Different ways of adding to Dictionary

Question

What is the difference in Dictionary add key  value  and Dictionary key    value   I ve noticed that the last version does not throw an ArgumentException when inserting a duplicate key  but is there any reason to prefer the first version   Edit  Does anyone have an authoritative source of information about this  I ve tried MSDN  but it is as always a wild goose chase

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Given the  most than probable similarities in performance  use whatever feel more correct and readable to the piece of code you re using   I feel an operation that describes an addition  being the presence of the key already a really rare exception is best represented with the add  Semantically it makes more sense   The dict key    value represents better a substitution  If I see that code I half expect the key to already be in the dictionary anyway

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To insert the Value into the Dictionary   Dictionary lt string  string gt  dDS1   new Dictionary lt string  string gt      Declaration  dDS1 Add  VEqpt    aaaa     adding key and value into the dictionary  string Count   dDS1  VEqpt     assigning the value of dictionary key to Count variable  dDS1  VEqpt     Count    bbbb    assigning the value to key

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One is assigning a value while the other is adding to the Dictionary a new Key and Value

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Yes  that is the difference  the Add method throws an exception if the key already exists   The reason to use the Add method is exactly this  If the dictionary is not supposed to contain the key already  you usually want the exception so that you are made aware of the problem

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The first version will add a new KeyValuePair to the dictionary  throwing if key is already in the dictionary  The second  using the indexer  will add a new pair if the key doesn t exist  but overwrite the value of the key if it already exists in the dictionary   IDictionary lt string  string gt  strings   new Dictionary lt string  string gt      strings  foo      bar              strings  foo       bar  strings  foo     string Empty      strings  foo      string empty strings Add  foo    bar            throws

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To answer the question first we need to take a look at the purpose of a dictionary and underlying technology     Dictionary is the list of KeyValuePair lt Tkey  Tvalue gt  where each value is represented by its unique key  Let s say we have a list of your favorite foods  Each value  food name  is represented by its unique key  a position   how much you like this food    Example code   Dictionary lt int  string gt  myDietFavorites   new Dictionary lt int  string gt            1   Burger          2   Fries          3   Donuts        Let s say you want to stay healthy  you ve changed your mind and you want to replace your favorite  Burger  with salad  Your list is still a list of your favorites  you won t change the nature of the list  Your favorite will remain number one on the list  only it s value will change  This is when you call this      your key stays 1  you only replace the value assigned to this key   you alter existing record in your dictionary   myDietFavorites 1     Salad     But don t forget you re the programmer  and from now on you finishes your sentences with   you refuse to use emojis because they would throw compilation error and all list of favorites is 0 index based   Your diet changed too  So you alter your list again     you don t want to replace Salad  you want to add this new fancy 0   position to your list  It wasn t there before so you can either define it   myDietFavorites 0     Pizza      or Add it   myDietFavorites Add 0   Pizza      There are two possibilities with defining  you either want to give a new definition for something not existent before or you want to change definition which already exists    Add method allows you to add a record but only under one condition  key for this definition may not exist in your dictionary   Now we are going to look under the hood  When you are making a dictionary your compiler make a reservation for the bucket  spaces in memory to store your records    Bucket don t store keys in the way you define them  Each key is hashed before going to the bucket  defined by Microsoft   worth mention that value part stays unchanged   I ll use the CRC32 hashing algorithm to simplify my example  When you defining   myDietFavorites 0     Pizza     What is going to the bucket is db2dc565  Pizza   simplified    When you alter the value in with   myDietFavorites 0     Spaghetti     You hash your 0 which is again db2dc565 then you look up this value in your bucket to find if it s there  If it s there you simply rewrite the value assigned to the key  If it s not there you ll place your value in the bucket   When you calling Add function on your dictionary like   myDietFavorite Add 0   Chocolate      You hash your 0 to compare it s value to ones in the bucket  You may place it in the bucket only if it s not there   It s crucial to know how it works especially if you work with dictionaries of string or char type of key  It s case sensitive because of undergoing hashing  So for example  name      Name   Let s use our CRC32 to depict this    Value for  name  is  e04112b1 Value for  Name  is  1107fb5b

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The performance is almost a 100  identical  You can check this out by opening the class in Reflector net  This is the This indexer   public TValue this TKey key        get               int index   this FindEntry key           if  index  gt   0                        return this entries index  value                    ThrowHelper ThrowKeyNotFoundException            return default TValue             set               this Insert key  value  false             And this is the Add method   public void Add TKey key  TValue value        this Insert key  value  true       I won t post the entire Insert method as it s rather long  however the method declaration is this   private void Insert TKey key  TValue value  bool add    And further down in the function  this happens   if   this entries i  hashCode    num   amp  amp  this comparer Equals this entries i  key  key         if  add                ThrowHelper ThrowArgumentException ExceptionResource Argument AddingDuplicate           Which checks if the key already exists  and if it does and the parameter add is true  it throws the exception   So for all purposes and intents the performance is the same   Like a few other mentions  it s all about whether you need the check  for attempts at adding the same key twice   Sorry for the lengthy post  I hope it s okay

User · Answer

Dictionary Add key  value  and Dictionary key    value have different purposes    Use the Add method to add new key value pair  existing keys will not be replaced  an ArgumentException is thrown   Use the indexer if you don t care whether the key already exists in the dictionary  in other words  add the key value pair if the the key is not in the dictionary or replace the value for the specified key if the key is already in the dictionary

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