Filter Linq EXCEPT on properties

Question

This may seem silly  but all the examples I ve found for using Except in linq use two lists or arrays of only strings or integers and filters them based on the matches  for example   var excludes   users Except matches     I want to use exclude to keep my code short and simple  but can t seem to find out how to do the following   class AppMeta       public int Id   get  set       var excludedAppIds   new List lt int gt   2  3  5  6   var unfilteredApps   new List lt AppMeta gt                                                        new AppMeta  Id   1                              new AppMeta  Id   2                              new AppMeta  Id   3                              new AppMeta  Id   4                              new AppMeta  Id   5                               How do I get a list of AppMeta back that filters on excludedAppIds

User · Answer

I use an extension method for Except  that allows you to compare Apples with Oranges as long as they both have something common that can be used to compare them  like an Id or Key   public static class ExtensionMethods       public static IEnumerable lt TA gt  Except lt TA  TB  TK gt           this IEnumerable lt TA gt  a          IEnumerable lt TB gt  b          Func lt TA  TK gt  selectKeyA          Func lt TB  TK gt  selectKeyB           IEqualityComparer lt TK gt  comparer   null                return a Where aItem   gt   b Select bItem   gt  selectKeyB bItem   Contains selectKeyA aItem   comparer              then use it something like this   var filteredApps   unfilteredApps Except excludedAppIds  a   gt  a Id  b   gt  b     the extension is very similar to ColinE  s answer  it s just packaged up into a neat extension that can be reused without to much mental overhead

User · Answer

ColinE s answer is simple and elegant  If your lists are larger and provided that the excluded apps list is sorted  BinarySearch lt T gt  may prove faster than Contains   EXAMPLE   unfilteredApps Where i   gt  excludedAppIds BinarySearch i Id   lt  0

User · Answer

MoreLinq has something useful for this MoreLinq Source MoreEnumerable ExceptBy  https   github com gsscoder morelinq blob master MoreLinq ExceptBy cs  namespace MoreLinq       using System      using System Collections Generic      using System Linq       static partial class MoreEnumerable                    lt summary gt              Returns the set of elements in the first sequence which aren t             in the second sequence  according to a given key selector               lt  summary gt               lt remarks gt              This is a set operation  if multiple elements in  lt paramref name  first   gt  have             equal keys  only the first such element is returned              This operator uses deferred execution and streams the results  although             a set of keys from  lt paramref name  second   gt  is immediately selected and retained               lt  remarks gt               lt typeparam name  TSource  gt The type of the elements in the input sequences  lt  typeparam gt               lt typeparam name  TKey  gt The type of the key returned by  lt paramref name  keySelector   gt   lt  typeparam gt               lt param name  first  gt The sequence of potentially included elements  lt  param gt               lt param name  second  gt The sequence of elements whose keys may prevent elements in              lt paramref name  first   gt  from being returned  lt  param gt               lt param name  keySelector  gt The mapping from source element to key  lt  param gt               lt returns gt A sequence of elements from  lt paramref name  first   gt  whose key was not also a key for             any element in  lt paramref name  second   gt   lt  returns gt           public static IEnumerable lt TSource gt  ExceptBy lt TSource  TKey gt  this IEnumerable lt TSource gt  first              IEnumerable lt TSource gt  second              Func lt TSource  TKey gt  keySelector                        return ExceptBy first  second  keySelector  null                           lt summary gt              Returns the set of elements in the first sequence which aren t             in the second sequence  according to a given key selector               lt  summary gt               lt remarks gt              This is a set operation  if multiple elements in  lt paramref name  first   gt  have             equal keys  only the first such element is returned              This operator uses deferred execution and streams the results  although             a set of keys from  lt paramref name  second   gt  is immediately selected and retained               lt  remarks gt               lt typeparam name  TSource  gt The type of the elements in the input sequences  lt  typeparam gt               lt typeparam name  TKey  gt The type of the key returned by  lt paramref name  keySelector   gt   lt  typeparam gt               lt param name  first  gt The sequence of potentially included elements  lt  param gt               lt param name  second  gt The sequence of elements whose keys may prevent elements in              lt paramref name  first   gt  from being returned  lt  param gt               lt param name  keySelector  gt The mapping from source element to key  lt  param gt               lt param name  keyComparer  gt The equality comparer to use to determine whether or not keys are equal              If null  the default equality comparer for  lt c gt TSource lt  c gt  is used  lt  param gt               lt returns gt A sequence of elements from  lt paramref name  first   gt  whose key was not also a key for             any element in  lt paramref name  second   gt   lt  returns gt           public static IEnumerable lt TSource gt  ExceptBy lt TSource  TKey gt  this IEnumerable lt TSource gt  first              IEnumerable lt TSource gt  second              Func lt TSource  TKey gt  keySelector              IEqualityComparer lt TKey gt  keyComparer                        if  first    null  throw new ArgumentNullException  first                if  second    null  throw new ArgumentNullException  second                if  keySelector    null  throw new ArgumentNullException  keySelector                return ExceptByImpl first  second  keySelector  keyComparer                      private static IEnumerable lt TSource gt  ExceptByImpl lt TSource  TKey gt  this IEnumerable lt TSource gt  first              IEnumerable lt TSource gt  second              Func lt TSource  TKey gt  keySelector              IEqualityComparer lt TKey gt  keyComparer                        var keys   new HashSet lt TKey gt  second Select keySelector   keyComparer               foreach  var element in first                                var key   keySelector element                   if  keys Contains key                                         continue                                    yield return element                  keys Add key

User · Answer

public static class ExceptByProperty       public static List lt T gt  ExceptBYProperty lt T  TProperty gt  this List lt T gt  list  List lt T gt  list2  Expression lt Func lt T  TProperty gt  gt  propertyLambda                Type type   typeof T            MemberExpression member   propertyLambda Body as MemberExpression           if  member    null              throw new ArgumentException string Format                   Expression   0   refers to a method  not a property                    propertyLambda ToString               PropertyInfo propInfo   member Member as PropertyInfo          if  propInfo    null              throw new ArgumentException string Format                   Expression   0   refers to a field  not a property                    propertyLambda ToString               if  type    propInfo ReflectedType  amp  amp               type IsSubclassOf propInfo ReflectedType               throw new ArgumentException string Format                   Expresion   0   refers to a property that is not from type  1                     propertyLambda ToString                    type            Func lt T  TProperty gt  func   propertyLambda Compile            var ids   list2 Select lt T  TProperty gt  x   gt  func x   ToArray            return list Where i   gt   ids Contains   TProperty propInfo GetValue i  null     ToList             public class testClass       public int ID   get  set        public string Name   get  set        For Test this           List lt testClass gt  a   new List lt testClass gt             List lt testClass gt  b   new List lt testClass gt             a Add new testClass     ID   1             a Add new testClass     ID   2             a Add new testClass     ID   3             a Add new testClass     ID   4             a Add new testClass     ID   5              b Add new testClass     ID   3             b Add new testClass     ID   5             a Select lt testClass  int gt  x   gt  x ID            var items   a ExceptBYProperty b  u   gt  u ID

User · Answer

This is what LINQ needs  public static IEnumerable lt T gt  Except lt T  TKey gt  this IEnumerable lt T gt  items  IEnumerable lt T gt  other  Func lt T  TKey gt  getKey         return from item in items             join otherItem in other on getKey item              equals getKey otherItem  into tempItems             from temp in tempItems DefaultIfEmpty               where ReferenceEquals null  temp     temp Equals default T               select item

User · Answer

Try a simple where query  var filtered   unfilteredApps Where i   gt   excludedAppIds Contains i Id       The except method uses equality  your lists contain objects of different types  so none of the items they contain will be equal

User · Answer

Construct a List lt AppMeta gt  from the excluded List and use the Except Linq operator   var ex   excludedAppIds Select x   gt  new AppMeta Id   x   ToList                               var result   ex Except unfilteredApps  ToList

User · Answer

I like the Except extension methods  but the original question doesn t have symmetric key access and I prefer Contains  or the Any variation  to join  so with all credit to azuneca s answer   public static IEnumerable lt T gt  Except lt T  TKey gt  this IEnumerable lt TKey gt  items      IEnumerable lt T gt  other  Func lt T  TKey gt  getKey         return from item in items         where  other Contains getKey item           select item      Which can then be used like   var filteredApps   unfilteredApps Except excludedAppIds  ua   gt  ua Id     Also  this version allows for needing a mapping for the exception IEnumerable by using a Select   var filteredApps   unfilteredApps Except excludedApps Select a   gt  a Id   ua   gt  ua Id

[c#] Filter Linq EXCEPT on properties

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