Queryable.GroupBy<TSource, TKey> Method (IQueryable<TSource>, Expression<Func<TSource, TKey>>)
Groups the elements of a sequence according to a specified key selector function.
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 )
Parameters
- source
-
Type:
System.Linq.IQueryable<TSource>
An IQueryable<T> whose elements to group.
- keySelector
-
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.
Type Parameters
- TSource
The type of the elements of source.
- TKey
The type of the key returned by the function represented in keySelector.
| Exception | Condition |
|---|---|
| ArgumentNullException | 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) { Console.WriteLine("Age group: {0} Number of pets: {1}", ageGroup.Key, ageGroup.Count()); } } /* 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 */
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.1
Windows Phone
Available since 8.1