What s the difference between the ref and out keywords

Question

I m creating a function where I need to pass an object so that it can be modified by the function  What is the difference between   public void myFunction ref MyClass someClass    and  public void myFunction out MyClass someClass    Which should I use and why

User · Answer

The ref modifier means that    The value is already set and  The method can read and modify it    The out modifier means that    The Value isn t set and can t be read by the method until it is set  The method must set it before returning

User · Answer

ref is in and out   You should use out in preference wherever it suffices for your requirements

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I am going to try my hand at an explanation   I think we understand how the value types work right   Value types are  int  long  struct etc     When you send them in to a function without a ref command it COPIES the data  Anything you do to that data in the function only affects the copy  not the original   The ref command sends the ACTUAL data and any changes will affect the data outside the function    Ok on to the confusing part  reference types   Lets create a reference type   List lt string gt  someobject   new List lt string gt      When you new up someobject  two parts are created    The block of memory that holds data for someobject   A reference  pointer  to that block of data    Now when you send in someobject into a method without ref it COPIES the reference pointer  NOT the data   So you now have this    outside method  reference1   gt  someobject  inside method   reference2   gt  someobject   Two references pointing to the same object   If you modify a property on someobject using reference2 it will affect the same data pointed to by reference1       inside method   reference2 Add  SomeString      outside method  reference1 0      SomeString      this is true   If you null out reference2 or point it to new data it will not affect reference1 nor the data reference1 points to    inside method  reference2   new List lt string gt      outside method  reference1    null  reference1 0      SomeString    this is true  The references are now pointing like this  reference2   gt  new List lt string gt    reference1   gt  someobject   Now what happens when you send someobject by ref to a method  The actual reference to someobject gets sent to the method   So you now have only one reference to the data    outside method  reference1   gt  someobject   inside method   reference1   gt  someobject    But what does this mean   It acts exactly the same as sending someobject not by ref except for two main thing   1  When you null out the reference inside the method it will null the one outside the method     inside method   reference1   null    outside method  reference1    null     true   2  You can now point the reference to a completely different data location and the reference outside the function will now point to the new data location     inside method   reference1   new List lt string gt       outside method  reference1 Count    0    this is true

User · Answer

From the standpoint of a method which receives a parameter  the difference between ref and out is that C  requires that methods must write to every out parameter before returning  and must not do anything with such a parameter  other than passing it as an out parameter or writing to it  until it has been either passed as an out parameter to another method or written directly   Note that some other languages do not impose such requirements  a virtual or interface method which is declared in C  with an out parameter may be overridden in another language which does not impose any special restrictions on such parameters   From the standpoint of the caller  C  will in many circumstances assume when calling a method with an out parameter will cause the passed variable to be written without having been read first   This assumption may not be correct when calling methods written in other languages   For example   struct MyStruct             myStruct IDictionary lt int  MyStruct gt  d            d TryGetValue 23  out this            If myDictionary identifies an IDictionary lt TKey TValue gt  implementation written in a language other than C   even though MyStruct s   new MyStruct myDictionary   looks like an assignment  it could potentially leave s unmodified   Note that constructors written in VB NET  unlike those in C   make no assumptions about whether called methods will modify any out parameters  and clear out all fields unconditionally   The odd behavior alluded to above won t occur with code written entirely in VB or entirely in C   but can occur when code written in C  calls a method written in VB NET

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Since you re passing in a reference type  a class  there is no need use ref because per default only a reference to the actual object is passed and therefore you always change the object behind the reference   Example   public void Foo         MyClass myObject   new MyClass        myObject Name    Dog       Bar myObject       Console WriteLine myObject Name      Writes  Cat      public void Bar MyClass someObject        someObject Name    Cat       As long you pass in a class you don t have to use ref if you want to change the object inside your method

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public static void Main string   args                  int a 10            change ref a             Console WriteLine a              Console Read             int b          change2 out b           Console WriteLine b           Console Read                 static void change ref int a                    a   20                static void change2 out int b                  b   20           you can check this code it will describe you its complete differnce when you use  ref  its mean that u already initialize that int string  but  when you use  out  it works in both conditions wheather u initialize that int string or not but u must initialize that int string in that function

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For those looking concise answer      Both ref and out keywords are used to pass-by-reference              A variable of ref keyword must have a value or must refer to an object   or null before its passing             Unlike ref  a variable of out keyword must have a value or must   refer to an object or null after its passing as well as no need   to have a value or refer to an object before passing

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Out   A return statement can be used for returning only one value from a function  However  using output parameters  you can return two values from a function  Output parameters are like reference parameters  except that they transfer data out of the method rather than into it   The following example illustrates this   using System   namespace CalculatorApplication      class NumberManipulator            public void getValue out int x                    int temp   5           x   temp                 static void Main string   args                   NumberManipulator n   new NumberManipulator                local variable definition             int a   100            Console WriteLine  Before method call  value of a    0    a                calling a function to get the value             n getValue out a             Console WriteLine  After method call  value of a    0    a            Console ReadLine                      ref  A reference parameter is a reference to a memory location of a variable  When you pass parameters by reference  unlike value parameters  a new storage location is not created for these parameters  The reference parameters represent the same memory location as the actual parameters that are supplied to the method   In C   you declare the reference parameters using the ref keyword  The following example demonstrates this   using System  namespace CalculatorApplication      class NumberManipulator            public void swap ref int x  ref int y                   int temp            temp   x     save the value of x             x   y       put y into x             y   temp     put temp into y                    static void Main string   args                   NumberManipulator n   new NumberManipulator                local variable definition             int a   100           int b   200            Console WriteLine  Before swap  value of a    0    a            Console WriteLine  Before swap  value of b    0    b                calling a function to swap the values             n swap ref a  ref b             Console WriteLine  After swap  value of a    0    a            Console WriteLine  After swap  value of b    0    b             Console ReadLine

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I am going to try my hand at an explanation   I think we understand how the value types work right   Value types are  int  long  struct etc     When you send them in to a function without a ref command it COPIES the data  Anything you do to that data in the function only affects the copy  not the original   The ref command sends the ACTUAL data and any changes will affect the data outside the function    Ok on to the confusing part  reference types   Lets create a reference type   List lt string gt  someobject   new List lt string gt      When you new up someobject  two parts are created    The block of memory that holds data for someobject   A reference  pointer  to that block of data    Now when you send in someobject into a method without ref it COPIES the reference pointer  NOT the data   So you now have this    outside method  reference1   gt  someobject  inside method   reference2   gt  someobject   Two references pointing to the same object   If you modify a property on someobject using reference2 it will affect the same data pointed to by reference1       inside method   reference2 Add  SomeString      outside method  reference1 0      SomeString      this is true   If you null out reference2 or point it to new data it will not affect reference1 nor the data reference1 points to    inside method  reference2   new List lt string gt      outside method  reference1    null  reference1 0      SomeString    this is true  The references are now pointing like this  reference2   gt  new List lt string gt    reference1   gt  someobject   Now what happens when you send someobject by ref to a method  The actual reference to someobject gets sent to the method   So you now have only one reference to the data    outside method  reference1   gt  someobject   inside method   reference1   gt  someobject    But what does this mean   It acts exactly the same as sending someobject not by ref except for two main thing   1  When you null out the reference inside the method it will null the one outside the method     inside method   reference1   null    outside method  reference1    null     true   2  You can now point the reference to a completely different data location and the reference outside the function will now point to the new data location     inside method   reference1   new List lt string gt       outside method  reference1 Count    0    this is true

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As well as allowing you to reassign someone else s variable to a different instance of a class  return multiple values etc  using ref or out lets someone else know what you need from them and what you intend to do with the variable they provide   You don t need ref or out if all you re going to do is modify things inside the MyClass instance that is passed in the argument someClass     The calling method will see changes like someClass Message    Hello World  whether you use ref  out or nothing Writing someClass   new MyClass   inside myFunction someClass  swaps out the object seen by the someClass in the scope of the myFunction method only  The calling method still knows about the original MyClass instance it created and passed to your method  You need ref or out if you plan on swapping the someClass out for a whole new object and want the calling method to see your change   Writing someClass   new MyClass   inside myFunction out someClass  changes the object seen by the method that called myFunction    Other programmers exist  And they want to know what you re going to do with their data  Imagine you re writing a library that will be used by millions of developers  You want them to know what you re going to do with their variables when they call your methods    Using ref makes a statement of  Pass a variable assigned to some value when you call my method  Be aware that I might change it out for something else entirely during the course of my method  Do not expect your variable to be pointing to the old object when I m done  Using out makes a statement of  Pass a placeholder variable to my method  It doesn t matter whether it has a value or not  the compiler will force me to assign it to a new value  I absolutely guarantee that the object pointed to by your variable before you called my method  will be different by the time I m done   By the way  in C 7 2 there s an in modifier too  And that prevents the method from swapping out the passed in instance for a different instance  Think of it like saying to those millions of developers  pass me your original variable reference  and I promise not to swap your carefully crafted data out for something else   in has some peculiarities  and in some cases such as where an implicit conversion might be required to make your short compatible with an in int the compiler will temporarily make an int  widen your short to it  pass it by reference and finish up  It can do this because you ve declared you re not going to mess with it     Microsoft did this with the  TryParse methods on the numeric types   int i   98234957  bool success   int TryParse  123   out i     By flagging the parameter as out they re actively declaring here  we are definitely going to change your painstakingly crafted value of 98234957 out for something else   Of course  they kinda have to  for things like parsing value types because if the parse method wasn t allowed to swap out the value type for something else it wouldn t work very well   But imagine there was some fictitious method in some library you re creating   public void PoorlyNamedMethod out SomeClass x    You can see it s an out  and you can thus know that if you spend hours crunching numbers  creating the perfect SomeClass   SomeClass x   SpendHoursMakingMeAPerfectSomeClass      now give it to the library PoorlyNamedMethod out x     Well that was a waste of time  taking all those hours to make that perfect class  It s definitely going to be tossed away and replaced by PoorlyNamedMethod

User · Answer

Let s say Dom shows up at Peter s cubicle about the memo about the TPS reports    If Dom were a ref argument  he would have a printed copy of the memo    If Dom were an out argument  he d make Peter print a new copy of the memo for him to take with him

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Mind well that the reference parameter which is passed inside the function is directly worked on   For example       public class MyClass               public string Name   get  set               public void Foo                 MyClass myObject   new MyClass            myObject Name    Dog           Bar myObject           Console WriteLine myObject Name      Writes  Dog              public void Bar MyClass someObject                MyClass myTempObject   new MyClass            myTempObject Name    Cat           someObject   myTempObject          This will write Dog  not Cat  Hence you should directly work on someObject

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Since you re passing in a reference type  a class  there is no need use ref because per default only a reference to the actual object is passed and therefore you always change the object behind the reference   Example   public void Foo         MyClass myObject   new MyClass        myObject Name    Dog       Bar myObject       Console WriteLine myObject Name      Writes  Cat      public void Bar MyClass someObject        someObject Name    Cat       As long you pass in a class you don t have to use ref if you want to change the object inside your method

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The ref modifier means that    The value is already set and  The method can read and modify it    The out modifier means that    The Value isn t set and can t be read by the method until it is set  The method must set it before returning

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Baker   That s because the first one changes your string-reference to point to  Baker   Changing the reference is possible because you passed it via the ref keyword     a reference to a reference to a string   The Second call gets a copy of the reference to the string   string looks some kind of special at first  But string is just a reference class and if you define   string s    Able     then s is a reference to a string class that contains the text  Able   Another assignment to the same variable via  s    Baker     does not change the original string but just creates a new instance and let s point to that instance   You can try it with the following little code example   string s    Able   string s2   s  s    Baker   Console WriteLine s2     What do you expect  What you will get is still  Able  because you just set the reference in s to another instance while s2 points to the original instance   EDIT  string is also immutable which means there is simply no method or property that modifies an existing string instance  you can try to find one in the docs but you won t fins any  -     All string manipulation methods return a new string instance   That s why you often get a better performance when using the StringBuilder class

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The ref modifier means that    The value is already set and  The method can read and modify it    The out modifier means that    The Value isn t set and can t be read by the method until it is set  The method must set it before returning

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Extending the Dog  Cat example  The second method with ref changes the object referenced by the caller  Hence  Cat           public static void Foo                 MyClass myObject   new MyClass            myObject Name    Dog           Bar myObject           Console WriteLine myObject Name      Writes  Dog            Bar ref myObject           Console WriteLine myObject Name      Writes  Cat               public static void Bar MyClass someObject                MyClass myTempObject   new MyClass            myTempObject Name    Cat           someObject   myTempObject             public static void Bar ref MyClass someObject                MyClass myTempObject   new MyClass            myTempObject Name    Cat           someObject   myTempObject

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Below I have shown an example using both Ref and out  Now  you all will be cleared about ref and out    In below mentioned example when i comment   myRefObj   new myClass   Name    ref outside called        line  will get an error saying  Use of unassigned local variable  myRefObj    but there is no such error in out   Where to use Ref  when we are calling a procedure with an in parameter and the same parameter will be used to store the output of that proc   Where to use Out  when we are calling a procedure with no in parameter and teh same param will be used to return the value from that proc  Also note the output  public partial class refAndOutUse   System Web UI Page       protected void Page Load object sender  EventArgs e                myClass myRefObj          myRefObj   new myClass   Name    ref outside called     lt br  gt              myRefFunction ref myRefObj           Response Write myRefObj Name     ref inside function          myClass myOutObj          myOutFunction out myOutObj           Response Write myOutObj Name     out inside function            void myRefFunction ref myClass refObj                refObj Name    ref inside function  lt br  gt            Response Write refObj Name     ref inside function           void myOutFunction out myClass outObj                outObj   new myClass   Name    out inside function  lt br  gt               Response Write outObj Name     out inside function          public class myClass       public string Name   get  set

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ref tells the compiler that the object is initialized before entering the function  while out tells the compiler that the object will be initialized inside the function    So while ref is two-ways  out is out-only

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Baker   That s because the first one changes your string-reference to point to  Baker   Changing the reference is possible because you passed it via the ref keyword     a reference to a reference to a string   The Second call gets a copy of the reference to the string   string looks some kind of special at first  But string is just a reference class and if you define   string s    Able     then s is a reference to a string class that contains the text  Able   Another assignment to the same variable via  s    Baker     does not change the original string but just creates a new instance and let s point to that instance   You can try it with the following little code example   string s    Able   string s2   s  s    Baker   Console WriteLine s2     What do you expect  What you will get is still  Able  because you just set the reference in s to another instance while s2 points to the original instance   EDIT  string is also immutable which means there is simply no method or property that modifies an existing string instance  you can try to find one in the docs but you won t fins any  -     All string manipulation methods return a new string instance   That s why you often get a better performance when using the StringBuilder class

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Let s say Dom shows up at Peter s cubicle about the memo about the TPS reports    If Dom were a ref argument  he would have a printed copy of the memo    If Dom were an out argument  he d make Peter print a new copy of the memo for him to take with him

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Ref  The ref keyword is used to pass an argument as a reference  This means that when value of that parameter is changed in the method  it gets reflected in the calling method  An argument that is passed using a ref keyword must be initialized in the calling method before it is passed to the called method   Out  The out keyword is also used to pass an argument like ref keyword  but the argument can be passed without assigning any value to it  An argument that is passed using an out keyword must be initialized in the called method before it returns back to calling method   public class Example    public static void Main        int val1   0    must be initialized   int val2    optional   Example1 ref val1    Console WriteLine val1      Example2 out val2    Console WriteLine val2         static void Example1 ref int value       value   1      static void Example2 out int value       value   2            Output     1     2        Ref and out in method overloading  Both ref and out cannot be used in method overloading simultaneously  However  ref and out are treated differently at run-time but they are treated same at compile time  CLR doesn t differentiates between the two while it created IL for ref and out

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For those that learn by example  like me  here s what Anthony Kolesov is saying   I ve created some minimal examples of ref  out  and others to illustrate the point  I m not covering best practices  just examples to understand the differences   https   gist github com 2upmedia 6d98a57b68d849ee7091

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Baker   That s because the first one changes your string-reference to point to  Baker   Changing the reference is possible because you passed it via the ref keyword     a reference to a reference to a string   The Second call gets a copy of the reference to the string   string looks some kind of special at first  But string is just a reference class and if you define   string s    Able     then s is a reference to a string class that contains the text  Able   Another assignment to the same variable via  s    Baker     does not change the original string but just creates a new instance and let s point to that instance   You can try it with the following little code example   string s    Able   string s2   s  s    Baker   Console WriteLine s2     What do you expect  What you will get is still  Able  because you just set the reference in s to another instance while s2 points to the original instance   EDIT  string is also immutable which means there is simply no method or property that modifies an existing string instance  you can try to find one in the docs but you won t fins any  -     All string manipulation methods return a new string instance   That s why you often get a better performance when using the StringBuilder class

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Extending the Dog  Cat example  The second method with ref changes the object referenced by the caller  Hence  Cat           public static void Foo                 MyClass myObject   new MyClass            myObject Name    Dog           Bar myObject           Console WriteLine myObject Name      Writes  Dog            Bar ref myObject           Console WriteLine myObject Name      Writes  Cat               public static void Bar MyClass someObject                MyClass myTempObject   new MyClass            myTempObject Name    Cat           someObject   myTempObject             public static void Bar ref MyClass someObject                MyClass myTempObject   new MyClass            myTempObject Name    Cat           someObject   myTempObject

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If you want to pass your parameter as a ref then you should initialize it before passing parameter to the function else compiler itself will show the error But in case of out parameter you don t need to initialize the object parameter before passing it to the method You can initialize the object in the calling method itself

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If you want to pass your parameter as a ref then you should initialize it before passing parameter to the function else compiler itself will show the error But in case of out parameter you don t need to initialize the object parameter before passing it to the method You can initialize the object in the calling method itself

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ref tells the compiler that the object is initialized before entering the function  while out tells the compiler that the object will be initialized inside the function    So while ref is two-ways  out is out-only

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Out   A return statement can be used for returning only one value from a function  However  using output parameters  you can return two values from a function  Output parameters are like reference parameters  except that they transfer data out of the method rather than into it   The following example illustrates this   using System   namespace CalculatorApplication      class NumberManipulator            public void getValue out int x                    int temp   5           x   temp                 static void Main string   args                   NumberManipulator n   new NumberManipulator                local variable definition             int a   100            Console WriteLine  Before method call  value of a    0    a                calling a function to get the value             n getValue out a             Console WriteLine  After method call  value of a    0    a            Console ReadLine                      ref  A reference parameter is a reference to a memory location of a variable  When you pass parameters by reference  unlike value parameters  a new storage location is not created for these parameters  The reference parameters represent the same memory location as the actual parameters that are supplied to the method   In C   you declare the reference parameters using the ref keyword  The following example demonstrates this   using System  namespace CalculatorApplication      class NumberManipulator            public void swap ref int x  ref int y                   int temp            temp   x     save the value of x             x   y       put y into x             y   temp     put temp into y                    static void Main string   args                   NumberManipulator n   new NumberManipulator                local variable definition             int a   100           int b   200            Console WriteLine  Before swap  value of a    0    a            Console WriteLine  Before swap  value of b    0    b                calling a function to swap the values             n swap ref a  ref b             Console WriteLine  After swap  value of a    0    a            Console WriteLine  After swap  value of b    0    b             Console ReadLine

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I may not be so good at this  but surely strings  even though they are technically reference types and live on the heap  are passed by value  not reference           string a    Hello            string b    goodbye            b   a    attempt to make b point to a  won t work           a    testing            Console WriteLine b     this will produce  hello   NOT  testing        This why you need ref if you want changes to exist outside of the scope of the function making them  you aren t passing a reference otherwise   As far as I am aware you only need ref for structs value types and string itself  as string is a reference type that pretends it is but is not a value type   I could be completely wrong here though  I am new

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ref  and out work just  like passing by references and passing by pointers as in C      For ref  the argument must declared and initialized   For out  the argument must declared but  may or may not be initialized          double nbr   6     if not initialized we get error         double dd   doit square ref nbr            double Half nbr      fine as passed by out  but inside the calling  method you initialize it         doit math routines nbr  out Half nbr

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public static void Main string   args                  int a 10            change ref a             Console WriteLine a              Console Read             int b          change2 out b           Console WriteLine b           Console Read                 static void change ref int a                    a   20                static void change2 out int b                  b   20           you can check this code it will describe you its complete differnce when you use  ref  its mean that u already initialize that int string  but  when you use  out  it works in both conditions wheather u initialize that int string or not but u must initialize that int string in that function

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To illustrate the many excellent explanations  I developed the following console app   using System  using System Collections Generic   namespace CSharpDemos     class Program         static void Main string   args              List lt string gt  StringList   new List lt string gt     Hello           List lt string gt  StringListRef   new List lt string gt     Hallo            AppendWorld StringList         Console WriteLine StringList 0    StringList 1           HalloWelt ref StringListRef         Console WriteLine StringListRef 0    StringListRef 1           CiaoMondo out List lt string gt  StringListOut         Console WriteLine StringListOut 0    StringListOut 1               static void AppendWorld List lt string gt  LiStri              LiStri Add   World           LiStri   new List lt string gt       Hola     Mundo            Console WriteLine LiStri 0    LiStri 1               static void HalloWelt ref List lt string gt  LiStriRef         LiStriRef   new List lt string gt    LiStriRef 0     Welt             static void CiaoMondo out List lt string gt  LiStriOut         LiStriOut   new List lt string gt     Ciao     Mondo                 Output    Hola Mundo  Hello World  Hallo Welt  Ciao Mondo        AppendWorld  A copy of StringList named LiStri is passed  At the start of the method  this copy references the original list and therefore can be used to modify this list  Later LiStri references another List lt string gt  object inside the method which doesn t affect the original list  HalloWelt  LiStriRef is an alias of the already initialized ListStringRef  The passed List lt string gt  object is used to initialize a new one  therefore ref was necessary  CiaoMondo  LiStriOut is an alias of ListStringOut and must be initialized    So  if a method just modifies the object referenced by the passed variable  the compiler will not let you use out and you should not use ref because it would confuse not the compiler but the reader of the code  If the method will make the passed argument reference another object  use ref for an already initialized object and out for methods that must initialize a new object for the passed argument  Besides that  ref and out behave the same

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Since you re passing in a reference type  a class  there is no need use ref because per default only a reference to the actual object is passed and therefore you always change the object behind the reference   Example   public void Foo         MyClass myObject   new MyClass        myObject Name    Dog       Bar myObject       Console WriteLine myObject Name      Writes  Cat      public void Bar MyClass someObject        someObject Name    Cat       As long you pass in a class you don t have to use ref if you want to change the object inside your method

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I may not be so good at this  but surely strings  even though they are technically reference types and live on the heap  are passed by value  not reference           string a    Hello            string b    goodbye            b   a    attempt to make b point to a  won t work           a    testing            Console WriteLine b     this will produce  hello   NOT  testing        This why you need ref if you want changes to exist outside of the scope of the function making them  you aren t passing a reference otherwise   As far as I am aware you only need ref for structs value types and string itself  as string is a reference type that pretends it is but is not a value type   I could be completely wrong here though  I am new

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ref is in and out   You should use out in preference wherever it suffices for your requirements

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Below I have shown an example using both Ref and out  Now  you all will be cleared about ref and out    In below mentioned example when i comment   myRefObj   new myClass   Name    ref outside called        line  will get an error saying  Use of unassigned local variable  myRefObj    but there is no such error in out   Where to use Ref  when we are calling a procedure with an in parameter and the same parameter will be used to store the output of that proc   Where to use Out  when we are calling a procedure with no in parameter and teh same param will be used to return the value from that proc  Also note the output  public partial class refAndOutUse   System Web UI Page       protected void Page Load object sender  EventArgs e                myClass myRefObj          myRefObj   new myClass   Name    ref outside called     lt br  gt              myRefFunction ref myRefObj           Response Write myRefObj Name     ref inside function          myClass myOutObj          myOutFunction out myOutObj           Response Write myOutObj Name     out inside function            void myRefFunction ref myClass refObj                refObj Name    ref inside function  lt br  gt            Response Write refObj Name     ref inside function           void myOutFunction out myClass outObj                outObj   new myClass   Name    out inside function  lt br  gt               Response Write outObj Name     out inside function          public class myClass       public string Name   get  set

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Baker   That s because the first one changes your string-reference to point to  Baker   Changing the reference is possible because you passed it via the ref keyword     a reference to a reference to a string   The Second call gets a copy of the reference to the string   string looks some kind of special at first  But string is just a reference class and if you define   string s    Able     then s is a reference to a string class that contains the text  Able   Another assignment to the same variable via  s    Baker     does not change the original string but just creates a new instance and let s point to that instance   You can try it with the following little code example   string s    Able   string s2   s  s    Baker   Console WriteLine s2     What do you expect  What you will get is still  Able  because you just set the reference in s to another instance while s2 points to the original instance   EDIT  string is also immutable which means there is simply no method or property that modifies an existing string instance  you can try to find one in the docs but you won t fins any  -     All string manipulation methods return a new string instance   That s why you often get a better performance when using the StringBuilder class

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ref means that the value in the ref parameter is already set  the method can read and modify it  Using the ref keyword is the same as saying that the caller is responsible for initializing the value of the parameter     out tells the compiler that the initialization of object is the responsibility of the function  the function has to assign to the out parameter   It s not allowed to leave it unassigned

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For those looking concise answer      Both ref and out keywords are used to pass-by-reference              A variable of ref keyword must have a value or must refer to an object   or null before its passing             Unlike ref  a variable of out keyword must have a value or must   refer to an object or null after its passing as well as no need   to have a value or refer to an object before passing

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ref and out behave similarly except following differences    ref variable must be initialized before use  out variable can be used without assignment out parameter must be treated as an unassigned value by the function that uses it  So  we can use initialized out parameter in the calling code  but the value will be lost when the function executes

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Authoring Time   1  We create the calling method Main    2  it creates a List object  which is a reference type object  and stores it in the variable myList  public sealed class Program        public static Main                  List lt int gt  myList   new List lt int gt      During Runtime   3  Runtime allocates a memory on stack at  00  wide enough to store an address   00   myList  since variable names are really just aliases for memory locations   4  Runtime creates a list object on heap at memory location  FF  all these addresses are for example sakes   5  Runtime would then store the starting address  FF of the object at  00 or in words  stores the reference of the List object in the pointer myList  Back to Authoring Time   6  We then pass the List object as argument myParamList to the called method modifyMyList and assign a new List object to it List lt int gt  myList   new List lt int gt      List lt int gt  newList   ModifyMyList myList   public List lt int gt  ModifyMyList List lt int gt  myParamList        myParamList   new List lt int gt          return myParamList     During Runtime   7  Runtime starts the call routine for the called method and as part of it  checks the type of parameters   8  Upon finding the reference type  it allocates a memory on stack at  04 for aliasing the parameter variable myParamList   9  It then stores the value  FF in it as well   10  Runtime creates a list object on heap at memory location  004 and replaces  FF in  04 with this value or dereferenced the original List object and pointed to the new List object in this method  The address in  00 is not altered and retains the reference to  FF or the original myList pointer is not disturbed    The ref keyword is a compiler directive to skip the generation of runtime code for  8  and  9  which means there will be no heap allocation for method parameters  It will use the original  00 pointer to operate on the object at  FF  If the original pointer is not initialised  the runtime will halt complaining it can   t proceed since the variable isn   t initialised The out keyword is a compiler directive which pretty much is the same as ref with a slight modification at  9  and  10   The compiler expects the argument to be uninitialised and will continue with  8    4  and  5  to create an object on heap and to stores its starting address in the argument variable  No uninitialised error will be thrown and any previous reference stored will be lost

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To illustrate the many excellent explanations  I developed the following console app   using System  using System Collections Generic   namespace CSharpDemos     class Program         static void Main string   args              List lt string gt  StringList   new List lt string gt     Hello           List lt string gt  StringListRef   new List lt string gt     Hallo            AppendWorld StringList         Console WriteLine StringList 0    StringList 1           HalloWelt ref StringListRef         Console WriteLine StringListRef 0    StringListRef 1           CiaoMondo out List lt string gt  StringListOut         Console WriteLine StringListOut 0    StringListOut 1               static void AppendWorld List lt string gt  LiStri              LiStri Add   World           LiStri   new List lt string gt       Hola     Mundo            Console WriteLine LiStri 0    LiStri 1               static void HalloWelt ref List lt string gt  LiStriRef         LiStriRef   new List lt string gt    LiStriRef 0     Welt             static void CiaoMondo out List lt string gt  LiStriOut         LiStriOut   new List lt string gt     Ciao     Mondo                 Output    Hola Mundo  Hello World  Hallo Welt  Ciao Mondo        AppendWorld  A copy of StringList named LiStri is passed  At the start of the method  this copy references the original list and therefore can be used to modify this list  Later LiStri references another List lt string gt  object inside the method which doesn t affect the original list  HalloWelt  LiStriRef is an alias of the already initialized ListStringRef  The passed List lt string gt  object is used to initialize a new one  therefore ref was necessary  CiaoMondo  LiStriOut is an alias of ListStringOut and must be initialized    So  if a method just modifies the object referenced by the passed variable  the compiler will not let you use out and you should not use ref because it would confuse not the compiler but the reader of the code  If the method will make the passed argument reference another object  use ref for an already initialized object and out for methods that must initialize a new object for the passed argument  Besides that  ref and out behave the same

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They re pretty much the same - the only difference is that a variable you pass as an out parameter doesn t need to be initialised  and the method using the ref parameter has to set it to something   int x     Foo out x      OK  int y     Foo ref y      Error   Ref parameters are for data that might be modified  out parameters are for data that s an additional output for the function  eg int TryParse  that are already using the return value for something

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Authoring Time   1  We create the calling method Main    2  it creates a List object  which is a reference type object  and stores it in the variable myList  public sealed class Program        public static Main                  List lt int gt  myList   new List lt int gt      During Runtime   3  Runtime allocates a memory on stack at  00  wide enough to store an address   00   myList  since variable names are really just aliases for memory locations   4  Runtime creates a list object on heap at memory location  FF  all these addresses are for example sakes   5  Runtime would then store the starting address  FF of the object at  00 or in words  stores the reference of the List object in the pointer myList  Back to Authoring Time   6  We then pass the List object as argument myParamList to the called method modifyMyList and assign a new List object to it List lt int gt  myList   new List lt int gt      List lt int gt  newList   ModifyMyList myList   public List lt int gt  ModifyMyList List lt int gt  myParamList        myParamList   new List lt int gt          return myParamList     During Runtime   7  Runtime starts the call routine for the called method and as part of it  checks the type of parameters   8  Upon finding the reference type  it allocates a memory on stack at  04 for aliasing the parameter variable myParamList   9  It then stores the value  FF in it as well   10  Runtime creates a list object on heap at memory location  004 and replaces  FF in  04 with this value or dereferenced the original List object and pointed to the new List object in this method  The address in  00 is not altered and retains the reference to  FF or the original myList pointer is not disturbed    The ref keyword is a compiler directive to skip the generation of runtime code for  8  and  9  which means there will be no heap allocation for method parameters  It will use the original  00 pointer to operate on the object at  FF  If the original pointer is not initialised  the runtime will halt complaining it can   t proceed since the variable isn   t initialised The out keyword is a compiler directive which pretty much is the same as ref with a slight modification at  9  and  10   The compiler expects the argument to be uninitialised and will continue with  8    4  and  5  to create an object on heap and to stores its starting address in the argument variable  No uninitialised error will be thrown and any previous reference stored will be lost

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ref tells the compiler that the object is initialized before entering the function  while out tells the compiler that the object will be initialized inside the function    So while ref is two-ways  out is out-only

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They re pretty much the same - the only difference is that a variable you pass as an out parameter doesn t need to be initialised  and the method using the ref parameter has to set it to something   int x     Foo out x      OK  int y     Foo ref y      Error   Ref parameters are for data that might be modified  out parameters are for data that s an additional output for the function  eg int TryParse  that are already using the return value for something

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Since you re passing in a reference type  a class  there is no need use ref because per default only a reference to the actual object is passed and therefore you always change the object behind the reference   Example   public void Foo         MyClass myObject   new MyClass        myObject Name    Dog       Bar myObject       Console WriteLine myObject Name      Writes  Cat      public void Bar MyClass someObject        someObject Name    Cat       As long you pass in a class you don t have to use ref if you want to change the object inside your method

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ref tells the compiler that the object is initialized before entering the function  while out tells the compiler that the object will be initialized inside the function    So while ref is two-ways  out is out-only

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Mind well that the reference parameter which is passed inside the function is directly worked on   For example       public class MyClass               public string Name   get  set               public void Foo                 MyClass myObject   new MyClass            myObject Name    Dog           Bar myObject           Console WriteLine myObject Name      Writes  Dog              public void Bar MyClass someObject                MyClass myTempObject   new MyClass            myTempObject Name    Cat           someObject   myTempObject          This will write Dog  not Cat  Hence you should directly work on someObject

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ref is in and out   You should use out in preference wherever it suffices for your requirements

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The ref modifier means that    The value is already set and  The method can read and modify it    The out modifier means that    The Value isn t set and can t be read by the method until it is set  The method must set it before returning

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ref and out behave similarly except following differences    ref variable must be initialized before use  out variable can be used without assignment out parameter must be treated as an unassigned value by the function that uses it  So  we can use initialized out parameter in the calling code  but the value will be lost when the function executes

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out   In C   a method can return only one value  If you like to return more than one value  you can use the out keyword  The out modifier return as return-by-reference  The simplest answer is that the keyword    out    is used to get the value from the method    You don t need to initialize the value in the calling function  You must assign the value in the called function  otherwise the compiler will report an error    ref   In C   when you pass a value type such as int  float  double etc  as an argument to the method parameter  it is passed by value  Therefore  if you modify the parameter value  it does not affect argument in the method call  But if you mark the parameter with    ref    keyword  it will reflect in the actual variable     You need to initialize the variable before you call the function  It   s not mandatory to assign any value to the ref parameter in the method  If you don   t change the value  what is the need to mark it as    ref

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ref means that the value in the ref parameter is already set  the method can read and modify it  Using the ref keyword is the same as saying that the caller is responsible for initializing the value of the parameter     out tells the compiler that the initialization of object is the responsibility of the function  the function has to assign to the out parameter   It s not allowed to leave it unassigned

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From the standpoint of a method which receives a parameter  the difference between ref and out is that C  requires that methods must write to every out parameter before returning  and must not do anything with such a parameter  other than passing it as an out parameter or writing to it  until it has been either passed as an out parameter to another method or written directly   Note that some other languages do not impose such requirements  a virtual or interface method which is declared in C  with an out parameter may be overridden in another language which does not impose any special restrictions on such parameters   From the standpoint of the caller  C  will in many circumstances assume when calling a method with an out parameter will cause the passed variable to be written without having been read first   This assumption may not be correct when calling methods written in other languages   For example   struct MyStruct             myStruct IDictionary lt int  MyStruct gt  d            d TryGetValue 23  out this            If myDictionary identifies an IDictionary lt TKey TValue gt  implementation written in a language other than C   even though MyStruct s   new MyStruct myDictionary   looks like an assignment  it could potentially leave s unmodified   Note that constructors written in VB NET  unlike those in C   make no assumptions about whether called methods will modify any out parameters  and clear out all fields unconditionally   The odd behavior alluded to above won t occur with code written entirely in VB or entirely in C   but can occur when code written in C  calls a method written in VB NET

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Ref  The ref keyword is used to pass an argument as a reference  This means that when value of that parameter is changed in the method  it gets reflected in the calling method  An argument that is passed using a ref keyword must be initialized in the calling method before it is passed to the called method   Out  The out keyword is also used to pass an argument like ref keyword  but the argument can be passed without assigning any value to it  An argument that is passed using an out keyword must be initialized in the called method before it returns back to calling method   public class Example    public static void Main        int val1   0    must be initialized   int val2    optional   Example1 ref val1    Console WriteLine val1      Example2 out val2    Console WriteLine val2         static void Example1 ref int value       value   1      static void Example2 out int value       value   2            Output     1     2        Ref and out in method overloading  Both ref and out cannot be used in method overloading simultaneously  However  ref and out are treated differently at run-time but they are treated same at compile time  CLR doesn t differentiates between the two while it created IL for ref and out

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As well as allowing you to reassign someone else s variable to a different instance of a class  return multiple values etc  using ref or out lets someone else know what you need from them and what you intend to do with the variable they provide   You don t need ref or out if all you re going to do is modify things inside the MyClass instance that is passed in the argument someClass     The calling method will see changes like someClass Message    Hello World  whether you use ref  out or nothing Writing someClass   new MyClass   inside myFunction someClass  swaps out the object seen by the someClass in the scope of the myFunction method only  The calling method still knows about the original MyClass instance it created and passed to your method  You need ref or out if you plan on swapping the someClass out for a whole new object and want the calling method to see your change   Writing someClass   new MyClass   inside myFunction out someClass  changes the object seen by the method that called myFunction    Other programmers exist  And they want to know what you re going to do with their data  Imagine you re writing a library that will be used by millions of developers  You want them to know what you re going to do with their variables when they call your methods    Using ref makes a statement of  Pass a variable assigned to some value when you call my method  Be aware that I might change it out for something else entirely during the course of my method  Do not expect your variable to be pointing to the old object when I m done  Using out makes a statement of  Pass a placeholder variable to my method  It doesn t matter whether it has a value or not  the compiler will force me to assign it to a new value  I absolutely guarantee that the object pointed to by your variable before you called my method  will be different by the time I m done   By the way  in C 7 2 there s an in modifier too  And that prevents the method from swapping out the passed in instance for a different instance  Think of it like saying to those millions of developers  pass me your original variable reference  and I promise not to swap your carefully crafted data out for something else   in has some peculiarities  and in some cases such as where an implicit conversion might be required to make your short compatible with an in int the compiler will temporarily make an int  widen your short to it  pass it by reference and finish up  It can do this because you ve declared you re not going to mess with it     Microsoft did this with the  TryParse methods on the numeric types   int i   98234957  bool success   int TryParse  123   out i     By flagging the parameter as out they re actively declaring here  we are definitely going to change your painstakingly crafted value of 98234957 out for something else   Of course  they kinda have to  for things like parsing value types because if the parse method wasn t allowed to swap out the value type for something else it wouldn t work very well   But imagine there was some fictitious method in some library you re creating   public void PoorlyNamedMethod out SomeClass x    You can see it s an out  and you can thus know that if you spend hours crunching numbers  creating the perfect SomeClass   SomeClass x   SpendHoursMakingMeAPerfectSomeClass      now give it to the library PoorlyNamedMethod out x     Well that was a waste of time  taking all those hours to make that perfect class  It s definitely going to be tossed away and replaced by PoorlyNamedMethod

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For those that learn by example  like me  here s what Anthony Kolesov is saying   I ve created some minimal examples of ref  out  and others to illustrate the point  I m not covering best practices  just examples to understand the differences   https   gist github com 2upmedia 6d98a57b68d849ee7091

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ref is in and out   You should use out in preference wherever it suffices for your requirements

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out   In C   a method can return only one value  If you like to return more than one value  you can use the out keyword  The out modifier return as return-by-reference  The simplest answer is that the keyword    out    is used to get the value from the method    You don t need to initialize the value in the calling function  You must assign the value in the called function  otherwise the compiler will report an error    ref   In C   when you pass a value type such as int  float  double etc  as an argument to the method parameter  it is passed by value  Therefore  if you modify the parameter value  it does not affect argument in the method call  But if you mark the parameter with    ref    keyword  it will reflect in the actual variable     You need to initialize the variable before you call the function  It   s not mandatory to assign any value to the ref parameter in the method  If you don   t change the value  what is the need to mark it as    ref

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ref  and out work just  like passing by references and passing by pointers as in C      For ref  the argument must declared and initialized   For out  the argument must declared but  may or may not be initialized          double nbr   6     if not initialized we get error         double dd   doit square ref nbr            double Half nbr      fine as passed by out  but inside the calling  method you initialize it         doit math routines nbr  out Half nbr

[c#] What's the difference between the 'ref' and 'out' keywords?

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