
I once worked at a place where the recommended practice was to use public fields instead of properties when the equivalent property def would just have been accessing a field, as in :
get { return _afield; }
set { _afield = value; }
Their reasoning was that the public field could be converted into a property later in future if required. It seemed a little strange to me at the time. Judging by these posts, it looks like not many here would agree either. What might you have said to try to change things ?
Edit : I should add that all of the code base at this place was compiled at the same time, so they might have thought that changing the public interface of classes (by changing a public field to a property) was not a problem.
Technically, i don't think that there is a difference, because properties are just wrappers around fields created by the user or automatically created by the compiler.The purpose of properties is to enforce encapsuation and to offer a lightweight method-like feature.
It's just a bad practice to declare fields as public, but it does not have any issues.
Fields are ordinary member variables or member instances of a class. Properties are an abstraction to get and set their values. Properties are also called accessors because they offer a way to change and retrieve a field if you expose a field in the class as private. Generally, you should declare your member variables private, then declare or define properties for them.
class SomeClass
{
int numbera; //Field
//Property
public static int numbera { get; set;}
}
If you are going to use thread primitives you are forced to use fields. Properties can break your threaded code. Apart from that, what cory said is correct.
My design of a field is that a field needs to be modified only by its parent, hence the class. Result the variable becomes private, then to be able to give the right to read the classes / methods outside I go through the system of property with only the Get. The field is then retrieved by the property and read-only! If you want to modify it you have to go through methods (for example the constructor) and I find that thanks to this way of making you secure, we have better control over our code because we "flange". One could very well always put everything in public so every possible case, the notion of variables / methods / classes etc ... in my opinion is just an aid to the development, maintenance of the code. For example, if a person resumes a code with public fields, he can do anything and therefore things "illogical" in relation to the objective, the logic of why the code was written. It's my point of view.
When i use a classic model private field / public readonly properties,for 10 privates fields i should write 10 publics properties! The code can be really big faster. I discover the private setter and now i only use public properties with a private setter.
The setter create in background a private field.
That why my old classic programming style was:
public class MyClass
{
private int _id;
public int ID { get { return _id; } }
public MyClass(int id)
{
_id = id;
}
}
My new programming style:
public class MyClass
{
public int ID { get; private set; }
public MyClass(int id)
{
ID = id;
}
}
Basic and general difference is:
Fields
ALWAYS give both get and set access
CAN NOT cause side effects (throwing exceptions, calling methods, changing fields except the one being got/set, etc)
Properties
NOT ALWAYS give both get and set access
CAN cause side effects
Properties encapsulate fields, thus enabling you to perform additional processing on the value to be set or retrieved. It is typically overkill to use properties if you will not be doing any pre- or postprocessing on the field value.
IMO, Properties are just the "SetXXX()" "GetXXX()" functions/methods/interfaces pairs we used before, but they are more concise and elegant.
Traditionally private fields are set via getter and setter methods. For the sake of less code you can use properties to set fields instead.
when you have a class which is "Car". The properties are color,shape..
Where as fields are variables defined within the scope of a class.
From Wikipedia -- Object-oriented programming:
Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which are data structures that contain data, in the form of fields, often known as attributes; and code, in the form of procedures, often known as methods. (emphasis added)
Properties are actually part of an object's behavior, but are designed to give consumers of the object the illusion/abstraction of working with the object's data.
Properties are special kind of class member, In properties we use a predefined Set or Get method.They use accessors through which we can read, written or change the values of the private fields.
For example, let us take a class named Employee, with private fields for name, age and Employee_Id. We cannot access these fields from outside the class , but we can access these private fields through properties.
Why do we use properties?
Making the class field public & exposing it is risky, as you will not have control what gets assigned & returned.
To understand this clearly with an example lets take a student class who have ID, passmark, name. Now in this example some problem with public field
ID should not be -ve.
Name can not be set to null
Pass mark should be read only.
If student name is missing No Name should be return.
To remove this problem We use Get and set method.
// A simple example
public class student
{
public int ID;
public int passmark;
public string name;
}
public class Program
{
public static void Main(string[] args)
{
student s1 = new student();
s1.ID = -101; // here ID can't be -ve
s1.Name = null ; // here Name can't be null
}
}
Now we take an example of get and set method
public class student
{
private int _ID;
private int _passmark;
private string_name ;
// for id property
public void SetID(int ID)
{
if(ID<=0)
{
throw new exception("student ID should be greater then 0");
}
this._ID = ID;
}
public int getID()
{
return_ID;
}
}
public class programme
{
public static void main()
{
student s1 = new student ();
s1.SetID(101);
}
// Like this we also can use for Name property
public void SetName(string Name)
{
if(string.IsNullOrEmpty(Name))
{
throw new exeception("name can not be null");
}
this._Name = Name;
}
public string GetName()
{
if( string.IsNullOrEmpty(This.Name))
{
return "No Name";
}
else
{
return this._name;
}
}
// Like this we also can use for Passmark property
public int Getpassmark()
{
return this._passmark;
}
}
Additional info:
By default, get and set accessors are as accessible as the property itself.
You can control/restrict accessor accessibility individually (for get and set) by applying more restrictive access modifiers on them.
Example:
public string Name
{
get
{
return name;
}
protected set
{
name = value;
}
}
Here get is still publicly accessed (as the property is public), but set is protected (a more restricted access specifier).
Think about it : You have a room and a door to enter this room. If you want to check how who is coming in and secure your room, then you should use properties otherwise they won't be any door and every one easily come in w/o any regulation
class Room {
public string sectionOne;
public string sectionTwo;
}
Room r = new Room();
r.sectionOne = "enter";
People is getting in to sectionOne pretty easily, there wasn't any checking
class Room
{
private string sectionOne;
private string sectionTwo;
public string SectionOne
{
get
{
return sectionOne;
}
set
{
sectionOne = Check(value);
}
}
}
Room r = new Room();
r.SectionOne = "enter";
Now you checked the person and know about whether he has something evil with him
Fields are the variables in classes. Fields are the data which you can encapsulate through the use of access modifiers.
Properties are similar to Fields in that they define states and the data associated with an object.
Unlike a field a property has a special syntax that controls how a person reads the data and writes the data, these are known as the get and set operators. The set logic can often be used to do validation.
Properties are used to expose field. They use accessors(set, get) through which the values of the private fields can be read, written or manipulated.
Properties do not name the storage locations. Instead, they have accessors that read, write, or compute their values.
Using properties we can set validation on the type of data that is set on a field.
For example we have private integer field age on that we should allow positive values since age cannot be negative.
We can do this in two ways using getter and setters and using property.
Using Getter and Setter
// field
private int _age;
// setter
public void set(int age){
if (age <=0)
throw new Exception();
this._age = age;
}
// getter
public int get (){
return this._age;
}
Now using property we can do the same thing. In the value is a key word
private int _age;
public int Age{
get{
return this._age;
}
set{
if (value <= 0)
throw new Exception()
}
}
Auto Implemented property if we don't logic in get and set accessors we can use auto implemented property.
When use auto-implemented property compiles creates a private, anonymous field that can only be accessed through get and set accessors.
public int Age{get;set;}
Abstract Properties
An abstract class may have an abstract property, which should be implemented in the derived class
public abstract class Person
{
public abstract string Name
{
get;
set;
}
public abstract int Age
{
get;
set;
}
}
// overriden something like this
// Declare a Name property of type string:
public override string Name
{
get
{
return name;
}
set
{
name = value;
}
}
We can privately set a property
In this we can privately set the auto property(set with in the class)
public int MyProperty
{
get; private set;
}
You can achieve same with this code. In this property set feature is not available as we have to set value to field directly.
private int myProperty;
public int MyProperty
{
get { return myProperty; }
}
I have a class with a number of fields which are normally calculated in the constructor from other data in the class. They are not serialized to XML because any changes to the rest of the data will likely require their recalculation.
Is there a way I can set up a function call to be triggered on deserialization?
What you are describing is [OnDeserialized]
XmlSerializer does not support serialization callback methods (at least, not in MS .NET; mono may be different). Depending on your needs, you could try DataContractSerializer which does support serialization callbacks (as do a number of other serializers). Otherwise, your best approach may be to just have your own public method that you call manually.
Another option is to manually implement IXmlSerializable, but this is hard.
Since an object that can be XML serialized need a public parameterless constructor, it seems you have a hole in your class design even before you hit XML serialization.
Personally I would go with lazy calculation of those fields. Store a flag inside the class whether you have calculated the fields or not, and set that field to a value signifying "out of date" when any of the properties that are used in the calculation is changed. Then, in the properties that return the calculated values, check if you need to recalculate before returning the value.
This would then work regardless of XML serialization or not.
example:
[XmlType("test")]
public class TestClass
{
private int _A;
private int? _B;
public TestClass()
: this(0)
{
}
public TestClass(int a)
{
_A = a;
}
[XmlAttribute("a")]
public int A
{
get { return _A; }
set { _A = value; _B = null; }
}
[XmlIgnore]
public int B
{
get { if (_B == null) Recalculate(); return _B; }
set { _B = value; }
}
private void Recalculate()
{
_B = _A + 1;
}
}
-- Edit:
Just confirmed that, as the commenter below says, the xml serialisation process doesn't hit the 'OnDeserialized'-attribute-declared method. Which is a shame.
-- Previous response:
Yes indeed, take a look here.
Specifically of interest would be the OnDeserialized attribute, discussed here.
It may be worth noting that, depending on the serialization method used (I think) your no-arg constructor will be called. Obviously, this will happen before anything else is set. So it's probably not entirely useful.
You can implement IDeserializationCallback interface
I have a class library containing several structures each consisting of several value and reference types. Most of the value types are mandatory, a few value types and all reference types are optional. All structures are XmlSerializable (which is mandatory).
As far as the class library is targeted to mobile devices I want to reduce the memory footprint. My first idea was to use Nullable<T> for the value types, but this increases the memory size by 4 bytes per Nullable<T>. My second idea is to pack all optional value types into a separate structure that is only instantiated when any of its members is needed. But this would force me to implement IXmlSerializable on the "main" structure.
Are there any other approaches to "shrink" the structures?
[EDIT]
Beg your pardon for this bad question. I think I have to clarify some things and get more specific:
The class library is designed to serialize data info GPX (GPS Exchange Format). The structures are e.g. Waypoint or Track. They have mandatory fields as latitude, longitude etc. Optional fields are Vertical/Horizontal/Position Dilution of Precision, a description, a link.
The library is mainly targeted to Mobile devices such as PDAs. RAM is short, but plenty of non-volatile Memory is available.
Code examples cannot be shown as far as there are none. I want to think about several pitfalls before starting the implementation.
Here is a technique to aggressively reduce in memory overhead whilst allowing Xml Serialization.
Update: the orignal inline linked list idea is more efficient for 1 and 2 entries than a standard list with count construct but the use of fixed size optionals for zero, one and two cases is even more efficient.
Proviso:
This is predicated on you knowing that you really do need to shave the memory, as such
(since you haven't done any coding yet) this may well be a massively premature
optimization.
Also this design is predicated on the optional fields being very rare.
I use double as a 'placeholder' whatever format best allows you to represent the precision/units involved should be used.
public class WayPoint
{
// consumes IntPtr.Size fixed cost
private IOptional optional = OptionalNone.Default;
public double Latitude { get; set; }
public double Longitude { get; set; }
public double Vertical
{
get { return optional.Get<double>("Vertical") ?? 0.0; }
set { optional = optional.Set<double>("Vertical", value); }
}
[XmlIgnore] // need this pair for every value type
public bool VerticalSpecified
{
get { return optional.Get<double>("Vertical").HasValue; }
}
public void ClearVertical()
{
optional = optional.Clear<double>("Vertical");
}
public string Description // setting to null clears it
{
get { return optional.GetRef<string>("Description"); }
set { optional = optional.SetRef<string>("Description", value); }
}
// Horizontal, Position, DilutionOfPrecision etc.
}
The real heavy lifting is done here:
internal interface IOptional
{
T? Get<T>(string id) where T : struct;
T GetRef<T>(string id) where T : class;
IOptional Set<T>(string id, T value);
IOptional Clear(string id);
}
internal sealed class OptionalNone : IOptional
{
public static readonly OptionalNone Default = new OptionalNone();
public T? Get<T>(string id) where T : struct
{
return null;
}
public T GetRef<T>(string id) where T : class
{
return null;
}
public IOptional Set<T>(string id, T value)
{
if (value == null)
return Clear(id);
return new OptionalWithOne<T>(id, value);
}
public IOptional Clear(string id)
{
return this; // no effect
}
}
The fixed size ones become more interesting to write, there is no point writing these as structs as they would be boxed to be placed within the IOptional field within the WayPoint class.
internal sealed class OptionalWithOne<X> : IOptional
{
private string id1;
private X value1;
public OptionalWithOne(string id, X value)
{
this.id1 = id;
this.value1 = value;
}
public T? Get<T>(string id) where T : struct
{
if (string.Equals(id, this.id1))
return (T)(object)this.value1;
return null;
}
public T GetRef<T>(string id) where T : class
{
if (string.Equals(id, this.id1))
return (T)(object)this.value1;
return null;
}
public IOptional Set<T>(string id, T value)
{
if (string.Equals(id, this.id1))
{
if (value == null)
return OptionalNone.Default;
this.value1 = (X)(object)value;
return this;
}
else
{
if (value == null)
return this;
return new OptionalWithTwo<X,T>(this.id1, this.value1, id, value);
}
}
public IOptional Clear(string id)
{
if (string.Equals(id, this.id1))
return OptionalNone.Default;
return this; // no effect
}
}
Then for two (you can extend this idea as far as you want but as you can see the code gets unpleasant quickly.
internal sealed class OptionalWithTwo<X,Y> : IOptional
{
private string id1;
private X value1;
private string id2;
private Y value2;
public OptionalWithTwo(
string id1, X value1,
string id2, Y value2)
{
this.id1 = id1;
this.value1 = value1;
this.id2 = id2;
this.value2 = value2;
}
public T? Get<T>(string id) where T : struct
{
if (string.Equals(id, this.id1))
return (T)(object)this.value1;
if (string.Equals(id, this.id2))
return (T)(object)this.value2;
return null;
}
public T GetRef<T>(string id) where T : class
{
if (string.Equals(id, this.id1))
return (T)(object)this.value1;
if (string.Equals(id, this.id2))
return (T)(object)this.value2;
return null;
}
public IOptional Set<T>(string id, T value)
{
if (string.Equals(id, this.id1))
{
if (value == null)
return Clear(id);
this.value1 = (X)(object)value;
return this;
}
else if (string.Equals(id, this.id2))
{
if (value == null)
return Clear(id);
this.value2 = (Y)(object)value;
return this;
}
else
{
if (value == null)
return this;
return new OptionalWithMany(
this.id1, this.value1,
this.id2, this.value2,
id, value);
}
}
public IOptional Clear(string id)
{
if (string.Equals(id, this.id1))
return new OptionalWithOne<Y>(this.id2, this.value2);
if (string.Equals(id, this.id2))
return new OptionalWithOne<X>(this.id1, this.value1);
return this; // no effect
}
}
Before finally ending with the relatively inefficient
internal sealed class OptionalWithMany : IOptional
{
private List<string> ids = new List<string>();
// this boxes, if you had a restricted set of data types
// you could do a per type list and map between them
// it is assumed that this is sufficiently uncommon that you don't care
private List<object> values = new List<object>();
public OptionalWithMany(
string id1, object value1,
string id2, object value2,
string id3, object value3)
{
this.ids.Add(id1);
this.values.Add(value1);
this.ids.Add(id2);
this.values.Add(value2);
this.ids.Add(id3);
this.values.Add(value3);
}
public T? Get<T>(string id) where T : struct
{
for (int i= 0; i < this.values.Count;i++)
{
if (string.Equals(id, this.ids[i]))
return (T)this.values[i];
}
return null;
}
public T GetRef<T>(string id) where T : class
{
for (int i= 0; i < this.values.Count;i++)
{
if (string.Equals(id, this.ids[i]))
return (T)this.values[i];
}
return null;
}
public IOptional Set<T>(string id, T value)
{
for (int i= 0; i < this.values.Count;i++)
{
if (string.Equals(id, this.ids[i]))
{
if (value == null)
return Clear(id);
this.values[i] = value;
return this;
}
}
if (value != null)
{
this.ids.Add(id);
this.values.Add(value);
}
return this;
}
public IOptional Clear(string id)
{
for (int i= 0; i < this.values.Count;i++)
{
if (string.Equals(id, this.ids[i]))
{
this.ids.RemoveAt(i);
this.values.RemoveAt(i);
return ShrinkIfNeeded();
}
}
return this; // no effect
}
private IOptional ShrinkIfNeeded()
{
if (this.ids.Count == 2)
{
//return new OptionalWithTwo<X,Y>(
// this.ids[0], this.values[0],
// this.ids[1], this.values[1]);
return (IOptional)
typeof(OptionalWithTwo<,>).MakeGenericType(
// this is a bit risky.
// your value types may not use inhertence
this.values[0].GetType(),
this.values[1].GetType())
.GetConstructors().First().Invoke(
new object[]
{
this.ids[0], this.values[0],
this.ids[1], this.values[1]
});
}
return this;
}
}
OptionalWithMany could be written rather better than this but it gives you the idea.
With restricted type support you could do a global Key -> value map per type 'heap' like so:
internal struct Key
{
public readonly OptionalWithMany;
public readonly string Id;
// define equality and hashcode as per usual
}
Then simply store the list of Id's currently in use within OptionalToMany. Shrinking would be slightly more complex (but better from a type point of view since you would scan each global 'heap' till you found the matching entry and use the type of the heap to construct the OptionalWithTwo. This would allow polymorphism in the property values.
Regardless of the internals the primary benefit of this is that the public surface of the WayPoint class hides all this entirely.
You can then set up the class however you want for serialization though attributes, IXmlSerializable (which would remove the need for the annoying xxxSpecified properties).
I used strings as Id's for simplicity in my example.
If you really care about size and speed you should change the Id's to be enumerations. Given packing behaviour this won't save you much even if you can fit all needed values into a byte but it would give you compile time sanity checking. The strings are all compile time constants so occupy next to no space (but are slower for checking equality).
I urge you to only do something like this after you check that it is needed. The plus side is that this does not limit your xml serialization so you can mould it to whatever format you desire. Also the public face of the 'data packet' can be kept clean (except for the xxxSpecified junk).
If you want to avoid the xxxSpecified hassles and you know you have some 'out of band' values you can use the following trick:
[DefaultValue(double.MaxValue)]
public double Vertical
{
get { return optional.Get<double>("Vertical") ?? double.MaxValue; }
set { optional = optional.Set<double>("Vertical", value); }
}
public void ClearVertical()
{
optional = optional.ClearValue<double>("Vertical");
}
However the rest of you API must be capable of detecting these special values. In general I would say that the specified route is better.
If a particular set of properties become 'always available' on certain devices, or in certain modes you should switch to alternate classes where the properties for those are simple ones. Since the xml form will be identical this means they can interoperate simply and easily but memory usage in those cases will be much less.
If the number of these groups becomes large you may even consider a code-gen scenario (at runtime even, though this increases your support burden considerably)
For some serious fun:
apply Flyweight and store all instances in a bitmap? With a small memory device you don't need 4 byte pointers.
[Edit] With Flyweight, you can have a separate storage strategy for each field. I do not suggest to directly store the string value in the bitmap, but you could store an index.
The type is not stored in the bitmap, but in the unique object factory.
It is probably good to know that the XmlSerializer doesn't care about your internal object layout, it only cares about your public fields and properties. You can hide the internal memory optimizations behind your property accessors, and the XmlSerializer wouldn't even know.
For instance, if you know that you usually have only 2 references set, but on occasion more, you can store the two frequent ones as part of your main object, and hide the infrequent ones inside an object[] or ListDictionary or a specialized private class of your own making. However, be careful that each indirect container object also contains overhead, as it needs to be a reference type. Or when you have 8 nullable integers as part of your public contract, internally you could use 8 regular integers and a single byte containing the is-this-int-null status as its bits.
If you want to specialize even further, perhaps create specialized subclasses depending on the available data, you would have to go the route of IXmlSerializable, but usually that's not really needed.
You can do a couple of things:
Make sure to use the smallest type possible for a particular value. For example, if you look at the schema, dgpsStationType has a min value of 0, and a max of 1023. This can be stored as a ushort. Reduce the size of these items when possible.
Make sure that your fields are 4-byte aligned. The end resulting size of your structure will be some multiple of 4-bytes in size (assuming 32-bit). The default layout for a class has the items stored sequentially. If the fields are not packed correctly, the compiler will have wasted space making sure that your fields are 4-byte aligned. You can specify the layout explicitly using StructLayoutAttribute.
Bad Example: these fields in a class take up 12 bytes. The int must take up 4 contiguous bytes, and the other members must be 4-byte aligned.
public class Bad {
byte a;
byte b;
int c;
ushort u;
}
Better Example: these fields in a class take up 8 bytes. These fields are packed efficiently.
public class Better {
byte a;
byte b;
ushort u;
int c;
}
Reduce the size of your object graph. Each reference type takes up 8 bytes of overhead. If you've got a deep graph, that's a lot of overhead. Pull everything you can into functions that operate on data in you main class. Think more 'C' like, and less OOD.
Its still a good idea to lazy-load some optional parameters, but you should draw your line clearly. Create 1 or maybe 2 sets of 'optional' values that can be loaded or null. Each set will mandate a reference type, and its overhead.
Use structs where you can. Be careful of value-type semantics though, they can be tricky.
Consider not implementing ISerializable. Interface methods are by definition virtual. Any class with virtual methods contains a reference to a vtable (another 4 bytes). Instead implement xml serialization manually in an external class.
Build your own serializing in order to minimize your structure. And serialize to binary and not xml.
Something along the lines of:
internal void Save(BinaryWriter w)
{
w.Write(this.id);
w.Write(this.name);
byte[] bytes = Encoding.UTF8.GetBytes(this.MyString);
w.Write(bytes.Length);
w.Write(bytes);
w.Write(this.tags.Count); // nested struct/class
foreach (Tag tag in this.tags)
{
tag.Save(w);
}
}
and have a constructor which builds it back up
public MyClass(BinaryReader reader)
{
this.id = reader.ReadUInt32();
etc.
}
Some sort of binary serialization will often do much better than XML serialization. You'll have to try it out for your specific data structures to see if you gain much.
Check out MSDN an example using BinaryFormatter.