Why use the ref keyword when passing an object

Question

If I am passing an object to a method  why should I use the ref keyword  Isn t this the default behaviour anyway   For example   class Program       static void Main string   args                TestRef t   new TestRef            t Something    Foo            DoSomething t           Console WriteLine t Something              static public void DoSomething TestRef t                t Something    Bar             public class TestRef       public string Something   get  set        The output is  Bar  which means that the object was passed as a reference

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With ref you can write   static public void DoSomething ref TestRef t        t   new TestRef        And t will be changed after the method has completed

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Think of variables  e g  foo  of reference types  e g  List lt T gt   as holding object identifiers of the form  Object  24601    Suppose the statement foo   new List lt int gt   1 5 7 9   causes foo to hold  Object  24601   a list with four items    Then calling foo Length will ask Object  24601 for its length  and it will respond 4  so foo Length will equal 4   If foo is passed to a method without using ref  that method might make changes to Object  24601   As a consequence of such changes  foo Length might no longer equal 4   The method itself  however  will be unable to change foo  which will continue to hold  Object  24601    Passing foo as a ref parameter will allow the called method to make changes not just to Object  24601  but also to foo itself   The method might create a new Object  8675309 and store a reference to that in foo   If it does so  foo would no longer hold  Object  24601   but instead  Object  8675309    In practice  reference-type variables don t hold strings of the form  Object  8675309   they don t even hold anything that can meaningfully converted into a number   Even though each reference-type variable will hold some bit pattern  there is no fixed relationship between the bit patterns stored in such variables and the objects they identify   There is no way code could extract information from an object or a reference to it  and later determine whether another reference identified the same object  unless the code either held or knew of a reference that identified the original object

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Pass a ref if you want to change what the object is  TestRef t   new TestRef    t Something    quot Foo quot   DoSomething ref t    void DoSomething ref TestRef t      t   new TestRef      t Something    quot Not just a changed t  but a completely different TestRef object quot      After calling DoSomething  t does not refer to the original new TestRef  but refers to a completely different object  This may be useful too if you want to change the value of an immutable object  e g  a string  You cannot change the value of a string once it has been created  But by using a ref  you could create a function that changes the string for another one that has a different value  It is not a good idea to use ref unless it is needed  Using ref gives the method freedom to change the argument for something else  callers of the method will need to be coded to ensure they handle this possibility  Also  when the parameter type is an object  then object variables always act as references to the object  This means that when the ref keyword is used you ve got a reference to a reference  This allows you to do things as described in the example given above  But  when the parameter type is a primitive value  e g  int   then if this parameter is assigned to within the method  the value of the argument that was passed in will be changed after the method returns  int x   1  Change ref x   Debug Assert x    5   WillNotChange x   Debug Assert x    5      Note  x doesn t become 10  void Change ref int x      x   5     void WillNotChange int x      x   10

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Pass a ref if you want to change what the object is  TestRef t   new TestRef    t Something    quot Foo quot   DoSomething ref t    void DoSomething ref TestRef t      t   new TestRef      t Something    quot Not just a changed t  but a completely different TestRef object quot      After calling DoSomething  t does not refer to the original new TestRef  but refers to a completely different object  This may be useful too if you want to change the value of an immutable object  e g  a string  You cannot change the value of a string once it has been created  But by using a ref  you could create a function that changes the string for another one that has a different value  It is not a good idea to use ref unless it is needed  Using ref gives the method freedom to change the argument for something else  callers of the method will need to be coded to ensure they handle this possibility  Also  when the parameter type is an object  then object variables always act as references to the object  This means that when the ref keyword is used you ve got a reference to a reference  This allows you to do things as described in the example given above  But  when the parameter type is a primitive value  e g  int   then if this parameter is assigned to within the method  the value of the argument that was passed in will be changed after the method returns  int x   1  Change ref x   Debug Assert x    5   WillNotChange x   Debug Assert x    5      Note  x doesn t become 10  void Change ref int x      x   5     void WillNotChange int x      x   10

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By using the ref keyword with reference types you are effectively passing a reference to the reference  In many ways it s the same as using the out keyword but with the minor difference that there s no guarantee that the method will actually assign anything to the ref ed parameter

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In  NET when you pass any parameter to a method  a copy is created  In value types means that any modification you make to the value is at the method scope  and is lost when you exit the method   When passing a Reference Type  a copy is also made  but it is a copy of a reference  i e  now you have TWO references in memory to the same object  So  if you use the reference to modify the object  it gets modified  But if you modify the reference itself - we must remember it is a copy - then any changes are also lost upon exiting the method   As people have said before  an assignment is a modification of the reference  thus is lost   public void Method1 object obj        obj   new Object        public void Method2 object obj       obj    privateObject       The methods above does not modifies the original object   A little modification of your example   using System       class Program                       static void Main string   args                                TestRef t   new TestRef                    t Something    Foo                    DoSomething t                   Console WriteLine t Something                               static public void DoSomething TestRef t                                t   new TestRef                    t Something    Bar                                  public class TestRef           private string s          public string Something                         get  return s                set   s   value

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In  NET when you pass any parameter to a method  a copy is created  In value types means that any modification you make to the value is at the method scope  and is lost when you exit the method   When passing a Reference Type  a copy is also made  but it is a copy of a reference  i e  now you have TWO references in memory to the same object  So  if you use the reference to modify the object  it gets modified  But if you modify the reference itself - we must remember it is a copy - then any changes are also lost upon exiting the method   As people have said before  an assignment is a modification of the reference  thus is lost   public void Method1 object obj        obj   new Object        public void Method2 object obj       obj    privateObject       The methods above does not modifies the original object   A little modification of your example   using System       class Program                       static void Main string   args                                TestRef t   new TestRef                    t Something    Foo                    DoSomething t                   Console WriteLine t Something                               static public void DoSomething TestRef t                                t   new TestRef                    t Something    Bar                                  public class TestRef           private string s          public string Something                         get  return s                set   s   value

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Since TestRef is a class  which are reference objects   you can change the contents inside t without passing it as a ref  However  if you pass t as a ref  TestRef can change what the original t refers to  i e  make it point to a different object

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If you re passing a value  however  things are different  You can force a value to be passed by reference  This allows you to pass an integer to a method  for example  and have the method modify the integer on your behalf

User · Answer

Since TestRef is a class  which are reference objects   you can change the contents inside t without passing it as a ref  However  if you pass t as a ref  TestRef can change what the original t refers to  i e  make it point to a different object

User · Answer

You need to distinguish between  quot passing a reference by value quot   and  quot passing a parameter argument by reference quot   I ve written a reasonably long article on the subject to avoid having to write carefully each time this comes up on newsgroups

User · Answer

This is like passing a pointer to a pointer in C   In  NET this will allow you to change what the original T refers to  personally though I think if you are doing that in  NET you have probably got a design issue

User · Answer

In  NET when you pass any parameter to a method  a copy is created  In value types means that any modification you make to the value is at the method scope  and is lost when you exit the method   When passing a Reference Type  a copy is also made  but it is a copy of a reference  i e  now you have TWO references in memory to the same object  So  if you use the reference to modify the object  it gets modified  But if you modify the reference itself - we must remember it is a copy - then any changes are also lost upon exiting the method   As people have said before  an assignment is a modification of the reference  thus is lost   public void Method1 object obj        obj   new Object        public void Method2 object obj       obj    privateObject       The methods above does not modifies the original object   A little modification of your example   using System       class Program                       static void Main string   args                                TestRef t   new TestRef                    t Something    Foo                    DoSomething t                   Console WriteLine t Something                               static public void DoSomething TestRef t                                t   new TestRef                    t Something    Bar                                  public class TestRef           private string s          public string Something                         get  return s                set   s   value

User · Answer

This is like passing a pointer to a pointer in C   In  NET this will allow you to change what the original T refers to  personally though I think if you are doing that in  NET you have probably got a design issue

User · Answer

Think of variables  e g  foo  of reference types  e g  List lt T gt   as holding object identifiers of the form  Object  24601    Suppose the statement foo   new List lt int gt   1 5 7 9   causes foo to hold  Object  24601   a list with four items    Then calling foo Length will ask Object  24601 for its length  and it will respond 4  so foo Length will equal 4   If foo is passed to a method without using ref  that method might make changes to Object  24601   As a consequence of such changes  foo Length might no longer equal 4   The method itself  however  will be unable to change foo  which will continue to hold  Object  24601    Passing foo as a ref parameter will allow the called method to make changes not just to Object  24601  but also to foo itself   The method might create a new Object  8675309 and store a reference to that in foo   If it does so  foo would no longer hold  Object  24601   but instead  Object  8675309    In practice  reference-type variables don t hold strings of the form  Object  8675309   they don t even hold anything that can meaningfully converted into a number   Even though each reference-type variable will hold some bit pattern  there is no fixed relationship between the bit patterns stored in such variables and the objects they identify   There is no way code could extract information from an object or a reference to it  and later determine whether another reference identified the same object  unless the code either held or knew of a reference that identified the original object

User · Answer

ref mimics  or behaves  as a global area just for two scopes    Caller Callee

User · Answer

In  NET when you pass any parameter to a method  a copy is created  In value types means that any modification you make to the value is at the method scope  and is lost when you exit the method   When passing a Reference Type  a copy is also made  but it is a copy of a reference  i e  now you have TWO references in memory to the same object  So  if you use the reference to modify the object  it gets modified  But if you modify the reference itself - we must remember it is a copy - then any changes are also lost upon exiting the method   As people have said before  an assignment is a modification of the reference  thus is lost   public void Method1 object obj        obj   new Object        public void Method2 object obj       obj    privateObject       The methods above does not modifies the original object   A little modification of your example   using System       class Program                       static void Main string   args                                TestRef t   new TestRef                    t Something    Foo                    DoSomething t                   Console WriteLine t Something                               static public void DoSomething TestRef t                                t   new TestRef                    t Something    Bar                                  public class TestRef           private string s          public string Something                         get  return s                set   s   value

User · Answer

This is like passing a pointer to a pointer in C   In  NET this will allow you to change what the original T refers to  personally though I think if you are doing that in  NET you have probably got a design issue

User · Answer

You need to distinguish between  quot passing a reference by value quot   and  quot passing a parameter argument by reference quot   I ve written a reasonably long article on the subject to avoid having to write carefully each time this comes up on newsgroups

User · Answer

Pass a ref if you want to change what the object is  TestRef t   new TestRef    t Something    quot Foo quot   DoSomething ref t    void DoSomething ref TestRef t      t   new TestRef      t Something    quot Not just a changed t  but a completely different TestRef object quot      After calling DoSomething  t does not refer to the original new TestRef  but refers to a completely different object  This may be useful too if you want to change the value of an immutable object  e g  a string  You cannot change the value of a string once it has been created  But by using a ref  you could create a function that changes the string for another one that has a different value  It is not a good idea to use ref unless it is needed  Using ref gives the method freedom to change the argument for something else  callers of the method will need to be coded to ensure they handle this possibility  Also  when the parameter type is an object  then object variables always act as references to the object  This means that when the ref keyword is used you ve got a reference to a reference  This allows you to do things as described in the example given above  But  when the parameter type is a primitive value  e g  int   then if this parameter is assigned to within the method  the value of the argument that was passed in will be changed after the method returns  int x   1  Change ref x   Debug Assert x    5   WillNotChange x   Debug Assert x    5      Note  x doesn t become 10  void Change ref int x      x   5     void WillNotChange int x      x   10

User · Answer

Ref denotes whether the function can get its hands on the object itself  or only on its value   Passing by reference is not bound to a language  it s a parameter binding strategy next to pass-by-value  pass by name  pass by need etc     A sidenote  the class name TestRef is a hideously bad choice in this context

User · Answer

ref mimics  or behaves  as a global area just for two scopes    Caller Callee

User · Answer

If you re passing a value  however  things are different  You can force a value to be passed by reference  This allows you to pass an integer to a method  for example  and have the method modify the integer on your behalf

User · Answer

Pass a ref if you want to change what the object is  TestRef t   new TestRef    t Something    quot Foo quot   DoSomething ref t    void DoSomething ref TestRef t      t   new TestRef      t Something    quot Not just a changed t  but a completely different TestRef object quot      After calling DoSomething  t does not refer to the original new TestRef  but refers to a completely different object  This may be useful too if you want to change the value of an immutable object  e g  a string  You cannot change the value of a string once it has been created  But by using a ref  you could create a function that changes the string for another one that has a different value  It is not a good idea to use ref unless it is needed  Using ref gives the method freedom to change the argument for something else  callers of the method will need to be coded to ensure they handle this possibility  Also  when the parameter type is an object  then object variables always act as references to the object  This means that when the ref keyword is used you ve got a reference to a reference  This allows you to do things as described in the example given above  But  when the parameter type is a primitive value  e g  int   then if this parameter is assigned to within the method  the value of the argument that was passed in will be changed after the method returns  int x   1  Change ref x   Debug Assert x    5   WillNotChange x   Debug Assert x    5      Note  x doesn t become 10  void Change ref int x      x   5     void WillNotChange int x      x   10

User · Answer

With ref you can write   static public void DoSomething ref TestRef t        t   new TestRef        And t will be changed after the method has completed

User · Answer

By using the ref keyword with reference types you are effectively passing a reference to the reference  In many ways it s the same as using the out keyword but with the minor difference that there s no guarantee that the method will actually assign anything to the ref ed parameter

User · Answer

Ref denotes whether the function can get its hands on the object itself  or only on its value   Passing by reference is not bound to a language  it s a parameter binding strategy next to pass-by-value  pass by name  pass by need etc     A sidenote  the class name TestRef is a hideously bad choice in this context

User · Answer

Ref denotes whether the function can get its hands on the object itself  or only on its value   Passing by reference is not bound to a language  it s a parameter binding strategy next to pass-by-value  pass by name  pass by need etc     A sidenote  the class name TestRef is a hideously bad choice in this context

User · Answer

Since TestRef is a class  which are reference objects   you can change the contents inside t without passing it as a ref  However  if you pass t as a ref  TestRef can change what the original t refers to  i e  make it point to a different object

User · Answer

You need to distinguish between  quot passing a reference by value quot   and  quot passing a parameter argument by reference quot   I ve written a reasonably long article on the subject to avoid having to write carefully each time this comes up on newsgroups

User · Answer

By using the ref keyword with reference types you are effectively passing a reference to the reference  In many ways it s the same as using the out keyword but with the minor difference that there s no guarantee that the method will actually assign anything to the ref ed parameter

User · Answer

If you re passing a value  however  things are different  You can force a value to be passed by reference  This allows you to pass an integer to a method  for example  and have the method modify the integer on your behalf

User · Answer

You need to distinguish between  quot passing a reference by value quot   and  quot passing a parameter argument by reference quot   I ve written a reasonably long article on the subject to avoid having to write carefully each time this comes up on newsgroups

User · Answer

By using the ref keyword with reference types you are effectively passing a reference to the reference  In many ways it s the same as using the out keyword but with the minor difference that there s no guarantee that the method will actually assign anything to the ref ed parameter

User · Answer

Ref denotes whether the function can get its hands on the object itself  or only on its value   Passing by reference is not bound to a language  it s a parameter binding strategy next to pass-by-value  pass by name  pass by need etc     A sidenote  the class name TestRef is a hideously bad choice in this context

User · Answer

Since TestRef is a class  which are reference objects   you can change the contents inside t without passing it as a ref  However  if you pass t as a ref  TestRef can change what the original t refers to  i e  make it point to a different object

User · Answer

If you re passing a value  however  things are different  You can force a value to be passed by reference  This allows you to pass an integer to a method  for example  and have the method modify the integer on your behalf

User · Answer

This is like passing a pointer to a pointer in C   In  NET this will allow you to change what the original T refers to  personally though I think if you are doing that in  NET you have probably got a design issue

User · Answer

With ref you can write   static public void DoSomething ref TestRef t        t   new TestRef        And t will be changed after the method has completed

User · Answer

With ref you can write   static public void DoSomething ref TestRef t        t   new TestRef        And t will be changed after the method has completed

[c#] Why use the 'ref' keyword when passing an object?

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