Distinct by property of class with LINQ

Question

I have a collection   List lt Car gt  cars   new List lt Car gt       Cars are uniquely identified by their property CarCode   I have three cars in the collection  and two with identical CarCodes   How can I use LINQ to convert this collection to Cars with unique CarCodes

User · Answer

Use MoreLINQ  which has a DistinctBy method     IEnumerable lt Car gt  distinctCars   cars DistinctBy car   gt  car CarCode      This is only for LINQ to Objects  mind you

User · Answer

You can check out my PowerfulExtensions library  Currently it s in a very young stage  but already you can use methods like Distinct  Union  Intersect  Except on any number of properties   This is how you use it   using PowerfulExtensions Linq      var distinct   myArray Distinct x   gt  x A  x   gt  x B

User · Answer

You can implement an IEqualityComparer and use that in your Distinct extension   class CarEqualityComparer   IEqualityComparer lt Car gt         region IEqualityComparer lt Car gt  Members      public bool Equals Car x  Car y                return x CarCode Equals y CarCode              public int GetHashCode Car obj                return obj CarCode GetHashCode                endregion     And then  var uniqueCars   cars Distinct new CarEqualityComparer

User · Answer

Another way to accomplish the same thing     List lt Car gt  distinticBy   cars      Select car   gt  car CarCode       Distinct        Select code   gt  cars First car   gt  car CarCode    code        ToList      It s possible to create an extension method to do this in a more generic way  It would be interesting if someone could evalute performance of this  DistinctBy  against the GroupBy approach

User · Answer

You can use grouping  and get the first car from each group   List lt Car gt  distinct     cars    GroupBy car   gt  car CarCode     Select g   gt  g First       ToList

User · Answer

Same approach as Guffa but as an extension method   public static IEnumerable lt T gt  DistinctBy lt T  TKey gt  this IEnumerable lt T gt  items  Func lt T  TKey gt  property        return items GroupBy property  Select x   gt  x First         Used as   var uniqueCars   cars DistinctBy x   gt  x CarCode

User · Answer

You can t effectively use Distinct on a collection of objects  without additional work   I will explain why   The documentation says      It uses the default equality comparer  Default  to compare values    For objects that means it uses the default equation method to compare objects  source   That is on their hash code  And since your objects don t implement the GetHashCode   and Equals methods  it will check on the reference of the object  which are not distinct

User · Answer

I think the best option in Terms of performance  or in any terms  is to Distinct using the The IEqualityComparer interface   Although implementing each time a new comparer for each class is cumbersome and produces boilerplate code   So here is an extension method which produces a new IEqualityComparer on the fly for any class using reflection   Usage   var filtered   taskList DistinctBy t   gt  t TaskExternalId  ToArray      Extension Method Code  public static class LinqExtensions       public static IEnumerable lt T gt  DistinctBy lt T  TKey gt  this IEnumerable lt T gt  items  Func lt T  TKey gt  property                GeneralPropertyComparer lt T  TKey gt  comparer   new GeneralPropertyComparer lt T TKey gt  property           return items Distinct comparer              public class GeneralPropertyComparer lt T TKey gt    IEqualityComparer lt T gt        private Func lt T  TKey gt  expr   get  set        public GeneralPropertyComparer  Func lt T  TKey gt  expr                this expr   expr            public bool Equals T left  T right                var leftProp   expr Invoke left           var rightProp   expr Invoke right           if  leftProp    null  amp  amp  rightProp    null              return true          else if  leftProp    null   rightProp    null              return false          else             return leftProp Equals rightProp             public int GetHashCode T obj                var prop   expr Invoke obj           return  prop  null   0 prop GetHashCode

User · Answer

Another extension method for Linq-to-Objects  without using GroupBy            lt summary gt          Returns the set of items  made distinct by the selected value           lt  summary gt           lt typeparam name  TSource  gt The type of the source  lt  typeparam gt           lt typeparam name  TResult  gt The type of the result  lt  typeparam gt           lt param name  source  gt The source collection  lt  param gt           lt param name  selector  gt A function that selects a value to determine unique results  lt  param gt           lt returns gt IEnumerable amp lt TSource amp gt   lt  returns gt      public static IEnumerable lt TSource gt  Distinct lt TSource  TResult gt  this IEnumerable lt TSource gt  source  Func lt TSource  TResult gt  selector                HashSet lt TResult gt  set   new HashSet lt TResult gt              foreach var item in source                        var selectedValue   selector item                if  set Add selectedValue                   yield return item

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