Use of Finalize Dispose method in C

Question

C  2008  I have been working on this for a while now  and I am still confused about the use of finalize and dispose methods in code  My questions are below    I know that we only need a finalizer while disposing unmanaged resources  However  if there are managed resources that make calls to unmanaged resources  would it still need to implement a finalizer  However  if I develop a class that doesn t use any unmanaged resource -  directly or indirectly  should I implement the IDisposable to allow the clients of that class to use the  using statement     Would it be feasible to implement IDisposable just to enable clients of your class to use the using statement   using myClass objClass   new myClass             Do stuff here    I have developed this simple code below to demonstrate the Finalize dispose use   public class NoGateway   IDisposable       private WebClient wc   null       public NoGateway                 wc   new WebClient            wc DownloadStringCompleted    wc DownloadStringCompleted                 Start the Async call to find if NoGateway is true or false     public void NoGatewayStatus                    Start the Async s download                Do other work here         wc DownloadStringAsync new Uri www xxxx xxx               private void wc DownloadStringCompleted object sender  DownloadStringCompletedEventArgs e                   Do work here               Dispose of the NoGateway object     public void Dispose                 wc DownloadStringCompleted -  wc DownloadStringCompleted          wc Dispose            GC SuppressFinalize this              Question about the source code    Here I have not added the finalizer  and normally the finalizer will be called by the GC  and the finalizer will call the Dispose  As I don t have the finalizer  when do I call the Dispose method  Is it the client of the class that has to call it   So my class in the example is called NoGateway and the client could use and dispose of the class like this     using NoGateway objNoGateway   new NoGateway             Do stuff here        Would the Dispose method be automatically called when execution reaches the end of the using block  or does the client have to manually call the dispose method  i e   NoGateway objNoGateway   new NoGateway       Do stuff with object objNoGateway Dispose       finished with it  I am using the WebClient class in my NoGateway class  Because WebClient implements the IDisposable interface  does this mean that WebClient indirectly uses unmanaged resources  Is there a hard and fast rule to follow this  How do I know that a class uses unmanaged resources

User · Accepted Answer

The recommended IDisposable pattern is here. When programming a class that uses IDisposable, generally you should use two patterns:

When implementing a sealed class that doesn't use unmanaged resources, you simply implement a Dispose method as with normal interface implementations:

public sealed class A : IDisposable
{
    public void Dispose()
    {
        // get rid of managed resources, call Dispose on member variables...
    }
}

When implementing an unsealed class, do it like this:

public class B : IDisposable
{    
    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    protected virtual void Dispose(bool disposing)
    {
        if (disposing)
        {
            // get rid of managed resources
        }   
        // get rid of unmanaged resources
    }

    // only if you use unmanaged resources directly in B
    //~B()
    //{
    //    Dispose(false);
    //}
}

Notice that I haven't declared a finalizer in B; you should only implement a finalizer if you have actual unmanaged resources to dispose. The CLR deals with finalizable objects differently to non-finalizable objects, even if SuppressFinalize is called.

So, you shouldn't declare a finalizer unless you have to, but you give inheritors of your class a hook to call your Dispose and implement a finalizer themselves if they use unmanaged resources directly:

public class C : B
{
    private IntPtr m_Handle;

    protected override void Dispose(bool disposing)
    {
        if (disposing)
        {
            // get rid of managed resources
        }
        ReleaseHandle(m_Handle);

        base.Dispose(disposing);
    }

    ~C() {
        Dispose(false);
    }
}

If you're not using unmanaged resources directly (SafeHandle and friends doesn't count, as they declare their own finalizers), then don't implement a finalizer, as the GC deals with finalizable classes differently, even if you later suppress the finalizer. Also note that, even though B doesn't have a finalizer, it still calls SuppressFinalize to correctly deal with any subclasses that do implement a finalizer.

When a class implements the IDisposable interface, it means that somewhere there are some unmanaged resources that should be got rid of when you've finished using the class. The actual resources are encapsulated within the classes; you don't need to explicitly delete them. Simply calling Dispose() or wrapping the class in a using(...) {} will make sure any unmanaged resources are got rid of as necessary.

User · Answer

Note that any IDisposable implementation should follow the below pattern  IMHO    I developed this pattern based on info from several excellent  NET  gods  the  NET Framework Design Guidelines  note that MSDN does not follow this for some reason     The  NET Framework Design Guidelines were written by Krzysztof Cwalina  CLR Architect at the time  and Brad Abrams  I believe the CLR Program Manager at the time  and Bill Wagner   Effective C   and  More Effective C    just take a look for these on Amazon com   Note that you should NEVER implement a Finalizer unless your class directly contains  not inherits  UNmanaged resources   Once you implement a Finalizer in a class  even if it is never called  it is guaranteed to live for an extra collection  It is automatically placed on the Finalization Queue  which runs on a single thread    Also  one very important note   all code executed within a Finalizer  should you need to implement one  MUST be thread-safe AND exception-safe   BAD things will happen otherwise    i e  undetermined behavior and in the case of an exception  a fatal unrecoverable application crash    The pattern I ve put together  and written a code snippet for  follows    region IDisposable implementation    TODO remember to make this class inherit from IDisposable - gt   className    IDisposable     Default initialization for a bool is  false  private bool IsDisposed   get  set          lt summary gt      Implementation of Dispose according to  NET Framework Design Guidelines       lt  summary gt       lt remarks gt Do not make this method virtual      A derived class should not be able to override this method       lt  remarks gt  public void Dispose         Dispose  true            This object will be cleaned up by the Dispose method         Therefore  you should call GC SupressFinalize to        take this object off the finalization queue         and prevent finalization code for this object        from executing a second time          Always use SuppressFinalize   in case a subclass        of this type implements a finalizer      GC SuppressFinalize  this            lt summary gt      Overloaded Implementation of Dispose       lt  summary gt       lt param name  isDisposing  gt  lt  param gt       lt remarks gt       lt para gt  lt list type  bulleted  gt Dispose bool isDisposing  executes in two distinct scenarios       lt item gt If  lt paramref name  isDisposing   gt  equals true  the method has been called directly     or indirectly by a user s code  Managed and unmanaged resources     can be disposed  lt  item gt       lt item gt If  lt paramref name  isDisposing   gt  equals false  the method has been called by the      runtime from inside the finalizer and you should not reference      other objects  Only unmanaged resources can be disposed  lt  item gt  lt  list gt  lt  para gt       lt  remarks gt  protected virtual void Dispose  bool isDisposing            TODO If you need thread safety  use a lock around these         operations  as well as in your methods that use the resource      try               if   this IsDisposed                         if  isDisposing                                    TODO Release all managed resources here                   end                                TODO Release all unmanaged resources here                   TODO explicitly set root references to null to expressly tell the GarbageCollector                that the resources have been disposed of and its ok to release the memory allocated for them                        finally                  explicitly call the base class Dispose implementation         base Dispose  isDisposing             this IsDisposed   true             TODO Uncomment this code if this class will contain members which are UNmanaged            lt summary gt Finalizer for  className  lt  summary gt         lt remarks gt This finalizer will run only if the Dispose method does not get called        It gives your base class the opportunity to finalize        DO NOT provide finalizers in types derived from this class        All code executed within a Finalizer MUST be thread-safe  lt  remarks gt        className                 Dispose  false           endregion IDisposable implementation   Here is the code for implementing IDisposable in a derived class   Note that you do not need to explicitly list inheritance from IDisposable in the definition of the derived class   public DerivedClass   BaseClass  IDisposable  remove the IDisposable because it is inherited from BaseClass    protected override void Dispose  bool isDisposing         try               if    this IsDisposed                         if   isDisposing                                    Release all managed resources here                                    finally                  explicitly call the base class Dispose implementation         base Dispose  isDisposing              I ve posted this implementation on my blog at  How to Properly Implement the Dispose Pattern

User · Answer

1  WebClient is a managed type  so you don t need a finalizer   The finalizer is needed in the case your users don t Dispose   of your NoGateway class and the native type  which is not collected by the GC  needs to be cleaned up after   In this case  if the user doesn t call Dispose    the contained WebClient will be disposed by the GC right after the NoGateway does   2  Indirectly yes  but you shouldn t have to worry about it   Your code is correct as stands and you cannot prevent your users from forgetting to Dispose   very easily

User · Answer

nobody answered the question about whether you should implement IDisposable even though you dont need it   Short answer   No  Long answer   This would allow a consumer of your class to use  using   The question I would ask is - why would they do it  Most devs will not use  using  unless they know that they must - and how do they know  Either   its obviuos the them from experience  a socket class for example  its documented they are cautious and can see that the class implements IDisposable   So by implementing IDisposable you are telling devs  at least some  that this class wraps up something that must be released  They will use  using  - but there are other cases where using is not possible  the scope of object is not local   and they will have to start worrying about the lifetime of the objects in those other cases - I would worry for sure  But this is not necessary  You implement Idisposable to enable them to use using  but they wont use using unless you tell them to   So dont do it

User · Answer

I agree with pm100  and should have explicitly said this in my earlier post    You should never implement IDisposable in a class unless you need it   To be very specific  there are about 5 times when you would ever need should implement IDisposable    Your class explicitly contains  i e  not via inheritance  any managed resources which implement IDisposable and should be cleaned up once your class is no longer used   For example  if your class contains an instance of a Stream  DbCommand  DataTable  etc  Your class explicitly contains any managed resources which implement a Close   method - e g  IDataReader  IDbConnection  etc   Note that some of these classes do implement IDisposable by having Dispose   as well as a Close   method  Your class explicitly contains an unmanaged resource - e g  a COM object  pointers  yes  you can use pointers in managed C  but they must be declared in  unsafe  blocks  etc  In the case of unmanaged resources  you should also make sure to call System Runtime InteropServices Marshal ReleaseComObject   on the RCW   Even though the RCW is  in theory  a managed wrapper  there is still reference counting going on under the covers  If your class subscribes to events using strong references  You need to unregister detach yourself from the events  Always to make sure these are not null first before trying to unregister detach them   Your class contains any combination of the above      A recommended alternative to working with COM objects and having to use Marshal ReleaseComObject   is to use the System Runtime InteropServices SafeHandle class   The BCL  Base Class Library Team  has a good blog post about it here http   blogs msdn com bclteam archive 2005 03 16 396900 aspx  One very important note to make is that if you are working with WCF and cleaning up resources  you should ALMOST ALWAYS avoid the  using  block   There are plenty of blog posts out there and some on MSDN about why this is a bad idea  I have also posted about it here - Don t use  using    with a WCF proxy

User · Answer

using NoGateway objNoGateway   new NoGateway      is equivalent to   try       NoGateway   new NoGateway       finally       NoGateway Dispose        A finalizer is called upon the GC destroying your object  This can be at a totally different time than when you leave your method  The Dispose of IDisposable is called immediately after you leave the using block  Hence the pattern is usually to use using to free ressources immediately after you don t need them anymore

User · Answer

If you are using other managed objects that are using unmanaged resources  it is not your responsibility to ensure those are finalized  Your responsibility is to call Dispose on those objects when Dispose is called on your object  and it stops there  If your class doesn t use any scarce resources  I fail to see why you would make your class implement IDisposable  You should only do so if you re    Know you will have scarce resources in your objects soon  just not now  and I mean that as in  we re still developing  it will be here before we re done   not as in  I think we ll need this   Using scarce resources  Yes  the code that uses your code must call the Dispose method of your object  And yes  the code that uses your object can use using as you ve shown   2 again   It is likely that the WebClient uses either unmanaged resources  or other managed resources that implement IDisposable  The exact reason  however  is not important  What is important is that it implements IDisposable  and so it falls on you to act upon that knowledge by disposing of the object when you re done with it  even if it turns out WebClient uses no other resources at all

User · Answer

Some aspects of another answer are slightly incorrect for 2 reasons   First   using NoGateway objNoGateway   new NoGateway      actually is equivalent to   try       NoGateway   new NoGateway      finally       if NoGateway    null                NoGateway Dispose              This may sound ridiculous since the  new  operator should never return  null  unless you have an OutOfMemory exception   But consider the following cases  1  You call a FactoryClass that returns an IDisposable resource or 2  If you have a type that may or may not inherit from IDisposable depending on its implementation - remember that I ve seen the IDisposable pattern implemented incorrectly many times at many clients where developers just add a Dispose   method without inheriting from IDisposable  bad  bad  bad    You could also have the case of an IDisposable resource being returned from a property or method  again bad  bad  bad - don t  give away your IDisposable resources   using IDisposable objNoGateway   new NoGateway   as IDisposable        if  NoGateway    null                      If the  as  operator returns null  or property or method returning the resource   and your code in the  using  block protects against  null   your code will not blow up when trying to call Dispose on a null object because of the  built-in  null check   The second reason your reply is not accurate is because of the following stmt      A finalizer is called upon the GC destroying your object   First  Finalization  as well as GC itself  is non-deterministic   THe CLR determines when it will call a finalizer   i e  the developer code has no idea   If the IDisposable pattern is implemented correctly  as I ve posted above  and GC SuppressFinalize   has been called  the the Finalizer will NOT be called   This is one of the big reasons to properly implement the pattern correctly   Since there is only 1 Finalizer thread per managed process  regardless of the number of logical processors  you can easily degrade performance by backing up or even hanging the Finalizer thread by forgetting to call GC SuppressFinalize     I ve posted a correct implementation of the Dispose Pattern on my blog  How to Properly Implement the Dispose Pattern

User · Answer

From what I know  it s highly recommended NOT to use the Finalizer   Destructor   public  MyClass         dont use this     Mostly  this is due to not knowing when or IF it will be called  The dispose method is much better  especially if you us using or dispose directly   using is good  use it

User · Answer

The official pattern to implement IDisposable is hard to understand  I believe this one is better   public class BetterDisposableClass   IDisposable      public void Dispose         CleanUpManagedResources        CleanUpNativeResources        GC SuppressFinalize this          protected virtual void CleanUpManagedResources                       protected virtual void CleanUpNativeResources                        BetterDisposableClass         CleanUpNativeResources             An even better solution is to have a rule that you always have to create a wrapper class for any unmanaged resource that you need to handle   public class NativeDisposable   IDisposable      public void Dispose         CleanUpNativeResource        GC SuppressFinalize this          protected virtual void CleanUpNativeResource                        NativeDisposable         CleanUpNativeResource             With SafeHandle and its derivatives  these classes should be very rare   The result for disposable classes that don t deal directly with unmanaged resources  even in the presence of inheritance  is powerful  they don t need to be concerned with unmanaged resources anymore  They ll be simple to implement and to understand   public class ManagedDisposable   IDisposable      public virtual void Dispose            dispose of managed resources

User · Answer

Dispose pattern   public abstract class DisposableObject   IDisposable       public bool Disposed   get  private set              public void Dispose                 Dispose true           GC SuppressFinalize this               DisposableObject                 Dispose false              private void Dispose bool disposing                if   Disposed                        if  disposing                                DisposeManagedResources                               DisposeUnmanagedResources                Disposed   true                       protected virtual void DisposeManagedResources           protected virtual void DisposeUnmanagedResources           Example of inheritance   public class A   DisposableObject       public Component components a   get  set        private IntPtr handle a       protected override void DisposeManagedResources                 try                     Console WriteLine  A DisposeManagedResources              components a Dispose              components a   null                    finally                      base DisposeManagedResources                         protected override void DisposeUnmanagedResources                 try                     Console WriteLine  A DisposeUnmanagedResources              CloseHandle handle a             handle a   IntPtr Zero                    finally                      base DisposeUnmanagedResources                       public class B   A       public Component components b   get  set        private IntPtr handle b       protected override void DisposeManagedResources                 try                     Console WriteLine  B DisposeManagedResources              components b Dispose              components b   null                    finally                      base DisposeManagedResources                         protected override void DisposeUnmanagedResources                 try                     Console WriteLine  B DisposeUnmanagedResources              CloseHandle handle b             handle b   IntPtr Zero                    finally                      base DisposeUnmanagedResources

User · Answer

Using lambdas instead of IDisposable   I have never been thrilled with the whole using IDisposable idea  The problem is that it requires the caller to    know that they must use IDisposable remember to use  using     My new preferred method is to use a factory method and a lambda instead  Imagine I want to do something with a SqlConnection  something that should be wrapped in a using   Classically you would do  using  Var conn   Factory MakeConnection           conn Query            New way  Factory DoWithConnection  conn   gt        conn Query           In the first case the caller could simply not use the using syntax  IN the second case the user has no choice  There is no method that creates a SqlConnection object  the caller must invoke DoWithConnection   DoWithConnection looks like this  void DoWithConnection Action lt SqlConnection gt  action       using  var conn   MakeConnection                action conn            MakeConnection is now private

User · Answer

Pattern from msdn     public class BaseResource  IDisposable      private IntPtr handle     private Component Components     private bool disposed   false     public BaseResource                public void Dispose              Dispose true               GC SuppressFinalize this           protected virtual void Dispose bool disposing             if  this disposed                           if disposing                         Components Dispose                                 CloseHandle handle            handle   IntPtr Zero                 disposed   true                    BaseResource                   Dispose false           public void DoSomething              if this disposed                   throw new ObjectDisposedException                   public class MyResourceWrapper  BaseResource      private ManagedResource addedManaged     private NativeResource addedNative     private bool disposed   false     public MyResourceWrapper                protected override void Dispose bool disposing             if  this disposed                   try                        if disposing                                            addedManaged Dispose                                       CloseHandle addedNative               this disposed   true                      finally                        base Dispose disposing

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