When to use in vs ref vs out

Question

Someone asked me the other day when they should use the parameter keyword out instead of ref  While I  I think  understand the difference between the ref and out keywords  that has been asked before  and the best explanation seems to be that ref    in and out  what are some  hypothetical or code  examples where I should always use out and not ref    Since ref is more general  why do you ever want to use out  Is it just syntactic sugar

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Extra notes regarding C  7  In C  7 there s no need to predeclare variables using out  So a code like this   public void PrintCoordinates Point p      int x  y     have to  predeclare    p GetCoordinates out x  out y     WriteLine     x    y          Can be written like this   public void PrintCoordinates Point p      p GetCoordinates out int x  out int y     WriteLine     x    y          Source   What s new in C  7

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You re correct in that  semantically  ref provides both  in  and  out  functionality  whereas out only provides  out  functionality  There are some things to consider    out requires that the method accepting the parameter MUST  at some point before returning  assign a value to the variable  You find this pattern in some of the key value data storage classes like Dictionary lt K V gt   where you have functions like TryGetValue  This function takes an out parameter that holds what the value will be if retrieved  It wouldn t make sense for the caller to pass a value into this function  so out is used to guarantee that some value will be in the variable after the call  even if it isn t  real  data  in the case of TryGetValue where the key isn t present   out and ref parameters are marshaled differently when dealing with interop code   Also  as an aside  it s important to note that while reference types and value types differ in the nature of their value  every variable in your application points to a location of memory that holds a value  even for reference types  It just happens that  with reference types  the value contained in that location of memory is another memory location  When you pass values to a function  or do any other variable assignment   the value of that variable is copied into the other variable  For value types  that means that the entire content of the type is copied  For reference types  that means that the memory location is copied  Either way  it does create a copy of the data contained in the variable  The only real relevance that this holds deals with assignment semantics  when assigning a variable or passing by value  the default   when a new assignment is made to the original  or new  variable  it does not affect the other variable  In the case of reference types  yes  changes made to the instance are available on both sides  but that s because the actual variable is just a pointer to another memory location  the content of the variable--the memory location--didn t actually change   Passing with the ref keyword says that both the original variable and the function parameter will actually point to the same memory location  This  again  affects only assignment semantics  If a new value is assigned to one of the variables  then because the other points to the same memory location the new value will be reflected on the other side

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It depends on the compile context  See Example below     out and ref both denote variable passing by reference  yet ref requires the variable to be initialized before being passed  which can be an important difference in the context of Marshaling  Interop  UmanagedToManagedTransition or vice versa    MSDN warns      Do not confuse the concept of passing by reference with the concept of reference types  The two concepts are not the same  A method parameter can be modified by ref regardless of whether it is a value type or a reference type  There is no boxing of a value type when it is passed by reference    From the official MSDN Docs    out       The out keyword causes arguments to be passed by reference  This is similar to the ref keyword  except that ref requires that the variable be initialized before being passed    ref       The ref keyword causes an argument to be passed by reference  not by value  The effect of passing by reference is that any change to the parameter in the method is reflected in the underlying argument variable in the calling method  The value of a reference parameter is always the same as the value of the underlying argument variable    We can verify that the out and ref are indeed the same when the argument gets assigned   CIL Example   Consider the following example  static class outRefTest      public static int myfunc int x  x 0  return x        public static void myfuncOut out int x  x 0       public static void myfuncRef ref int x  x 0       public static void myfuncRefEmpty ref int x           Define other methods and classes here     in CIL  the instructions of myfuncOut and myfuncRef are identical as expected   outRefTest myfunc  IL 0000   nop          IL 0001   ldc i4 0     IL 0002   starg s     00  IL 0004   ldarg 0      IL 0005   stloc 0      IL 0006   br s        IL 0008 IL 0008   ldloc 0      IL 0009   ret           outRefTest myfuncOut  IL 0000   nop          IL 0001   ldarg 0      IL 0002   ldc i4 0     IL 0003   stind i4     IL 0004   ret           outRefTest myfuncRef  IL 0000   nop          IL 0001   ldarg 0      IL 0002   ldc i4 0     IL 0003   stind i4     IL 0004   ret           outRefTest myfuncRefEmpty  IL 0000   nop          IL 0001   ret            nop  no operation  ldloc  load local  stloc  stack local  ldarg  load argument  bs s  branch to target       See  List of CIL instructions

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Below are some notes which i pulled from this codeproject article on C  Out Vs Ref   It should be used only when we are expecting multiple outputs from a function or a method  A thought on structures can be also a good option for the same  REF and OUT are keywords which dictate how data is passed from caller to callee and vice versa  In REF data passes two way  From caller to callee and vice-versa     In Out data passes only one way from callee to caller  In this case if Caller tried to send data to the callee it will be overlooked   rejected    If you are a visual person then please see this yourtube video which demonstrates the difference practically https   www youtube com watch v lYdcY5zulXA  Below image shows the differences more visually

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How it sounds   out   only initialize fill a parameter  the parameter must be empty  return it out plain  ref   reference  standard parameter  maybe with value   but the function can modifiy it

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out is more constraint version of ref   In a method body  you need to assign to all out parameters before leaving the method  Also an values assigned to an out parameter is ignored  whereas ref requires them to be assigned   So out allows you to do   int a  b  c   foo out a  out b     where ref would require a and b to be assigned

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You need to use ref if you plan to read and write to the parameter  You need to use out if you only plan to write  In effect  out is for when you d need more than one return value  or when you don t want to use the normal return mechanism for output  but this should be rare    There are language mechanics that assist these use cases  Ref parameters must have been initialized before they are passed to a method  putting emphasis on the fact that they are read-write   and out parameters cannot be read before they are assigned a value  and are guaranteed to have been written to at the end of the method  putting emphasis on the fact that they are write only   Contravening to these principles results in a compile-time error   int x  Foo ref x      error  x is uninitialized  void Bar out int x         error  x was not written to   For instance  int TryParse returns a bool and accepts an out int parameter   int value  if  int TryParse numericString  out value            numericString was parsed into value  now do stuff      else          numericString couldn t be parsed        This is a clear example of a situation where you need to output two values  the numeric result and whether the conversion was successful or not  The authors of the CLR decided to opt for out here since they don t care about what the int could have been before   For ref  you can look at Interlocked Increment   int x   4  Interlocked Increment ref x     Interlocked Increment atomically increments the value of x  Since you need to read x to increment it  this is a situation where ref is more appropriate  You totally care about what x was before it was passed to Increment   In the next version of C   it will even be possible to declare variable in out parameters  adding even more emphasis on their output-only nature   if  int TryParse numericString  out int value             value  exists and was declared in the  if  statement   else          conversion didn t work   value  doesn t exist here

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An argument passed as ref must be initialized before passing to the method whereas out parameter needs not to be initialized before passing to a method

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How to use in or out or ref in C     All keywords in C  have the same functionality but with some boundaries  in arguments cannot be modified by the called method  ref arguments may be modified  ref must be initialized before being used by caller it can be read and updated in the method  out arguments must be modified by the caller  out arguments must be initialized in the method Variables passed as in arguments must be initialized before being passed in a method call  However  the called method may not assign a value or modify the argument    You can t use the in  ref  and out keywords for the following kinds of methods    Async methods  which you define by using the async modifier  Iterator methods  which include a yield return or yield break statement

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Basically both ref and out for passing object value between methods  The out keyword causes arguments to be passed by reference  This is like the ref keyword  except that ref requires that the variable be initialized before it is passed   out   Argument is not initialized and it must be initialized in the method  ref   Argument is already initialized and it can be read and updated in the method   What is the use of    ref    for reference-types    You can change the given reference to a different instance   Did you know    Although the ref and out keywords cause different run-time behavior  they are not considered part of the method signature at compile time  Therefore  methods cannot be overloaded if the only difference is that one method takes a ref argument and the other takes an out argument  You can t use the ref and out keywords for the following kinds of methods    Async methods  which you define by using the async modifier  Iterator methods  which include a yield return or yield break statement   Properties are not variables and therefore cannot be passed as out parameters

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You should use out unless you need ref   It makes a big difference when the data needs to be marshalled e g  to another process  which can be costly  So you want to avoid marshalling the initial value when the method doesn t make use of it   Beyond that  it also shows the reader of the declaration or the call whether the initial value is relevant  and potentially preserved   or thrown away   As a minor difference  an out parameter needs not be initialized   Example for out   string a  b  person GetBothNames out a  out b     where GetBothNames is a method to retrieve two values atomically  the method won t change behavior whatever a and b are  If the call goes to a server in Hawaii  copying the initial values from here to Hawaii is a waste of bandwidth  A similar snippet using ref   string a   String Empty  b   String Empty  person GetBothNames ref a  ref b     could confuse readers  because it looks like the initial values of a and b are relevant  though the method name would indicate they are not    Example for ref   string name   textbox Text  bool didModify   validator SuggestValidName ref name     Here the initial value is relevant to the method

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You can use the out contextual keyword in two contexts  each is a link to detailed information   as a parameter modifier or in generic type parameter declarations in interfaces and delegates  This topic discusses the parameter modifier  but you can see this other topic for information on the generic type parameter declarations   The out keyword causes arguments to be passed by reference  This is like the ref keyword  except that ref requires that the variable be initialized before it is passed  To use an out parameter  both the method definition and the calling method must explicitly use the out keyword  For example  C   class OutExample       static void Method out int i                i   44            static void Main                 int value          Method out value              value is now 44           Although variables passed as out arguments do not have to be initialized before being passed  the called method is required to assign a value before the method returns   Although the ref and out keywords cause different run-time behavior  they are not considered part of the method signature at compile time  Therefore  methods cannot be overloaded if the only difference is that one method takes a ref argument and the other takes an out argument  The following code  for example  will not compile  C   class CS0663 Example          Compiler error CS0663   Cannot define overloaded         methods that differ only on ref and out       public void SampleMethod out int i          public void SampleMethod ref int i          Overloading can be done  however  if one method takes a ref or out argument and the other uses neither  like this  C   class OutOverloadExample       public void SampleMethod int i          public void SampleMethod out int i    i   5        Properties are not variables and therefore cannot be passed as out parameters   For information about passing arrays  see Passing Arrays Using ref and out  C  Programming Guide    You can t use the ref and out keywords for the following kinds of methods   Async methods  which you define by using the async modifier   Iterator methods  which include a yield return or yield break statement    Example  Declaring an out method is useful when you want a method to return multiple values  The following example uses out to return three variables with a single method call  Note that the third argument is assigned to null  This enables methods to return values optionally  C   class OutReturnExample       static void Method out int i  out string s1  out string s2                i   44          s1    I ve been returned           s2   null            static void Main                 int value          string str1  str2          Method out value  out str1  out str2              value is now 44            str1 is now  I ve been returned             str2 is  still  null

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why do you ever want to use out    To let others know that the variable will be initialized when it returns from the called method    As mentioned above   for an out parameter  the calling method is required to assign a value before the method returns    example   Car car  SetUpCar out car   car drive        You know car is initialized

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It should be noted that in is a valid keyword as of C  ver 7 2   The in parameter modifier is available in C  7 2 and later  Previous versions generate compiler error CS8107   quot Feature  readonly references  is not available in C  7 0  Please use language version 7 2 or greater  quot   To configure the compiler language version  see Select the C  language version      The in keyword causes arguments to be passed by reference  It makes the formal parameter an alias for the argument  which must be a variable  In other words  any operation on the parameter is made on the argument  It is like the ref or out keywords  except that in arguments cannot be modified by the called method  Whereas ref arguments may be modified  out arguments must be modified by the called method  and those modifications are observable in the calling context

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Still feel the need for a good summary  this is what I came up with   Summary   When we are inside the function  this is how we specify the variable data access control   in   R  out   must W before R  ref   R W    Explanation   in    Function may only READ that variable     out   Variable must not be initialised first because  function MUST WRITE to it before READ   ref   Function may READ WRITE to that variable         Why is it named as such   Focusing on where data gets modified   in    Data must only be set before entering  in  function     out     Data must only be set before leaving  out  function     ref    Data must be set before entering  in  function  Data may be set before leaving  out  function

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Just to clarify on OP s comment that the use on ref and out is a  reference to a value type or struct declared outside the method   which has already been established in incorrect    Consider the use of ref on a StringBuilder  which is a reference type   private void Nullify StringBuilder sb  string message        sb Append message       sb   null        -- snip --  StringBuilder sb   new StringBuilder    string message    Hi Guy   Nullify sb  message   System Console WriteLine sb ToString         Output    Hi Guy   As apposed to this   private void Nullify ref StringBuilder sb  string message        sb Append message       sb   null        -- snip --  StringBuilder sb   new StringBuilder    string message    Hi Guy   Nullify ref sb  message   System Console WriteLine sb ToString         Output    NullReferenceException

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Use out to denote that the parameter is not being used  only set   This helps the caller understand that you re always initializing the parameter   Also  ref and out are not just for value types   They also let you reset the object that a reference type is referencing from within a method

[c#] When to use in vs ref vs out

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