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.
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 )
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.
- elementSelector
-
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
- TSource
The type of the elements of source.
- TKey
The type of the key returned by the function represented in keySelector.
- TElement
The type of the elements in each IGrouping<TKey, TElement>.
| Exception | Condition |
|---|---|
| ArgumentNullException | 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. 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 */
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