Select distinct using linq

Question

I have a class list of class  public class LinqTest   public int id   get  set    public string value   get  set        List lt LinqTest gt  myList   new List lt LinqTest gt     myList Add new LinqTest     id   1  value    a      myList Add new LinqTest     id   1  value    b      myList Add new LinqTest     id   2  value    c        I need to select only the distinct id s from that list  ie  my resultant list should only contain    id 1 value  a     id   2  value    c       How can I do this with linq   Edit  Input   id      value 1        a 1        b 2        c 3        d 3        e   Out put should be   id      value 1        a 2        c 3        d   ie  if there is a repetition of id  result should take the first occurrence only

User · Answer

Using morelinq you can use DistinctBy   myList DistinctBy x   gt  x id     Otherwise  you can use a group   myList GroupBy x   gt  x id         Select g   gt  g First

User · Answer

myList GroupBy test   gt  test id         Select grp   gt  grp First       Edit  as getting this IEnumerable lt  gt  into a List lt  gt  seems to be a mystery to many people  you can simply write   var result   myList GroupBy test   gt  test id                      Select grp   gt  grp First                        ToList      But one is often better off working with the IEnumerable rather than IList as the Linq above is lazily evaluated  it doesn t actually do all of the work until the enumerable is iterated  When you call ToList it actually walks the entire enumerable forcing all of the work to be done up front   And may take a little while if your enumerable is infinitely long    The flipside to this advice is that each time you enumerate such an IEnumerable the work to evaluate it has to be done afresh  So you need to decide for each case whether it is better to work with the lazily evaluated IEnumerable or to realize it into a List  Set  Dictionary or whatnot

User · Answer

myList GroupBy i   gt  i id  Select group   gt  group First

User · Answer

You should override Equals and GetHashCode meaningfully  in this case to compare the ID   public class LinqTest       public int id   get  set        public string value   get  set         public override bool Equals object obj                LinqTest obj2   obj as LinqTest          if  obj2    null  return false          return id    obj2 id             public override int GetHashCode                 return id            Now you can use Distinct   List lt LinqTest gt  uniqueIDs   myList Distinct   ToList

[c#] Select distinct using linq

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