List passed by ref - help me explain this behaviour

Question

Take a look at the following program   class Test       List lt int gt  myList   new List lt int gt          public void TestMethod                 myList Add 100           myList Add 50           myList Add 10            ChangeList myList            foreach  int i in myList                        Console WriteLine i                        private void ChangeList List lt int gt  myList                myList Sort             List lt int gt  myList2   new List lt int gt             myList2 Add 3           myList2 Add 4            myList   myList2            I assumed myList would have passed by ref  and the output would   3 4   The list is indeed  passed by ref   but only the sort function takes effect  The following statement myList   myList2  has no effect   So the output is in fact   10 50 100   Can you help me explain this behavior  If indeed myList is not passed-by-ref  as it appears from myList   myList2 not taking effect   how does myList Sort   take effect    I was assuming even that statement to not take effect and the output to be   100 50 10

User · Answer

Initially  it can be represented graphically as follow     Then  the sort is applied myList Sort      Finally  when you did  myList    myList2  you lost the one of the reference but not the original and the collection stayed sorted     If you use by reference  ref  then myList  and myList will become the same  only one reference    Note  I use myList  to represent the parameter that you use in ChangeList  because you gave the same name as the original

User · Answer

C  just does a shallow copy when it passes by value unless the object in question executes ICloneable  which apparently the List class does not      What this means is that it copies the List itself  but the references to the objects inside the list remain the same  that is  the pointers continue to reference the same objects as the original List     If you change the values of the things your new List references  you change the original List also  since it is referencing the same objects    However  you then change what myList references entirely  to a new List  and now only the original List is referencing those integers     Read the Passing Reference-Type Parameters section from this MSDN article on  Passing Parameters  for more information    How do I Clone a Generic List in C   from StackOverflow talks about how to make a deep copy of a List

User · Answer

This link will help you in understanding pass by reference in C   Basically when an object of reference type is passed by value to an method  only methods which are available on that object can modify the contents of object  For example List sort   method changes List contents but if you assign some other object to same variable  that assignment is local to that method  That is why myList remains unchanged  If we pass object of reference type by using ref keyword then we can assign some other object to same variable and that changes entire object itself   Edit  this is the updated version of the documentation linked above

User · Answer

You are passing a reference to the list  but your aren t passing the list variable by reference - so when you call ChangeList the value of the variable  i e  the reference - think  pointer   is copied - and changes to the value of the parameter inside ChangeList aren t seen by TestMethod   try   private void ChangeList ref List lt int gt  myList            ChangeList ref myList     This then passes a reference to the local-variable myRef  as declared in TestMethod   now  if you reassign the parameter inside ChangeList you are also reassigning the variable inside TestMethod

User · Answer

Here is an easy way to understand it   Your List is an object created on heap  The variable myList is a reference to that object  In C  you never pass objects  you pass their references by value  When you access the list object via the passed reference in ChangeList  while sorting  for example  the original list is changed  The assignment on the ChangeList method is made to the value of the reference  hence no changes are done to the original list  still on the heap but not referenced on the method variable anymore

User · Answer

While I agree with what everyone has said above  I have a different take on this code  Basically you re assigning the new list to the local variable myList not the global  if you change the signature of ChangeList List myList  to private void ChangeList   you ll see the output of 3  4   Here s my reasoning    Even though list is passed by reference  think of it as passing a pointer variable by value When you call ChangeList myList  you re passing the pointer to  Global myList  Now this is stored in the  local myList variable  So now your  local myList and  global myList are pointing to the same list  Now you do a sort    it works because  local myList is referencing the original  global myList Next you create a new list and assign the pointer to that your  local myList  But as soon as the function exits the  local myList variable is destroyed  HTH  class Test       List lt int gt  myList   new List lt int gt         public void TestMethod                  myList Add 100           myList Add 50           myList Add 10            ChangeList             foreach  int i in myList                        Console WriteLine i                        private void ChangeList                 myList Sort             List lt int gt  myList2   new List lt int gt             myList2 Add 3           myList2 Add 4            myList   myList2

User · Answer

Use the ref keyword  Look at the definitive reference here to understand passing parameters  To be specific  look at this  to understand the behavior of the code  EDIT  Sort works on the same reference  that is passed by value  and hence the values are ordered  However  assigning a new instance to the parameter won t work because parameter is passed by value  unless you put ref  Putting ref lets you change the pointer to the reference to a new instance of List in your case  Without ref  you can work on the existing parameter  but can t make it point to something else

User · Answer

There are two parts of memory allocated for an object of reference type  One in stack and one in heap  The part in stack  aka a pointer  contains reference to the part in heap - where the actual values are stored  When ref keyword is not use  just a copy of part in stack is created and passed to the method - reference to same part in heap  Therefore if you change something in heap part  those change will stayed  If you change the copied pointer - by assign it to refer to other place in heap - it will not affect to origin pointer outside of the method

[c#] List passed by ref - help me explain this behaviour

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