What is the difference between a field and a property

Question

In C   what makes a field different from a property  and when should a field be used instead of a property

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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

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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

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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

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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

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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

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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  lt   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

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Traditionally private fields are set via getter and setter methods   For the sake of less code you can use properties to set fields instead

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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  lt   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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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IMO  Properties are just the  SetXXX     GetXXX    functions methods interfaces pairs we used before  but they are more concise and elegant

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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

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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  lt  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  lt   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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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  amp  exposing it is risky  as you will not have control what gets assigned  amp  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 lt  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

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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

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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

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Traditionally private fields are set via getter and setter methods   For the sake of less code you can use properties to set fields instead

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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

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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

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The difference is clearly explained here However  just to summarize and highlight    The fields are encapsulated inside class for its internal operation whereas properties can be used for exposing the class to outside world in addition to other internal operations shown in the link shared Additionally  if you want to load certain methods or  user controls based on the value of a particular field  then the property will do it for you    For example    You can function below user control inside your asp net page by simply assigning a value to the Id preperty of control in your aspx page as given below    useMeId Id 5 ---call the property of user control  UseMe ascx    UseMe ascx   lt    Register Src    UseMe ascx  TagPrefix  uc  TagName  UseMe    gt   lt uc UseMe runat  Server  id  useMeId    gt   UseMe ascx cs  private int currentId   public int Id            get                  return currentId                set                  currentId   value         LoadInitialData currentId                Private void LoadinitialData int currentIdParam      your action

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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 lt 1900    MYearOfBirth gt 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  lt Value1 gt k  BackingField        CompilerGenerated       DebuggerBrowsable DebuggerBrowsableState Never       private int  lt Value2 gt k  BackingField       public int Value1                CompilerGenerated          get                       return  lt Value1 gt k  BackingField                     CompilerGenerated          set                        lt Value1 gt k  BackingField   value                       public int Value2                CompilerGenerated          get                       return  lt Value2 gt k  BackingField                     CompilerGenerated          set                        lt Value2 gt 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

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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

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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

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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  lt   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

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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  amp  exposing it is risky  as you will not have control what gets assigned  amp  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 lt  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

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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

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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

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The vast majority of cases it s going to be a property name that you access as opposed to a variable name  field  The reason for that is it s considered good practice in  NET and in C  in particular to protect every piece of data within a class  whether it s an instance variable or a static variable  class variable  because it s associated with a class    Protect all of those variables with corresponding properties which allow you to define  set and get accessors and do things like validation when you re manipulating those pieces of data    But in other cases like Math class  System namespace   there are a couple of static properties that are built into the class  one of which is the math constant PI   eg  Math PI   and because PI is a piece of data that is well-defined  we don t need to have multiple copies of PI  it always going to be the same value  So static variables are sometimes used to share data amongst object of a class  but the are also commonly used for constant information where you only need one copy of a piece of data

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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

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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  lt  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  lt  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

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The difference is clearly explained here However  just to summarize and highlight    The fields are encapsulated inside class for its internal operation whereas properties can be used for exposing the class to outside world in addition to other internal operations shown in the link shared Additionally  if you want to load certain methods or  user controls based on the value of a particular field  then the property will do it for you    For example    You can function below user control inside your asp net page by simply assigning a value to the Id preperty of control in your aspx page as given below    useMeId Id 5 ---call the property of user control  UseMe ascx    UseMe ascx   lt    Register Src    UseMe ascx  TagPrefix  uc  TagName  UseMe    gt   lt uc UseMe runat  Server  id  useMeId    gt   UseMe ascx cs  private int currentId   public int Id            get                  return currentId                set                  currentId   value         LoadInitialData currentId                Private void LoadinitialData int currentIdParam      your action

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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

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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  lt   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

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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

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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

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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  lt  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  lt   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

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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

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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  lt  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  lt  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

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IMO  Properties are just the  SetXXX     GetXXX    functions methods interfaces pairs we used before  but they are more concise and elegant

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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

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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

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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

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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 lt 1900    MYearOfBirth gt 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  lt Value1 gt k  BackingField        CompilerGenerated       DebuggerBrowsable DebuggerBrowsableState Never       private int  lt Value2 gt k  BackingField       public int Value1                CompilerGenerated          get                       return  lt Value1 gt k  BackingField                     CompilerGenerated          set                        lt Value1 gt k  BackingField   value                       public int Value2                CompilerGenerated          get                       return  lt Value2 gt k  BackingField                     CompilerGenerated          set                        lt Value2 gt 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

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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

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The vast majority of cases it s going to be a property name that you access as opposed to a variable name  field  The reason for that is it s considered good practice in  NET and in C  in particular to protect every piece of data within a class  whether it s an instance variable or a static variable  class variable  because it s associated with a class    Protect all of those variables with corresponding properties which allow you to define  set and get accessors and do things like validation when you re manipulating those pieces of data    But in other cases like Math class  System namespace   there are a couple of static properties that are built into the class  one of which is the math constant PI   eg  Math PI   and because PI is a piece of data that is well-defined  we don t need to have multiple copies of PI  it always going to be the same value  So static variables are sometimes used to share data amongst object of a class  but the are also commonly used for constant information where you only need one copy of a piece of data

User · Answer

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

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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

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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

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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

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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

User · Answer

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

User · Answer

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

User · Answer

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

[c#] What is the difference between a field and a property?

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