What is hashCode used for Is it unique

Question

I notice there is a getHashCode   method in every controls  items  in WP7  which return a sequence of number  Can I use this hashcode to identify an item  For example I want to identify a picture or a song in the device  and check it whereabout  This could be done if the hashcode given for specific items is unique   Can you help explain to me what is hashCode and getHashCode   use for

User · Accepted Answer

MSDN says      A hash code is a numeric value that is used to identify an object   during equality testing  It can also serve as an index for an object   in a collection       The GetHashCode method is suitable for use in hashing algorithms and   data structures such as a hash table       The default implementation of the GetHashCode method does not   guarantee unique return values for different objects  Furthermore  the    NET Framework does not guarantee the default implementation of the   GetHashCode method  and the value it returns will be the same between   different versions of the  NET Framework  Consequently  the default   implementation of this method must not be used as a unique object   identifier for hashing purposes       The GetHashCode method can be overridden by a derived type  Value   types must override this method to provide a hash function that is   appropriate for that type and to provide a useful distribution in a   hash table  For uniqueness  the hash code must be based on the value   of an instance field or property instead of a static field or   property       Objects used as a key in a Hashtable object must also override the   GetHashCode method because those objects must generate their own hash   code  If an object used as a key does not provide a useful   implementation of GetHashCode  you can specify a hash code provider   when the Hashtable object is constructed  Prior to the  NET Framework   version 2 0  the hash code provider was based on the   System Collections IHashCodeProvider interface  Starting with version   2 0  the hash code provider is based on the   System Collections IEqualityComparer interface    Basically  hash codes exist to make hashtables possible  Two equal objects are guaranteed to have equal hashcodes  Two unequal objects are not guaranteed to have unequal hashcodes  that s called a collision

User · Answer

GetHashCode   is used to help support using the object as a key for hash tables   A similar thing exists in Java etc   The goal is for every object to return a distinct hash code  but this often can t be absolutely guaranteed   It is required though that two logically equal objects return the same hash code   A typical hash table implementation starts with the hashCode value  takes a modulus  thus constraining the value within a range  and uses it as an index to an array of  buckets

User · Answer

After learning what it is all about  I thought to write a hopefully simpler explanation via analogy   Summary  What is a hashcode    It s a fingerprint  We can use this finger print to identify people of interest    Read below for more details   Think of a Hashcode as us trying to To Uniquely Identify Someone  I am a detective  on the look out for a criminal  Let us call him Mr Cruel   He was a notorious murderer when I was a kid -- he broke into a house kidnapped and murdered a poor girl  dumped her body and he s still out on the loose - but that s a separate matter   Mr Cruel has certain peculiar characteristics that I can use to uniquely identify him amongst a sea of people  We have 25 million people in Australia  One of them is Mr Cruel  How can we find him   Bad ways of Identifying Mr Cruel  Apparently Mr Cruel has blue eyes  That s not much help because almost half the population in Australia also has blue eyes   Good ways of Identifying Mr Cruel  What else can i use  I know  I will use a fingerprint   Advantages    It is really really hard for two people to have the same finger print  not impossible  but extremely unlikely   Mr Cruel s fingerprint will never change   Every single part of Mr Cruel s entire being  his looks  hair colour  personality  eating habits etc must  ideally  be reflected in his fingerprint  such that if he has a brother  who is very similar but not the same  - then both should have different finger prints  I say  should  because we cannot guarantee 100  that two people in this world will have different fingerprints   But we can always guarantee that Mr Cruel will always have the same finger print - and that his fingerprint will NEVER change    The above characteristics generally make for good hash functions   So what s the deal with  Collisions    So imagine if I get a lead and I find someone matching Mr Cruel s fingerprints  Does this mean I have found Mr Cruel           perhaps  I must take a closer look  If i am using SHA256  a hashing function  and I am looking in a small town with only 5 people - then there is a very good chance I found him  But if I am using MD5  another famous hashing function  and checking for fingerprints in a town with  2 1000 people  then it is a fairly good possibility that two entirely different people might have the same fingerprint   So what is the benefit of all this anyways   The only real benefit of hashcodes is if you want to put something in a hash table - and with hash tables you d want to find objects quickly - and that s where the hash code comes in  They allow you to find things in hash tables really quickly  It s a hack that massively improves performance  but at a small expense of accuracy   So let s imagine we have a hash table filled with people - 25 million suspects in Australia  Mr Cruel is somewhere in there      How can we find him really quickly  We need to sort through them all  to find a potential match  or to otherwise acquit potential suspects  You don t want to consider each person s unique characteristics because that would take too much time  What would you use instead  You d use a hashcode  A hashcode can tell you if two people are different  Whether Joe Bloggs is NOT Mr Cruel  If the prints don t match then you know it s definitely NOT Mr Cruel  But  if the finger prints do match then depending on the hash function you used  chances are already fairly good you found your man  But it s not 100   The only way you can be certain is to investigate further   i  did he she have an opportunity motive   ii  witnesses etc etc    When you are using computers if two objects have the same hash code value  then you again need to investigate further whether they are truly equal  e g  You d have to check whether the objects have e g  the same height  same weight etc  if the integers are the same  or if the customer id is a match  and then come to the conclusion whether they are the same  this is typically done perhaps by implementing an IComparer or IEquality interfaces   Key Summary  So basically a hashcode is a finger print      Two different people objects can theoretically still have the same fingerprint  Or in other words  If you have two fingerprints that are the same         then they need not both come from the same person object  Buuuuuut  the same person object will always return the same fingerprint  Which means that if two objects return different hash codes then you know for 100  certainty that those objects are different    It takes a good 3 minutes to get your head around the above  Perhaps read it a few times till it makes sense  I hope this helps someone because it took a lot of grief for me to learn it all

User · Answer

It s not unique to WP7--it s present on all  Net objects   It sort of does what you describe  but I would not recommend it as a unique identifier in your apps  as it is not guaranteed to be unique   Object GetHashCode Method

User · Answer

This is from the msdn article here   https   blogs msdn microsoft com tomarcher 2006 05 10 are-hash-codes-unique    While you will hear people state that hash codes generate a unique value for a given input  the fact is that  while difficult to accomplish  it is technically feasible to find two different data inputs that hash to the same value  However  the true determining factors regarding the effectiveness of a hash algorithm lie in the length of the generated hash code and the complexity of the data being hashed    So just use a hash algorithm suitable to your data size and it will have unique hashcodes

[c#] What is hashCode used for? Is it unique?

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