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

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

Usage Note

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.
                    Console.WriteLine(petGroup.Key);
                    // 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:

                8
                  Barley
                4
                  Boots
                  Daisy
                1
                  Whiskers
            */

.NET Framework

.NET Framework Client Profile

Portable Class Library

.NET for Windows Store apps

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.

Reference