Enumerable.GroupBy<TSource, TKey, TElement, TResult> Method (IEnumerable<TSource>, Func<TSource, TKey>, Func<TSource, TElement>, Func<TKey, IEnumerable<TElement>, TResult>)
Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. The elements of each group are projected by using a specified function.
Assembly: System.Core (in System.Core.dll)
public static IEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult>( this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, Func<TKey, IEnumerable<TElement>, TResult> resultSelector )
Parameters
- source
-
Type:
System.Collections.Generic.IEnumerable<TSource>
An IEnumerable<T> whose elements to group.
- keySelector
-
Type:
System.Func<TSource, TKey>
A function to extract the key for each element.
- elementSelector
-
Type:
System.Func<TSource, TElement>
A function to map each source element to an element in an IGrouping<TKey, TElement>.
- resultSelector
-
Type:
System.Func<TKey, IEnumerable<TElement>, TResult>
A function to create a result value from each group.
Return Value
Type: System.Collections.Generic.IEnumerable<TResult>A collection of elements of type TResult where each element represents a projection over a group and its key.
Type Parameters
- TSource
The type of the elements of source.
- TKey
The type of the key returned by keySelector.
- TElement
The type of the elements in each IGrouping<TKey, TElement>.
- TResult
The type of the result value returned by resultSelector.
In query expression syntax, a group by (Visual C#) or Group By Into (Visual Basic) clause translates to an invocation of GroupBy.
The following code example demonstrates how to use GroupBy<TSource, TKey, TElement, TResult>(IEnumerable<TSource>, Func<TSource, TKey>, Func<TSource, TElement>, Func<TKey, IEnumerable<TElement>, TResult>) to group the projected elements of a sequence and then project a sequence of results of type TResult.
class Pet { public string Name { get; set; } public double Age { get; set; } } public static void GroupByEx4() { // Create a list of pets. List<Pet> petsList = new List<Pet>{ new Pet { Name="Barley", Age=8.3 }, new Pet { Name="Boots", Age=4.9 }, new Pet { Name="Whiskers", Age=1.5 }, new Pet { Name="Daisy", Age=4.3 } }; // Group Pet.Age values by the Math.Floor of the age. // Then project an anonymous type from each group // that consists of the key, the count of the group's // elements, and the minimum and maximum age in the group. var query = petsList.GroupBy( pet => Math.Floor(pet.Age), pet => pet.Age, (baseAge, ages) => new { Key = baseAge, Count = ages.Count(), Min = ages.Min(), Max = ages.Max() }); // Iterate over each anonymous type. foreach (var result in query) { Console.WriteLine("\nAge group: " + result.Key); Console.WriteLine("Number of pets in this age group: " + result.Count); Console.WriteLine("Minimum age: " + result.Min); Console.WriteLine("Maximum age: " + result.Max); } /* This code produces the following output: Age group: 8 Number of pets in this age group: 1 Minimum age: 8.3 Maximum age: 8.3 Age group: 4 Number of pets in this age group: 2 Minimum age: 4.3 Maximum age: 4.9 Age group: 1 Number of pets in this age group: 1 Minimum age: 1.5 Maximum age: 1.5 */ }
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 7.0
Windows Phone
Available since 8.1