Avoid Adding duplicate elements to a List C

Question

string   lines3   new string 100   List lt string gt  lines2   new List lt string gt     lines3   Regex Split s1     s   s      if   lines2 Contains lines3 ToString           lines2 AddRange lines3 Distinct   ToArray         I have checked all the spaces etc but i still get duplicate values in my lines2 List  I have to remove my duplicate values here itself

User · Answer

You can use Enumerable Except to get distinct items from lines3 which is not in lines2   lines2 AddRange lines3 Except lines2      If lines2 contains all items from lines3 then nothing will be added  BTW internally Except uses Set lt string gt  to get distinct items from second sequence and to verify those items present in first sequence  So  it s pretty fast

User · Answer

not a good way but kind of quick fix  take a bool to check if in whole list there is any duplicate entry  bool containsKey  string newKey   public void addKey string newKey        foreach  string key in MyKeys                if  key    newKey                        containsKey   true                       if   containsKey                MyKeys add newKey             else               containsKey   false

User · Answer

You could use a simple Union   Distinct   var lines   lines2 Union lines3  Distinct      That will add all the items from the second list into the first list  and then return all the unique strings in the combined list  Not likely to perform well with large lists  but it s simple   Reference  http   msdn microsoft com en-us library bb341731 aspx

User · Answer

Use a HashSet lt string gt  instead of a List lt string gt   It is prepared to perform a better performance because you don t need to provide checks for any items  The collection will manage it for you  That is the difference between a list and a set  For sample   HashSet lt string gt  set   new HashSet lt string gt      set Add  a    set Add  a    set Add  b    set Add  c    set Add  b    set Add  c    set Add  a    set Add  d    set Add  e    set Add  e     var total   set Count    Total is 5 and the values are a  b  c  d  e   The implemention of List lt T gt  does not give you nativelly  You can do it  but you have to provide this control  For sample  this extension method   public static class CollectionExtensions       public static void AddItem lt T gt  this List lt T gt  list  T item               if   list Contains item                      list Add item                      and use it   var list   new List lt string gt     list AddItem 1   list AddItem 2   list AddItem 3   list AddItem 2   list AddItem 4   list AddItem 5

User · Answer

If you want to save distinct values into a collection you could try HashSet Class  It will automatically remove the duplicate values and save your coding time

User · Answer

Taking the hint from  Felipe Oriani  I made the extension which I would like to share here for good   public static class CollectionExtension       public static void AddUniqueItem lt T gt  this List lt T gt  list  T item  bool throwException                if   list Contains item                         list Add item                     else if throwException                        throw new InvalidOperationException  Item already exists in the list                        public static bool IsUnique lt T gt  this List lt T gt  list  IEqualityComparer lt T gt  comparer                return list Count    list Distinct comparer  Count              public static bool IsUnique lt T gt  this List lt T gt  list                return list Count    list Distinct   Count

User · Answer

Use a HashSet along with your List   List lt string gt  myList   new List lt string gt     HashSet lt string gt  myHashSet   new HashSet lt string gt      public void addToList string s        if  myHashSet Add s             myList Add s             myHashSet Add s  will return true if s is not exist in it

User · Answer

use HashSet it s better  take a look here   http   www dotnetperls com hashset

User · Answer

Your this check    if   lines2 Contains lines3 ToString       is invalid  You are checking if your lines2 contains System String   since lines3 ToString   will give you that  You need to check if item from lines3  exists in lines2 or not    You can iterate each item in lines3 check if it exists in the lines2 and then add it  Something like    foreach  string str in lines3        if   lines2 Contains str           lines2 Add str       Or if your lines2 is any empty list  then you can simply add the lines3 distinct values to the list like    lines2 AddRange lines3 Distinct       then your lines2 will contain distinct values

User · Answer

If you don t want duplicates in a list  use a HashSet   That way it will be clear to anyone else reading your code what your intention was and you ll have less code to write since HashSet already handles what you are trying to do

User · Answer

If your check would have worked  it would have either added all the items  or none at all  However  calling the ToString method on an array returns the name of the data type  not the contents of the array  and the Contains method can only look for a single item  not a collection of items anyway   You have to check each string in the array   string   lines3  List lt string gt  lines2   new List lt string gt      lines3   Regex Split s1     s   s      foreach  string s in lines3      if   lines2 Contains s         lines2 Add s           However  if you start with an empty list  you can use the Distinct method to remove the duplicates  and you only need a single line of code   List lt string gt  lines2   Regex Split s1     s   s    Distinct   ToList

[c#] Avoid Adding duplicate elements to a List C#

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