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Queryable.GroupBy<TSource, TKey, TElement> Method (IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TSource, TElement>>)

Groups the elements of a sequence according to a specified key selector function and projects the elements for each group by using a specified function.

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

public static IQueryable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(
	this IQueryable<TSource> source,
	Expression<Func<TSource, TKey>> keySelector,
	Expression<Func<TSource, TElement>> elementSelector

Type Parameters


The type of the elements of source.


The type of the key returned by the function represented in keySelector.


The type of the elements in each IGrouping<TKey, TElement>.


Type: System.Linq.IQueryable<TSource>
An IQueryable<T> whose elements to group.
Type: System.Linq.Expressions.Expression<Func<TSource, TKey>>
A function to extract the key for each element.
Type: System.Linq.Expressions.Expression<Func<TSource, TElement>>
A function to map each source element to an element in an IGrouping<TKey, TElement>.

Return Value

Type: System.Linq.IQueryable<IGrouping<TKey, TElement>>
An IQueryable<IGrouping<TKey, TElement>> in C# or IQueryable(Of IGrouping(Of TKey, TElement)) in Visual Basic where each IGrouping<TKey, TElement> contains a sequence of objects of type TElement and a key.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IQueryable<TSource>. 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).


source or keySelector or elementSelector 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 GroupBy<TSource, TKey, TElement>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TSource, TElement>>) method generates a MethodCallExpression that represents calling GroupBy<TSource, TKey, TElement>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TSource, TElement>>) 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 source parameter.

The query behavior that occurs as a result of executing an expression tree that represents calling GroupBy<TSource, TKey, TElement>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TSource, TElement>>) depends on the implementation of the type of the source parameter. The expected behavior is that it groups the elements of source by a key value that is obtained by invoking keySelector on each element. It invokes elementSelector on each element to obtain a result element.

The following code example demonstrates how to use GroupBy<TSource, TKey, TElement>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TSource, TElement>>) to group the elements of a sequence.

            class Pet
                public string Name { get; set; }
                public int Age { get; set; }

            public static void GroupByEx2()
                // Create a list of Pet objects.
                List<Pet> pets =
                    new List<Pet>{ new Pet { Name="Barley", Age=8 },
                                   new Pet { Name="Boots", Age=4 },
                                   new Pet { Name="Whiskers", Age=1 },
                                   new Pet { Name="Daisy", Age=4 } };

                // Group the pets using Pet.Age as the key.
                // Use Pet.Name as the value for each entry.
                IEnumerable<IGrouping<int, string>> query =
                    pets.AsQueryable().GroupBy(pet => pet.Age, pet => pet.Name);

                // Iterate over each IGrouping in the collection.
                foreach (IGrouping<int, string> petGroup in query)
                    // Print the key value of the IGrouping.
                    // Iterate over each value in the 
                    // IGrouping and print the value.
                    foreach (string name in petGroup)
                        Console.WriteLine("  {0}", name);

                This code produces the following output:


.NET Framework

Supported in: 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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