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


The .NET API Reference documentation has a new home. Visit the .NET API Browser on docs.microsoft.com to see the new experience.

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: 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.

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>.

Exception Condition

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:


Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.1
Windows Phone
Available since 8.1
Return to top