NET unique object identifier

Question

Is there a way of getting a unique identifier of an instance   GetHashCode   is the same for the two references pointing to the same instance  However  two different instances can  quite easily  get the same hash code   Hashtable hashCodesSeen   new Hashtable    LinkedList lt object gt  l   new LinkedList lt object gt     int n   0  while  true        object o   new object           Remember objects so that they don t get collected         This does not make any difference though        l AddFirst o       int hashCode   o GetHashCode        n        if  hashCodesSeen ContainsKey hashCode                    Same hashCode seen twice for DIFFERENT objects  n is as low as 5322           Console WriteLine  Hashcode seen twice      n          hashCode                 break            hashCodesSeen Add hashCode  null       I m writing a debugging addin  and I need to get some kind of ID for a reference which is unique during the run of the program   I already managed to get internal ADDRESS of the instance  which is unique until the garbage collector  GC  compacts the heap    moves the objects   changes the addresses    Stack nbsp Overflow question Default implementation for Object GetHashCode   might be related   The objects are not under my control as I am accessing objects in a program being debugged using the debugger API  If I was in control of the objects  adding my own unique identifiers would be trivial   I wanted the unique ID for building a hashtable ID -  object  to be able to lookup already seen objects  For now I solved it like this   Build a hashtable   hashCode  - gt   list of objects with hash code     hashCode   Find if object seen o        candidates   hashtable o GetHashCode       Objects with the same hashCode      If no candidates  the object is new     If some candidates  compare their addresses to o Address         If no address is equal  the hash code was just a coincidence  - gt  o is new         If some address equal  o already seen

User · Answer

You would have to assign such an identifier yourself, manually - either inside the instance, or externally.

For records related to a database, the primary key may be useful (but you can still get duplicates). Alternatively, either use a Guid, or keep your own counter, allocating using Interlocked.Increment (and make it large enough that it isn't likely to overflow).

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Checked out the ObjectIDGenerator class  This does what you re attempting to do  and what Marc Gravell describes   The ObjectIDGenerator keeps track of previously identified objects  When you ask for the ID of an object  the ObjectIDGenerator knows whether to return the existing ID  or generate and remember a new ID  The IDs are unique for the life of the ObjectIDGenerator instance  Generally  a ObjectIDGenerator life lasts as long as the Formatter that created it  Object IDs have meaning only within a given serialized stream  and are used for tracking which objects have references to others within the serialized object graph  Using a hash table  the ObjectIDGenerator retains which ID is assigned to which object  The object references  which uniquely identify each object  are addresses in the runtime garbage-collected heap  Object reference values can change during serialization  but the table is updated automatically so the information is correct  Object IDs are 64-bit numbers  Allocation starts from one  so zero is never a valid object ID  A formatter can choose a zero value to represent an object reference whose value is a null reference  Nothing in Visual Basic

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I know that this has been answered  but it s at least useful to note that you can use   http   msdn microsoft com en-us library system object referenceequals aspx  Which will not give you a  unique id  directly  but combined with WeakReferences  and a hashset   could give you a pretty easy way of tracking various instances

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It is possible to make a unique object identifier in Visual Studio  In the watch window  right-click the object variable and choose Make Object ID from the context menu   Unfortunately  this is a manual step  and I don t believe the identifier can be accessed via code

User · Answer

You can develop your own thing in a second  For instance      class Program               static void Main string   args                        var a   new object                var b   new object                Console WriteLine     a GetId    b GetId                          public static class MyExtensions                 this dictionary should use weak key references         static Dictionary lt object  int gt  d   new Dictionary lt object int gt             static int gid   0           public static int GetId this object o                        if  d ContainsKey o   return d o               return d o    gid                         You can choose what you will like to have as unique ID on your own  for instance  System Guid NewGuid   or simply integer for fastest access

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How about this method   Set a field in the first object to a new value   If the same field in the second object has the same value  it s probably the same instance   Otherwise  exit as different   Now set the field in the first object to a different new value   If the same field in the second object has changed to the different value  it s definitely the same instance   Don t forget to set field in the first object back to it s original value on exit   Problems

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RuntimeHelpers GetHashCode   may help  MSDN

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The information I give here is not new  I just added this for completeness   The idea of this code is quite simple    Objects need a unique ID  which isn t there by default  Instead  we have to rely on the next best thing  which is RuntimeHelpers GetHashCode to get us a sort-of unique ID To check uniqueness  this implies we need to use object ReferenceEquals However  we would still like to have a unique ID  so I added a GUID  which is by definition unique  Because I don t like locking everything if I don t have to  I don t use ConditionalWeakTable    Combined  that will give you the following code   public class UniqueIdMapper       private class ObjectEqualityComparer   IEqualityComparer lt object gt                public bool Equals object x  object y                        return object ReferenceEquals x  y                      public int GetHashCode object obj                        return RuntimeHelpers GetHashCode obj                        private Dictionary lt object  Guid gt  dict   new Dictionary lt object  Guid gt  new ObjectEqualityComparer         public Guid GetUniqueId object o                Guid id          if   dict TryGetValue o  out id                         id   Guid NewGuid                dict Add o  id                     return id            To use it  create an instance of the UniqueIdMapper and use the GUID s it returns for the objects     Addendum  So  there s a bit more going on here  let me write a bit down about ConditionalWeakTable    ConditionalWeakTable does a couple of things  The most important thing is that it doens t care about the garbage collector  that is  the objects that you reference in this table will be collected regardless  If you lookup an object  it basically works the same as the dictionary above   Curious no  After all  when an object is being collected by the GC  it checks if there are references to the object  and if there are  it collects them  So if there s an object from the ConditionalWeakTable  why will the referenced object be collected then   ConditionalWeakTable uses a small trick  which some other  NET structures also use  instead of storing a reference to the object  it actually stores an IntPtr  Because that s not a real reference  the object can be collected   So  at this point there are 2 problems to address  First  objects can be moved on the heap  so what will we use as IntPtr  And second  how do we know that objects have an active reference    The object can be pinned on the heap  and its real pointer can be stored  When the GC hits the object for removal  it unpins it and collects it  However  that would mean we get a pinned resource  which isn t a good idea if you have a lot of objects  due to memory fragmentation issues   This is probably not how it works  When the GC moves an object  it calls back  which can then update the references  This might be how it s implemented judging by the external calls in DependentHandle - but I believe it s slightly more sophisticated  Not the pointer to the object itself  but a pointer in the list of all objects from the GC is stored  The IntPtr is either an index or a pointer in this list  The list only changes when an object changes generations  at which point a simple callback can update the pointers  If you remember how Mark  amp  Sweep works  this makes more sense  There s no pinning  and removal is as it was before  I believe this is how it works in DependentHandle    This last solution does require that the runtime doesn t re-use the list buckets until they are explicitly freed  and it also requires that all objects are retrieved by a call to the runtime   If we assume they use this solution  we can also address the second problem  The Mark  amp  Sweep algorithm keeps track of which objects have been collected  as soon as it has been collected  we know at this point  Once the object checks if the object is there  it calls  Free   which removes the pointer and the list entry  The object is really gone   One important thing to note at this point is that things go horribly wrong if ConditionalWeakTable is updated in multiple threads and if it isn t thread safe  The result would be a memory leak  This is why all calls in ConditionalWeakTable do a simple  lock  which ensures this doesn t happen   Another thing to note is that cleaning up entries has to happen once in a while  While the actual objects will be cleaned up by the GC  the entries are not  This is why ConditionalWeakTable only grows in size  Once it hits a certain limit  determined by collision chance in the hash   it triggers a Resize  which checks if objects have to be cleaned up -- if they do  free is called in the GC process  removing the IntPtr handle   I believe this is also why DependentHandle is not exposed directly - you don t want to mess with things and get a memory leak as a result  The next best thing for that is a WeakReference  which also stores an IntPtr instead of an object  - but unfortunately doesn t include the  dependency  aspect   What remains is for you to toy around with the mechanics  so that you can see the dependency in action  Be sure to start it multiple times and watch the results   class DependentObject       public class MyKey   IDisposable               public MyKey bool iskey                        this iskey   iskey                     private bool disposed   false          private bool iskey           public void Dispose                         if   disposed                                disposed   true                  Console WriteLine  Cleanup  0    iskey                                     MyKey                         Dispose                         static void Main string   args                var dep   new MyKey true      also try passing this to cwt Add          ConditionalWeakTable lt MyKey  MyKey gt  cwt   new ConditionalWeakTable lt MyKey  MyKey gt             cwt Add new MyKey true   dep      try doing this 5 times f ex           GC Collect GC MaxGeneration           GC WaitForFullGCComplete             Console WriteLine  Wait            Console ReadLine       Put a breakpoint here and inspect cwt to see that the IntPtr is still there

User · Answer

The reference is the unique identifier for the object  I don t know of any way of converting this into anything like a string etc  The value of the reference will change during compaction  as you ve seen   but every previous value A will be changed to value B  so as far as safe code is concerned it s still a unique ID   If the objects involved are under your control  you could create a mapping using weak references  to avoid preventing garbage collection  from a reference to an ID of your choosing  GUID  integer  whatever   That would add a certain amount of overhead and complexity  however

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NET 4 and later only  Good news  everyone   The perfect tool for this job is built in  NET 4 and it s called ConditionalWeakTable lt TKey  TValue gt   This class    can be used to associate arbitrary data with managed object instances much like a dictionary  although it is not a dictionary  does not depend on memory addresses  so is immune to the GC compacting the heap does not keep objects alive just because they have been entered as keys into the table  so it can be used without making every object in your process live forever uses reference equality to determine object identity  moveover  class authors cannot modify this behavior so it can be used consistently on objects of any type can be populated on the fly  so does not require that you inject code inside object constructors

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If you are writing a module in your own code for a specific usage  majkinetor s method MIGHT have worked  But there are some problems   First  the official document does NOT guarantee that the GetHashCode   returns an unique identifier  see Object GetHashCode Method          You should not assume that equal hash codes imply object equality    Second  assume you have a very small amount of objects so that GetHashCode   will work in most cases  this method can be overridden by some types   For example  you are using some class C and it overrides GetHashCode   to always return 0  Then every object of C will get the same hash code  Unfortunately  Dictionary  HashTable and some other associative containers will make use this method      A hash code is a numeric value that is used to insert and identify an object in a hash-based collection such as the Dictionary lt TKey  TValue  class  the Hashtable class  or a type derived from the DictionaryBase class  The GetHashCode method provides this hash code for algorithms that need quick checks of object equality    So  this approach has great limitations   And even more  what if you want to build a general purpose library  Not only are you not able to modify the source code of the used classes  but their behavior is also unpredictable   I appreciate that Jon and Simon have posted their answers  and I will post a code example and a suggestion on performance below   using System  using System Diagnostics  using System Runtime CompilerServices  using System Runtime Serialization  using System Collections Generic    namespace ObjectSet       public interface IObjectSet                    lt summary gt  check the existence of an object   lt  summary gt               lt returns gt  true if object is exist  false otherwise   lt  returns gt          bool IsExist object obj                 lt summary gt  if the object is not in the set  add it in  else do nothing   lt  summary gt               lt returns gt  true if successfully added  false otherwise   lt  returns gt          bool Add object obj              public sealed class ObjectSetUsingConditionalWeakTable   IObjectSet                    lt summary gt  unit test on object set   lt  summary gt          internal static void Main                 Stopwatch sw   new Stopwatch                sw Start                ObjectSetUsingConditionalWeakTable objSet   new ObjectSetUsingConditionalWeakTable                for  int i   0  i  lt  10000000    i                    object obj   new object                    if  objSet IsExist obj     Console WriteLine  bug                         if   objSet Add obj     Console WriteLine  bug                         if   objSet IsExist obj     Console WriteLine  bug                                   sw Stop                Console WriteLine sw ElapsedMilliseconds                       public bool IsExist object obj                return objectSet TryGetValue obj  out tryGetValue out0                      public bool Add object obj                if  IsExist obj                     return false                else                   objectSet Add obj  null                   return true                                        lt summary gt  internal representation of the set   only use the key   lt  summary gt          private ConditionalWeakTable lt object  object gt  objectSet   new ConditionalWeakTable lt object  object gt                   lt summary gt  used to fill the out parameter of ConditionalWeakTable TryGetValue     lt  summary gt          private static object tryGetValue out0   null              Obsolete  It will crash if there are too many objects and ObjectSetUsingConditionalWeakTable get a better performance         public sealed class ObjectSetUsingObjectIDGenerator   IObjectSet                    lt summary gt  unit test on object set   lt  summary gt          internal static void Main                 Stopwatch sw   new Stopwatch                sw Start                ObjectSetUsingObjectIDGenerator objSet   new ObjectSetUsingObjectIDGenerator                for  int i   0  i  lt  10000000    i                    object obj   new object                    if  objSet IsExist obj     Console WriteLine  bug                         if   objSet Add obj     Console WriteLine  bug                         if   objSet IsExist obj     Console WriteLine  bug                                   sw Stop                Console WriteLine sw ElapsedMilliseconds                       public bool IsExist object obj                bool firstTime              idGenerator HasId obj  out firstTime               return  firstTime                     public bool Add object obj                bool firstTime              idGenerator GetId obj  out firstTime               return firstTime                           lt summary gt  internal representation of the set   lt  summary gt          private ObjectIDGenerator idGenerator   new ObjectIDGenerator              In my test  the ObjectIDGenerator will throw an exception to complain that there are too many objects when creating 10 000 000 objects  10x than in the code above  in the for loop   Also  the benchmark result is that the ConditionalWeakTable implementation is 1 8x faster than the ObjectIDGenerator implementation

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