How to use LINQ Distinct with multiple fields

Question

I have the following EF class derived from a database  simplified   class Product         public string ProductId       public string ProductName       public string CategoryId       public string CategoryName      ProductId is the Primary Key of the table    For a bad design decision made by the DB designer  I cannot modify it   I have CategoryId and CategoryName in this table   I need a DropDownList with  distinct  CategoryId as Value and CategoryName as Text  Therefore I applied the following code   product Select m   gt  new  m CategoryId  m CategoryName   Distinct      which logically it should create an anonymous object with CategoryId and CategoryName as properties  The Distinct   guarantees that there are no duplicates pair  CategoryId  CategoryName      But actually it does not work  As far as I understood the Distinct   works just when there is just one field in the collection otherwise it just ignores them   is it correct  Is there any workaround  Thanks   UPDATE  Sorry product is   List lt Product gt  product   new List lt Product gt       I found an alternative way to get the same result as Distinct     product GroupBy d   gt  new  d CategoryId  d CategoryName            Select m   gt  new  m Key CategoryId  m Key CategoryName

User · Answer

Distinct method returns distinct elements from a sequence.

If you take a look on its implementation with Reflector, you'll see that it creates DistinctIterator for your anonymous type. Distinct iterator adds elements to Set when enumerating over collection. This enumerator skips all elements which are already in Set. Set uses GetHashCode and Equals methods for defining if element already exists in Set.

How GetHashCode and Equals implemented for anonymous type? As it stated on msdn:

Equals and GetHashCode methods on anonymous types are defined in terms of the Equals and GetHashcode methods of the properties, two instances of the same anonymous type are equal only if all their properties are equal.

So, you definitely should have distinct anonymous objects, when iterating on distinct collection. And result does not depend on how many fields you use for your anonymous type.

User · Answer

This is my solution  it supports keySelectors of different types   public static IEnumerable lt TSource gt  DistinctBy lt TSource gt  this IEnumerable lt TSource gt  source  params Func lt TSource  object gt    keySelectors           initialize the table     var seenKeysTable   keySelectors ToDictionary x   gt  x  x   gt  new HashSet lt object gt              loop through each element in source     foreach  var element in source                   initialize the flag to true         var flag   true              loop through each keySelector a         foreach  var  keySelector  hashSet  in seenKeysTable                                               if all conditions are true             flag   flag  amp  amp  hashSet Add keySelector element                          if no duplicate key was added to table  then yield the list element         if  flag                        yield return element                      To use it   list DistinctBy d   gt  d CategoryId  d   gt  d CategoryName

User · Answer

I assume that you use distinct like a method call on a list  You need to use the result of the query as datasource for your DropDownList  for example by materializing it via ToList   var distinctCategories   product                          Select m   gt  new  m CategoryId  m CategoryName                            Distinct                            ToList    DropDownList1 DataSource       distinctCategories  DropDownList1 DataTextField     CategoryName   DropDownList1 DataValueField    CategoryId     Another way if you need the real objects instead of the anonymous type with only few properties is to use GroupBy with an anonymous type   List lt Product gt  distinctProductList   product      GroupBy m   gt  new  m CategoryId  m CategoryName        Select group   gt  group First        instead of First you can also apply your logic here what you want to take  for example an OrderBy      ToList      A third option is to use MoreLinq s DistinctBy

User · Answer

public List lt ItemCustom2 gt  GetBrandListByCat int id                 var OBJ    from a in db Items                    join b in db Brands on a BrandId equals b Id into abc1                    where  a ItemCategoryId    id                     from b in abc1 DefaultIfEmpty                      select new                                             ItemCategoryId   a ItemCategoryId                         Brand Name   b Name                         Brand Id   b Id                         Brand Pic   b Pic                         Distinct              List lt ItemCustom2 gt  ob   new List lt ItemCustom2 gt             foreach  var item in OBJ                        ItemCustom2 abc   new ItemCustom2                abc CategoryId   item ItemCategoryId              abc BrandId   item Brand Id              abc BrandName   item Brand Name              abc BrandPic   item Brand Pic              ob Add abc                     return ob

User · Answer

Answering the headline of the question  what attracted people here  and ignoring that the example used anonymous types      This solution will also work for non-anonymous types   It should not be needed for anonymous types   Helper class        lt summary gt      Allow IEqualityComparer to be configured within a lambda expression      From https   stackoverflow com questions 98033 wrap-a-delegate-in-an-iequalitycomparer      lt  summary gt       lt typeparam name  T  gt  lt  typeparam gt  public class LambdaEqualityComparer lt T gt    IEqualityComparer lt T gt        readonly Func lt T  T  bool gt   comparer      readonly Func lt T  int gt   hash            lt summary gt          Simplest constructor  provide a conversion to string for type T to use as a comparison key  GetHashCode   and Equals            https   stackoverflow com questions 98033 wrap-a-delegate-in-an-iequalitycomparer  user  orip           lt  summary gt           lt param name  toString  gt  lt  param gt      public LambdaEqualityComparer Func lt T  string gt  toString            this  t1  t2    gt  toString t1     toString t2   t   gt  toString t  GetHashCode                          lt summary gt          Constructor   Assumes T GetHashCode   is accurate           lt  summary gt           lt param name  comparer  gt  lt  param gt      public LambdaEqualityComparer Func lt T  T  bool gt  comparer            this comparer  t   gt  t GetHashCode                          lt summary gt          Constructor  provide a equality comparer and a hash           lt  summary gt           lt param name  comparer  gt  lt  param gt           lt param name  hash  gt  lt  param gt      public LambdaEqualityComparer Func lt T  T  bool gt  comparer  Func lt T  int gt  hash                 comparer   comparer           hash   hash             public bool Equals T x  T y                return  comparer x  y              public int GetHashCode T obj                return  hash obj                 Simplest usage   List lt Product gt  products   duplicatedProducts Distinct      new LambdaEqualityComparer lt Product gt  p   gt          String Format   0  1  2  3                p ProductId              p ProductName              p CategoryId              p CategoryName             ToList      The simplest  but not that efficient  usage is to map to a string representation so that custom hashing is avoided   Equal strings already have equal hash codes   Reference  Wrap a delegate in an IEqualityComparer

User · Answer

Use the Key keyword in your select will work  like below   product Select m   gt  new  Key m CategoryId  Key m CategoryName   Distinct     I realize this is bringing up an old thread but figured it might help some people  I generally code in VB NET when working with  NET so Key may translate differently into C

User · Answer

The Distinct   guarantees that there are no duplicates pair  CategoryId  CategoryName     - exactly that  Anonymous types  magically  implement Equals and GetHashcode  I assume another error somewhere  Case sensitivity  Mutable classes  Non-comparable fields

User · Answer

the solution to your problem looks like this   public class Category     public long CategoryId   get  set      public string CategoryName   get  set              public class CategoryEqualityComparer   IEqualityComparer lt Category gt       public bool Equals Category x  Category y         gt  x CategoryId Equals y CategoryId             amp  amp  x CategoryName  Equals y CategoryName    StringComparison OrdinalIgnoreCase       public int GetHashCode Mapping obj         gt  obj    null             0            obj CategoryId GetHashCode                obj CategoryName GetHashCode              var distinctCategories   product       Select     gt           new Category              CategoryId     CategoryId              CategoryName     CategoryName                  Distinct new CategoryEqualityComparer          ToList

User · Answer

Employee emp1   new Employee     ID   1  Name    Narendra1   Salary   11111  Experience   3  Age   30   Employee emp2   new Employee     ID   2  Name    Narendra2   Salary   21111  Experience   10  Age   38    Employee emp3   new Employee     ID   3  Name    Narendra3   Salary   31111  Experience   4  Age   33    Employee emp4   new Employee     ID   3  Name    Narendra4   Salary   41111  Experience   7  Age   33     List lt Employee gt  lstEmployee   new List lt Employee gt      lstEmployee Add emp1   lstEmployee Add emp2   lstEmployee Add emp3   lstEmployee Add emp4    var eemmppss lstEmployee Select cc  gt new  cc ID cc Age   Distinct

[c#] How to use LINQ Distinct() with multiple fields

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