Enumerable.GroupJoin Method (System.Linq)

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Download Microsoft Edge More info about Internet Explorer and Microsoft Edge GroupJoin<TOuter,TInner,TKey,TResult>(IEnumerable<TOuter>, IEnumerable<TInner>, Func<TOuter,TKey>, Func<TInner,TKey>, Func<TOuter,IEnumerable<TInner>, TResult>, IEqualityComparer<TKey>)

Correlates the elements of two sequences based on key equality and groups the results. A specified IEqualityComparer<T> is used to compare keys.

GroupJoin<TOuter,TInner,TKey,TResult>(IEnumerable<TOuter>, IEnumerable<TInner>, Func<TOuter,TKey>, Func<TInner,TKey>, Func<TOuter,IEnumerable<TInner>, TResult>)

Correlates the elements of two sequences based on equality of keys and groups the results. The default equality comparer is used to compare keys.

GroupJoin<TOuter,TInner,TKey,TResult>(IEnumerable<TOuter>, IEnumerable<TInner>, Func<TOuter,TKey>, Func<TInner,TKey>, Func<TOuter,IEnumerable<TInner>, TResult>, IEqualityComparer<TKey>)

public:
generic <typename TOuter, typename TInner, typename TKey, typename TResult>
[System::Runtime::CompilerServices::Extension]
 static System::Collections::Generic::IEnumerable<TResult> ^ GroupJoin(System::Collections::Generic::IEnumerable<TOuter> ^ outer, System::Collections::Generic::IEnumerable<TInner> ^ inner, Func<TOuter, TKey> ^ outerKeySelector, Func<TInner, TKey> ^ innerKeySelector, Func<TOuter, System::Collections::Generic::IEnumerable<TInner> ^, TResult> ^ resultSelector, System::Collections::Generic::IEqualityComparer<TKey> ^ comparer);

public static System.Collections.Generic.IEnumerable<TResult> GroupJoin<TOuter,TInner,TKey,TResult> (this System.Collections.Generic.IEnumerable<TOuter> outer, System.Collections.Generic.IEnumerable<TInner> inner, Func<TOuter,TKey> outerKeySelector, Func<TInner,TKey> innerKeySelector, Func<TOuter,System.Collections.Generic.IEnumerable<TInner>,TResult> resultSelector, System.Collections.Generic.IEqualityComparer<TKey> comparer);

public static System.Collections.Generic.IEnumerable<TResult> GroupJoin<TOuter,TInner,TKey,TResult> (this System.Collections.Generic.IEnumerable<TOuter> outer, System.Collections.Generic.IEnumerable<TInner> inner, Func<TOuter,TKey> outerKeySelector, Func<TInner,TKey> innerKeySelector, Func<TOuter,System.Collections.Generic.IEnumerable<TInner>,TResult> resultSelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer);

static member GroupJoin : seq<'Outer> * seq<'Inner> * Func<'Outer, 'Key> * Func<'Inner, 'Key> * Func<'Outer, seq<'Inner>, 'Result> * System.Collections.Generic.IEqualityComparer<'Key> -> seq<'Result>

<Extension()>
Public Function GroupJoin(Of TOuter, TInner, TKey, TResult) (outer As IEnumerable(Of TOuter), inner As IEnumerable(Of TInner), outerKeySelector As Func(Of TOuter, TKey), innerKeySelector As Func(Of TInner, TKey), resultSelector As Func(Of TOuter, IEnumerable(Of TInner), TResult), comparer As IEqualityComparer(Of TKey)) As IEnumerable(Of TResult)

Type Parameters

TOuter

Remarks

This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

If comparer is null , the default equality comparer, Default , is used to hash and compare keys.

GroupJoin produces hierarchical results, which means that elements from outer are paired with collections of matching elements from inner . GroupJoin enables you to base your results on a whole set of matches for each element of outer .

If there are no correlated elements in inner for a given element of outer , the sequence of matches for that element will be empty but will still appear in the results.

The resultSelector function is called only one time for each outer element together with a collection of all the inner elements that match the outer element. This differs from the Join method in which the result selector function is invoked on pairs that contain one element from outer and one element from inner .

GroupJoin preserves the order of the elements of outer , and for each element of outer , the order of the matching elements from inner .

GroupJoin has no direct equivalent in traditional relational database terms. However, this method does implement a superset of inner joins and left outer joins. Both of these operations can be written in terms of a grouped join. For more information, see Join operations .

GroupJoin<TOuter,TInner,TKey,TResult>(IEnumerable<TOuter>, IEnumerable<TInner>, Func<TOuter,TKey>, Func<TInner,TKey>, Func<TOuter,IEnumerable<TInner>, TResult>)

public:
generic <typename TOuter, typename TInner, typename TKey, typename TResult>
[System::Runtime::CompilerServices::Extension]
 static System::Collections::Generic::IEnumerable<TResult> ^ GroupJoin(System::Collections::Generic::IEnumerable<TOuter> ^ outer, System::Collections::Generic::IEnumerable<TInner> ^ inner, Func<TOuter, TKey> ^ outerKeySelector, Func<TInner, TKey> ^ innerKeySelector, Func<TOuter, System::Collections::Generic::IEnumerable<TInner> ^, TResult> ^ resultSelector);

public static System.Collections.Generic.IEnumerable<TResult> GroupJoin<TOuter,TInner,TKey,TResult> (this System.Collections.Generic.IEnumerable<TOuter> outer, System.Collections.Generic.IEnumerable<TInner> inner, Func<TOuter,TKey> outerKeySelector, Func<TInner,TKey> innerKeySelector, Func<TOuter,System.Collections.Generic.IEnumerable<TInner>,TResult> resultSelector);

static member GroupJoin : seq<'Outer> * seq<'Inner> * Func<'Outer, 'Key> * Func<'Inner, 'Key> * Func<'Outer, seq<'Inner>, 'Result> -> seq<'Result>

<Extension()>
Public Function GroupJoin(Of TOuter, TInner, TKey, TResult) (outer As IEnumerable(Of TOuter), inner As IEnumerable(Of TInner), outerKeySelector As Func(Of TOuter, TKey), innerKeySelector As Func(Of TInner, TKey), resultSelector As Func(Of TOuter, IEnumerable(Of TInner), TResult)) As IEnumerable(Of TResult)

Type Parameters

TOuter

A function to create a result element from an element from the first sequence and a collection of matching elements from the second sequence.

Returns

Examples

The following code example demonstrates how to use GroupJoin<TOuter,TInner,TKey,TResult>(IEnumerable<TOuter>, IEnumerable<TInner>, Func<TOuter,TKey>, Func<TInner,TKey>, Func<TOuter,IEnumerable<TInner>, TResult>) to perform a grouped join on two sequences.


   class Person
    public string Name { get; set; }
class Pet
    public string Name { get; set; }
    public Person Owner { get; set; }
public static void GroupJoinEx1()
    Person magnus = new Person { Name = "Hedlund, Magnus" };
    Person terry = new Person { Name = "Adams, Terry" };
    Person charlotte = new Person { Name = "Weiss, Charlotte" };
    Pet barley = new Pet { Name = "Barley", Owner = terry };
    Pet boots = new Pet { Name = "Boots", Owner = terry };
    Pet whiskers = new Pet { Name = "Whiskers", Owner = charlotte };
    Pet daisy = new Pet { Name = "Daisy", Owner = magnus };
    List<Person> people = new List<Person> { magnus, terry, charlotte };
    List<Pet> pets = new List<Pet> { barley, boots, whiskers, daisy };
    // Create a list where each element is an anonymous
    // type that contains a person's name and
    // a collection of names of the pets they own.
    var query =
        people.GroupJoin(pets,
                         person => person,
                         pet => pet.Owner,
                         (person, petCollection) =>
                                 OwnerName = person.Name,
                                 Pets = petCollection.Select(pet => pet.Name)
    foreach (var obj in query)
        // Output the owner's name.
        Console.WriteLine("{0}:", obj.OwnerName);
        // Output each of the owner's pet's names.
        foreach (string pet in obj.Pets)
            Console.WriteLine("  {0}", pet);
 This code produces the following output:
 Hedlund, Magnus:
   Daisy
 Adams, Terry:
   Barley
   Boots
 Weiss, Charlotte:
   Whiskers
   
    Structure Person
    Public Name As String
End Structure
Structure Pet
    Public Name As String
    Public Owner As Person
End Structure
Sub GroupJoinEx1()
    Dim magnus As New Person With {.Name = "Hedlund, Magnus"}
    Dim terry As New Person With {.Name = "Adams, Terry"}
    Dim charlotte As New Person With {.Name = "Weiss, Charlotte"}
    Dim barley As New Pet With {.Name = "Barley", .Owner = terry}
    Dim boots As New Pet With {.Name = "Boots", .Owner = terry}
    Dim whiskers As New Pet With {.Name = "Whiskers", .Owner = charlotte}
    Dim daisy As New Pet With {.Name = "Daisy", .Owner = magnus}
    Dim people As New List(Of Person)(New Person() {magnus, terry, charlotte})
    Dim pets As New List(Of Pet)(New Pet() {barley, boots, whiskers, daisy})
    ' Create a collection where each element is an anonymous type
    ' that contains a Person's name and a collection of names of
    ' the pets that are owned by them.
    Dim query =
people.GroupJoin(pets,
           Function(person) person,
           Function(pet) pet.Owner,
           Function(person, petCollection) _
               New With {.OwnerName = person.Name,
                         .Pets = petCollection.Select(
                                            Function(pet) pet.Name)})
    Dim output As New System.Text.StringBuilder
    For Each obj In query
        ' Output the owner's name.
        output.AppendLine(obj.OwnerName & ":")
        ' Output each of the owner's pet's names.
        For Each pet As String In obj.Pets
            output.AppendLine("  " & pet)
    ' Display the output.
    Console.WriteLine(output.ToString)
End Sub
' This code produces the following output:
' Hedlund, Magnus
'   Daisy
' Adams, Terry
'   Barley
'   Boots
' Weiss, Charlotte
'   Whiskers
    
     Remarks
    
    
     This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its
     
      GetEnumerator
     
     method directly or by using
     
      foreach
     
     in Visual C# or
     
      For Each
     
     in Visual Basic.
    
    
     The default equality comparer,
     
      Default
     
     , is used to hash and compare keys.
    
    
     
      GroupJoin
     
     produces hierarchical results, which means that elements from
     
      outer
     
     are paired with collections of matching elements from
     
      inner
     
     .
     
      GroupJoin
     
     enables you to base your results on a whole set of matches for each element of
     
      outer
     
     .
    
    
     If there are no correlated elements in
     
      inner
     
     for a given element of
     
      outer
     
     , the sequence of matches for that element will be empty but will still appear in the results.
    
    
     The
     
      resultSelector
     
     function is called only one time for each
     
      outer
     
     element together with a collection of all the
     
      inner
     
     elements that match the
     
      outer
     
     element. This differs from the
     
      Join
     
     method, in which the result selector function is invoked on pairs that contain one element from
     
      outer
     
     and one element from
     
      inner
     
     .
    
    
     
      GroupJoin
     
     preserves the order of the elements of
     
      outer
     
     , and for each element of
     
      outer
     
     , the order of the matching elements from
     
      inner
     
     .
    
    
     
      GroupJoin
     
     has no direct equivalent in traditional relational database terms. However, this method does implement a superset of inner joins and left outer joins. Both of these operations can be written in terms of a grouped join. For more information, see
     
      Join operations
     
     .
    
    
     In query expression syntax, a
     
      join ... into
     
     (Visual C#) or
     
      Group Join
     
     (Visual Basic) clause translates to an invocation of
     
      GroupJoin
     
     .