Check if two lists are equal

Question

I have a class as follows  public class Tag       public Int32 Id   get  set        public String Name   get  set       And I have two lists of tag  List lt Tag gt  tags1  List lt Tag gt  tags2   I used LINQ s select to get the Ids of each tags list  And then  List lt Int32 gt  ids1   new List lt Int32 gt    1  2  3  4    List lt Int32 gt  ids2   new List lt Int32 gt    1  2  3  4    List lt Int32 gt  ids3   new List lt Int32 gt    2  1  3  4    List lt Int32 gt  ids4   new List lt Int32 gt    1  2  3  5    List lt Int32 gt  ids5   new List lt Int32 gt    1  1  3  4     ids1 should be equal to ids2 and ids3     Both have the same numbers  ids1 should not be equal to ids4 and to ids5     I tried the following  var a   ints1 Equals ints2   var b   ints1 Equals ints3    But both give me false  What is the fastest way to check if the lists of tags are equal  UPDATE I am looking for POSTS which TAGS are exactly the same as the TAGS in a BOOK  IRepository repository   new Repository new Context      IList lt Tags gt  tags   new List lt Tag gt    new Tag   Id   1    new Tag   Id   2       Book book   new Book   Tags   new List lt Tag gt    new Tag   Id   1    new Tag   Id   2         var posts   repository    Include lt Post gt  x   gt  x Tags     Where x   gt  new HashSet lt Int32 gt  tags Select y   gt  y Id   SetEquals book Tags Select y   gt  y Id       ToList     I am using Entity Framework and I get the error   An exception of type  System NotSupportedException  occurred in mscorlib dll but was not handled in user code Additional information  LINQ to Entities does not recognize the method  Boolean SetEquals System Collections Generic IEnumerable 1 System Int32    method  and this method cannot be translated into a store expression   How do I solve this

User · Accepted Answer

Use SequenceEqual to check for sequence equality because Equals method checks for reference equality   var a   ints1 SequenceEqual ints2     Or if you don t care about elements order use Enumerable All method   var a   ints1 All ints2 Contains     The second version also requires another check for Count because it would return true even if ints2 contains more elements than ints1  So the more correct version would be something like this   var a   ints1 All ints2 Contains   amp  amp  ints1 Count    ints2 Count    In order to check inequality just reverse the result of All method   var a    ints1 All ints2 Contains

User · Answer

List lt T gt  equality does not check them element-by-element  You can use LINQ s SequenceEqual method for that   var a   ints1 SequenceEqual ints2     To ignore order  use SetEquals   var a   new HashSet lt int gt  ints1  SetEquals ints2     This should work  because you are comparing sequences of IDs  which do not contain duplicates  If it does  and you need to take duplicates into account  the way to do it in linear time is to compose a hash-based dictionary of counts  add one for each element of the first sequence  subtract one for each element of the second sequence  and check if the resultant counts are all zeros   var counts   ints1      GroupBy v   gt  v       ToDictionary g   gt  g Key  g   gt  g Count     var ok   true  foreach  var n in ints2        int c      if  counts TryGetValue n  out c             counts n    c-1        else           ok   false          break          var res   ok  amp  amp  counts Values All c   gt  c    0     Finally  if you are fine with an O N LogN  solution  you can sort the two sequences  and compare them for equality using SequenceEqual

User · Answer

Enumerable SequenceEqual FirstList OrderBy fElement   gt  fElement                             SecondList OrderBy sElement   gt  sElement

[c#] Check if two lists are equal

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