C 4 0 optional out ref arguments

Question

Does C  4 0 allow optional out or ref arguments

User · Accepted Answer

As already mentioned  this is simply not allowed and I think it makes a very good sense  However  to add some more details  here is a quote from the C  4 0 Specification  section 21 1      Formal parameters of constructors  methods  indexers and delegate types can be declared optional         fixed-parameter      nbsp   nbsp  attributesopt parameter-modifieropt type identifier default-argumentopt   default-argument      nbsp   nbsp     expression          A fixed-parameter with a default-argument is an optional parameter  whereas a fixed-parameter without a default-argument is a required parameter       A required parameter cannot appear after an optional parameter in a formal-parameter-list      A ref or out parameter cannot have a default-argument

User · Answer

There actually is a way to do this that is allowed by C   This gets back to C    and rather violates the nice Object-Oriented structure of C    USE THIS METHOD WITH CAUTION   Here s the way you declare and write your function with an optional parameter   unsafe public void OptionalOutParameter int  pOutParam   null        int lInteger   5         If the parameter is NULL  the caller doesn t care about this value      if  pOutParam    null                     If it isn t null  the caller has provided the address of an integer           pOutParam   lInteger     Dereference the pointer and assign the return value            Then call the function like this   unsafe   OptionalOutParameter         does nothing int MyInteger   0  unsafe   OptionalOutParameter  amp MyInteger        pass in the address of MyInteger    In order to get this to compile  you will need to enable unsafe code in the project options  This is a really hacky solution that usually shouldn t be used  but if you for some strange  arcane  mysterious  management-inspired decision  REALLY need an optional out parameter in C   then this will allow you to do just that

User · Answer

No  but another great alternative is having the method use a generic template class for optional parameters as follows   public class OptionalOut lt Type gt        public Type Result   get  set        Then you can use it as follows   public string foo string value  OptionalOut lt int gt  outResult   null              do something      if  outResult    null            outResult Result   100             return value     public void bar          string str    bar        string result      OptionalOut lt int gt  optional   new OptionalOut lt int gt              example  call without the optional out parameter     result   foo  str       Console WriteLine   Output was  0  with no optional value used   result           example  call it with optional parameter     result   foo  str  optional       Console WriteLine   Output was  0  with optional value of  1    result  optional Result           example  call it with named optional parameter     foo  str  outResult  optional       Console WriteLine   Output was  0  with optional value of  1    result  optional Result

User · Answer

void foo ref int  n        return null

User · Answer

ICYMI  Included on the new features for C  7 0 enumerated here    discards  is now allowed as out parameters in the form of a    to let you ignore out parameters you don   t care about   p GetCoordinates out var x  out        I only care about x  P S  if you re also confused with the part  out var x   read the new feature about  Out Variables  on the link as well

User · Answer

Use an overloaded method without the out parameter to call the one with the out parameter for C  6 0 and lower  I m not sure why a C  7 0 for  NET Core is even the correct answer for this thread when it was specifically asked if C  4 0 can have an optional out parameter  The answer is NO

User · Answer

What about like this   public bool OptionalOutParamMethod  Optional  ref string pOutParam        return true      You still have to pass a value to the parameter from C  but it is an optional ref param

User · Answer

No  but you can use a delegate  e g  Action  as an alternative   Inspired in part by Robin R s answer when facing a situation where I thought I wanted an optional out parameter  I instead used an Action delegate  I ve borrowed his example code to modify for use of Action lt int gt  in order to show the differences and similarities   public string foo string value  Action lt int gt  outResult   null              do something      outResult  Invoke 100        return value     public void bar          string str    bar        string result      int optional   0          example  call without the optional out parameter     result   foo  str       Console WriteLine   Output was  0  with no optional value used   result           example  call it with optional parameter     result   foo  str  x   gt  optional   x       Console WriteLine   Output was  0  with optional value of  1    result  optional           example  call it with named optional parameter     foo  str  outResult  x   gt  optional   x       Console WriteLine   Output was  0  with optional value of  1    result  optional       This has the advantage that the optional variable appears in the source as a normal int  the compiler wraps it in a closure class  rather than us wrapping it explicitly in a user-defined class    The variable needs explicit initialisation because the compiler cannot assume that the Action will be called before the function call exits   It s not suitable for all use cases  but worked well for my real use case  a function that provides data for a unit test  and where a new unit test needed access to some internal state not present in the return value

User · Answer

No  A workaround is to overload with another method that doesn t have out   ref parameters  and which just calls your current method  public bool SomeMethod out string input                  new overload public bool SomeMethod         string temp      return SomeMethod out temp       If you have C  7 0  you can simplify     new overload public bool SomeMethod         return SomeMethod out           declare out as an inline discard variable     Thanks  Oskar    Reiner for pointing this out

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