Do you need to dispose of objects and set them to null

Question

Do you need to dispose of objects and set them to null  or will the garbage collector clean them up when they go out of scope

User · Answer

Normally  there s no need to set fields to null  I d always recommend disposing unmanaged resources however   From experience I d also advise you to do the following    Unsubscribe from events if you no longer need them  Set any field holding a delegate or an expression to null if it s no longer needed    I ve come across some very hard to find issues that were the direct result of not following the advice above   A good place to do this is in Dispose    but sooner is usually better   In general  if a reference exists to an object the garbage collector  GC  may take a couple of generations longer to figure out that an object is no longer in use  All the while the object remains in memory   That may not be a problem until you find that your app is using a lot more memory than you d expect  When that happens  hook up a memory profiler to see what objects are not being cleaned up  Setting fields referencing other objects to null and clearing collections on disposal can really help the GC figure out what objects it can remove from memory  The GC will reclaim the used memory faster making your app a lot less memory hungry and faster

User · Answer

When an object implements IDisposable you should call Dispose  or Close  in some cases  that will call Dispose for you     You normally do not have to set objects to null  because the GC will know that an object will not be used anymore   There is one exception when I set objects to null  When I retrieve a lot of objects  from the database  that I need to work on  and store them in a collection  or array   When the  work  is done  I set the object to null  because the GC does not know I m finished working with it   Example   using  var db   GetDatabase             Retrieves array of keys     var keys   db GetRecords mySelection         for int i   0  i  lt  keys Length  i             var record   db GetRecord keys i           record DoWork           keys i    null     GC can dispose of key now           The record had gone out of scope automatically             and does not need any special treatment            end using   gt  db Dispose is called

User · Answer

Objects never go out of scope in C  as they do in C    They are dealt with by the Garbage Collector automatically when they are not used anymore  This is a more complicated approach than C   where the scope of a variable is entirely deterministic  CLR garbage collector actively goes through all objects that have been created and works out if they are being used    An object can go  out of scope  in one function but if its value is returned  then GC would look at whether or not the calling function holds onto the return value   Setting object references to null is unnecessary as garbage collection works by working out which objects are being referenced by other objects    In practice  you don t have to worry about destruction  it just works and it s great     Dispose must be called on all objects that implement IDisposable when you are finished working with them  Normally you would use a using block with those objects like so   using  var ms   new MemoryStream                  EDIT On variable scope  Craig has asked whether the variable scope has any effect on the object lifetime  To properly explain that aspect of CLR  I ll need to explain a few concepts from C   and C    Actual variable scope  In both languages the variable can only be used in the same scope as it was defined - class  function or a statement block enclosed by braces  The subtle difference  however  is that in C   variables cannot be redefined in a nested block   In C    this is perfectly legal   int iVal   8    iVal    8 if  iVal    8       int iVal   5        iVal    5     iVal    8   In C   however you get a a compiler error   int iVal   8  if iVal    8        int iVal   5    error CS0136  A local variable named  iVal  cannot be declared in this scope because it would give a different meaning to  iVal   which is already used in a  parent or current  scope to denote something else     This makes sense if you look at generated MSIL - all the variables used by the function are defined at the start of the function  Take a look at this function   public static void Scope         int iVal   8      if iVal    8            int iVal2   5            Below is the generated IL  Note that iVal2  which is defined inside the if block is actually defined at function level  Effectively this means that C  only has class and function level scope as far as variable lifetime is concerned    method public hidebysig static void  Scope   cil managed        Code size       19  0x13     maxstack  2    locals init   0  int32 iVal              1  int32 iVal2              2  bool CS 4 0000     Function IL - omitted      end of method Test2  Scope   C   scope and object lifetime  Whenever a C   variable  allocated on the stack  goes out of scope it gets destructed  Remember that in C   you can create objects on the stack or on the heap  When you create them on the stack  once execution leaves the scope  they get popped off the stack and gets destroyed   if  true      MyClass stackObj    created on the stack   MyClass heapObj   new MyClass      created on the heap   obj doSomething         lt -- stackObj is destroyed   heapObj still lives   When C   objects are created on the heap  they must be explicitly destroyed  otherwise it is a memory leak  No such problem with stack variables though   C  Object Lifetime  In CLR  objects  i e  reference types  are always created on the managed heap  This is further reinforced by object creation syntax  Consider this code snippet   MyClass stackObj    In C   this would create an instance on MyClass on the stack and call its default constructor  In C  it would create a reference to class MyClass that doesn t point to anything  The only way to create an instance of a class is by using new operator   MyClass stackObj   new MyClass      In a way  C  objects are a lot like objects that are created using new syntax in C   - they are created on the heap but unlike C   objects  they are managed by the runtime  so you don t have to worry about destructing them   Since the objects are always on the heap the fact that object references  i e  pointers  go out of scope becomes moot  There are more factors involved in determining if an object is to be collected than simply presence of references to the object   C  Object references  Jon Skeet compared object references in Java to pieces of string that are attached to the balloon  which is the object  Same analogy applies to C  object references  They simply point to a location of the heap that contains the object  Thus  setting it to null has no immediate effect on the object lifetime  the balloon continues to exist  until the GC  pops  it   Continuing down the balloon analogy  it would seem logical that once the balloon has no strings attached to it  it can be destroyed  In fact this is exactly how reference counted objects work in non-managed languages  Except this approach doesn t work for circular references very well  Imagine two balloons that are attached together by a string but neither balloon has a string to anything else  Under simple ref counting rules  they both continue to exist  even though the whole balloon group is  orphaned     NET objects are a lot like helium balloons under a roof  When the roof opens  GC runs  - the unused balloons float away  even though there might be groups of balloons that are tethered together    NET GC uses a combination of generational GC and mark and sweep  Generational approach involves the runtime favouring to inspect objects that have been allocated most recently  as they are more likely to be unused and mark and sweep involves runtime going through the whole object graph and working out if there are object groups that are unused  This adequately deals with circular dependency problem   Also   NET GC runs on another thread so called finalizer thread  as it has quite a bit to do and doing that on the main thread would interrupt your program

User · Answer

You never need to set objects to null in C   The compiler and runtime will take care of figuring out when they are no longer in scope   Yes  you should dispose of objects that implement IDisposable

User · Answer

Always call dispose  It is not worth the risk  Big managed enterprise applications should be treated with respect  No assumptions can be made or else it will come back to bite you   Don t listen to leppie   A lot of objects don t actually implement IDisposable  so you don t have to worry about them  If they genuinely go out of scope they will be freed automatically  Also I have never come across the situation where I have had to set something to null   One thing that can happen is that a lot of objects can be held open  This can greatly increase the memory usage of your application  Sometimes it is hard to work out whether this is actually a memory leak  or whether your application is just doing a lot of stuff   Memory profile tools can help with things like that  but it can be tricky   In addition always unsubscribe from events that are not needed  Also be careful with WPF binding and controls  Not a usual situation  but I came across a situation where I had a WPF control that was being bound to an underlying object  The underlying object was large and took up a large amount of memory  The WPF control was being replaced with a new instance  and the old one was still hanging around for some reason  This caused a large memory leak   In hindsite the code was poorly written  but the point is that you want to make sure that things that are not used go out of scope  That one took a long time to find with a memory profiler as it is hard to know what stuff in memory is valid  and what shouldn t be there

User · Answer

If the object implements IDisposable  then yes  you should dispose it   The object could be hanging on to native resources  file handles  OS objects  that might not be freed immediately otherwise   This can lead to resource starvation  file-locking issues  and other subtle bugs that could otherwise be avoided   See also Implementing a Dispose Method on MSDN

User · Answer

As others have said you definitely want to call Dispose if the class implements IDisposable  I take a fairly rigid position on this  Some might claim that calling Dispose on DataSet  for example  is pointless because they disassembled it and saw that it did not do anything meaningful  But  I think there are fallacies abound in that argument  Read this for an interesting debate by respected individuals on the subject  Then read my reasoning here why I think Jeffery Richter is in the wrong camp  Now  on to whether or not you should set a reference to null  The answer is no  Let me illustrate my point with the following code  public static void Main       Object a   new Object      Console WriteLine  quot object created quot      DoSomething a     Console WriteLine  quot object used quot      a   null    Console WriteLine  quot reference set to null quot       So when do you think the object referenced by a is eligible for collection  If you said after the call to a   null then you are wrong  If you said after the Main method completes then you are also wrong  The correct answer is that it is eligible for collection sometime during the call to DoSomething  That is right  It is eligible before the reference is set to null and perhaps even before the call to DoSomething completes  That is because the JIT compiler can recognize when object references are no longer dereferenced even if they are still rooted

User · Answer

I have to answer  too  The JIT generates tables together with the code from it s static analysis of variable usage  Those table entries are the  GC-Roots  in the current stack frame  As the instruction pointer advances  those table entries become invalid and so ready for garbage collection  Therefore  If it is a scoped variable  you don t need to set it to null - the GC will collect the object  If it is a member or a static variable  you have to set it to null

User · Answer

If they implement the IDisposable interface then you should dispose them  The garbage collector will take care of the rest   EDIT  best is to use the using command when working with disposable items   using var con   new SqlConnection

User · Answer

Objects will be cleaned up when they are no longer being used and when the garbage collector sees fit   Sometimes  you may need to set an object to null in order to make it go out of scope  such as a static field whose value you no longer need   but overall there is usually no need to set to null   Regarding disposing objects  I agree with  Andre   If the object is IDisposable it is a good idea to dispose it when you no longer need it  especially if the object uses unmanaged resources  Not disposing unmanaged resources will lead to memory leaks   You can use the using statement to automatically dispose an object once your program leaves the scope of the using statement   using  MyIDisposableObject obj   new MyIDisposableObject             use the object here      the object is disposed here   Which is functionally equivalent to   MyIDisposableObject obj  try       obj   new MyIDisposableObject      finally       if  obj    null                  IDisposable obj  Dispose

User · Answer

I agree with the common answer here that yes you should dispose and no you generally shouldn t set the variable to null    but I wanted to point out that dispose is NOT primarily about memory management  Yes  it can help  and sometimes does  with memory management  but it s primary purpose is to give you deterministic releasing of scarce resources   For example  if you open a hardware port  serial for example   a TCP IP socket  a file  in exclusive access mode  or even a database connection you have now prevented any other code from using those items until they are released  Dispose generally releases these items  along with GDI and other  os  handles etc  which there are 1000 s of available  but are still limited overall   If you don t call dipose on the owner object and explicitly release these resources  then try to open the same resource again in the future  or another program does  that open attempt will fail because your undisposed  uncollected object still has the item open  Of course  when the GC collects the item  if the Dispose pattern has been implemented correctly  the resource will get released    but you don t know when that will be  so you don t know when it s safe to re-open that resource  This is the primary issue Dispose  works around  Of course  releasing these handles often releases memory too  and never releasing them may never release that memory    hence all the talk about memory leaks  or delays in memory clean up   I have seen real world examples of this causing problems  For instance  I have seen ASP Net web applications that eventually fail to connect to the database  albeit for short periods of time  or until the web server process is restarted  because the sql server  connection pool is full     i e  so many connections have been created and not explicitly released in so short a period of time that no new connections can be created and many of the connections in the pool  although not active  are still referenced by undiposed and uncollected objects and so can t be reused  Correctly disposing the database connections where necessary ensures this problem doesn t happen  at least not unless you have very high concurrent access

[c#] Do you need to dispose of objects and set them to null?

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