How to check list A contains any value from list B

Question

List A     1  2  3  4   List B    2  5   How to check if list A contains any value from list B   e g  something like A contains a  a id   B id

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For faster and short solution you can use HashSet instead of List  a Overlaps b    Overlaps documentation This method is an O n  instead of O n 2  with two lists

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I ve profiled Justins two solutions  a Any a   gt  b Contains a   is fastest   using System  using System Collections Generic  using System Linq   namespace AnswersOnSO       public class Class1               public static void Main string   args                          How to check if list A contains any value from list B                e g  something like A contains a  gt a id   B id               var a   new List lt int gt   1 2 3 4               var b   new List lt int gt   2 5               var times   10000000               DateTime dtAny   DateTime Now              for  var i   0  i  lt  times  i                                  var aContainsBElements   a Any b Contains                             var timeAny    DateTime Now - dtAny  TotalSeconds               DateTime dtIntersect   DateTime Now              for  var i   0  i  lt  times  i                                  var aContainsBElements   a Intersect b  Any                              var timeIntersect    DateTime Now - dtIntersect  TotalSeconds                  timeAny  1 1470656 secs                timeIn   3 1431798 secs

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I write a faster method for it can make the small one to set  But I test it in some data that some time it s faster that Intersect but some time Intersect fast that my code       public static bool Contain lt T gt  List lt T gt  a  List lt T gt  b                if  a Count  lt   10  amp  amp  b Count  lt   10                        return a Any b Contains                      if  a Count  gt  b Count                        return Contain  IEnumerable lt T gt   b   IEnumerable lt T gt   a                     return Contain  IEnumerable lt T gt   a   IEnumerable lt T gt   b              public static bool Contain lt T gt  IEnumerable lt T gt  a  IEnumerable lt T gt  b                HashSet lt T gt  j   new HashSet lt T gt  a           return b Any j Contains           The Intersect calls Set that have not check the second size and this is the Intersect s code           Set lt TSource gt  set   new Set lt TSource gt  comparer           foreach  TSource element in second  set Add element           foreach  TSource element in first              if  set Remove element   yield return element    The difference in two methods is my method use HashSet and check the count and Intersect use set that is faster than HashSet  We dont warry its performance   The test       static void Main string   args                var a   Enumerable Range 0  100000           var b   Enumerable Range 10000000  1000           var t   new Stopwatch            t Start            Repeat     gt    Contain a  b               t Stop            Console WriteLine t ElapsedMilliseconds    490ms          var a1   Enumerable Range 0  100000  ToList            var a2   b ToList            t Restart            Repeat     gt    Contain a1  a2               t Stop             Console WriteLine t ElapsedMilliseconds    203ms          t Restart            Repeat     gt   a Intersect b  Any                t Stop            Console WriteLine t ElapsedMilliseconds    190ms          t Restart            Repeat     gt   b Intersect a  Any                t Stop            Console WriteLine t ElapsedMilliseconds    497ms          t Restart            a Any b Contains           t Stop            Console WriteLine t ElapsedMilliseconds    600ms             private static void Repeat Action a                for  int i   0  i  lt  100  i                          a

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I use this to count   int cnt   0   foreach  var lA in listA        if  listB Contains lA                 cnt

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If you didn t care about performance  you could try   a Any item   gt  b Contains item      or  as in the column using a method group a Any b Contains    But I would try this first   a Intersect b  Any

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You can Intersect the two lists   if  A Intersect B  Any

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You can check if a list is inside of another list with this  var list1   new List lt int gt    1  2  3  4  6    var list2   new List lt int gt    2  3    bool a   list1 Any c   gt  list2 Contains c

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Sorry  if this is irelevant  but will return list with matches using FindAll   in case you need this           private bool IsContain string cont                List lt string gt  ListToMatch  new List lt string gt      string1   string2             if  ListToMatch ToArray   Any cont Contains                         return false                    else             return true          And usage   List lt string gt  ListToCheck   new List lt string gt      string1   string2   string3   string4    List lt string gt  FinalList   ListToCheck FindAll IsContain     The final list contains only the matched elements string1 and string2 from list to check  Can easy be switched to int List

[c#] How to check list A contains any value from list B?

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