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

Groups the elements of a sequence according to a specified key selector function.

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

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

Type Parameters


The type of the elements of source.


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


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.

Return Value

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


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

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

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

         public static void GroupByEx1()
            // 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.
            var query = pets.AsQueryable().GroupBy(pet => pet.Age);

            // Iterate over each IGrouping in the collection.
            foreach (var ageGroup in query)
               outputBlock.Text += String.Format("Age group: {0}  Number of pets: {1}", ageGroup.Key, ageGroup.Count()) + "\n";

             This code produces the following output:

             Age group: 8  Number of pets: 1
             Age group: 4  Number of pets: 2
             Age group: 1  Number of pets: 1



Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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