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Queryable.GroupJoin<TOuter, TInner, TKey, TResult> Method (IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, IEnumerable<TInner>, TResult>>)

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

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)

public static IQueryable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(
	this IQueryable<TOuter> outer,
	IEnumerable<TInner> inner,
	Expression<Func<TOuter, TKey>> outerKeySelector,
	Expression<Func<TInner, TKey>> innerKeySelector,
	Expression<Func<TOuter, IEnumerable<TInner>, TResult>> resultSelector
)

Type Parameters

TOuter

The type of the elements of the first sequence.

TInner

The type of the elements of the second sequence.

TKey

The type of the keys returned by the key selector functions.

TResult

The type of the result elements.

Parameters

outer
Type: System.Linq.IQueryable<TOuter>

The first sequence to join.

inner
Type: System.Collections.Generic.IEnumerable<TInner>

The sequence to join to the first sequence.

outerKeySelector
Type: System.Linq.Expressions.Expression<Func<TOuter, TKey>>

A function to extract the join key from each element of the first sequence.

innerKeySelector
Type: System.Linq.Expressions.Expression<Func<TInner, TKey>>

A function to extract the join key from each element of the second sequence.

resultSelector
Type: System.Linq.Expressions.Expression<Func<TOuter, IEnumerable<TInner>, TResult>>

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

Return Value

Type: System.Linq.IQueryable<TResult>
An IQueryable<T> that contains elements of type TResult obtained by performing a grouped join on two sequences.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IQueryable<TOuter>. When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

ExceptionCondition
ArgumentNullException

outer or inner or outerKeySelector or innerKeySelector or resultSelector is null.

This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T, TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.

The GroupJoin<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, IEnumerable<TInner>, TResult>>) method generates a MethodCallExpression that represents calling GroupJoin<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, IEnumerable<TInner>, TResult>>) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery<TElement>(Expression) method of the IQueryProvider represented by the Provider property of the outer parameter.

The query behavior that occurs as a result of executing an expression tree that represents calling GroupJoin<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, IEnumerable<TInner>, TResult>>) depends on the implementation of the type of the outer parameter. The expected behavior is that the outerKeySelector and innerKeySelector functions are used to extract keys from outer and inner, respectively. These keys are compared for equality to match each element in outer with zero or more elements from inner. Then the resultSelector function is invoked to project a result object from each group of correlated elements.

The following code example demonstrates how to use GroupJoin<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<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 that are owned by them. 
                var query =
                    people.AsQueryable().GroupJoin(pets,
                                     person => person,
                                     pet => pet.Owner,
                                     (person, petCollection) =>
                                         new
                                         {
                                             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
            */

.NET Framework

Supported in: 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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