Enumerable.GroupBy<TSource, TKey, TResult> Method (IEnumerable<TSource>, Func<TSource, TKey>, Func<TKey, IEnumerable<TSource>, 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.

Type Parameters

• TSource
  The type of the elements of source.

• TKey
  The type of the key returned by keySelector.

• TResult
  The type of the result value returned by 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.

• resultSelector
  Type: System.Func<TKey, IEnumerable<TSource>, 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.

 Structure Pet
 Public Name As String
 Public Age As Double
 End Structure
 Public Sub GroupByEx3()
 ' Create a list of pets.
 Dim petsList As New List(Of Pet)(New Pet() _
 {New Pet With {.Name = "Barley", .Age = 8.3}, _
 New Pet With {.Name = "Boots", .Age = 4.9}, _
 New Pet With {.Name = "Whiskers", .Age = 1.5}, _
 New Pet With {.Name = "Daisy", .Age = 4.3}})
 ' Group Pet objects by the Math.Floor of their 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.
 Dim query = petsList.GroupBy( _
 Function(pet) Math.Floor(pet.Age), _
 Function(age, pets) New With _
 {.Key = age, _
 .Count = pets.Count(), _
 .Min = pets.Min(Function(pet) pet.Age), _
 .Max = pets.Max(Function(Pet) Pet.Age)} _
 )
 Dim output As New System.Text.StringBuilder
 ' Iterate over each anonymous type.
 For Each result In query
 output.AppendLine(vbCrLf & "Age group: " & result.Key)
 output.AppendLine("Number of pets in this age group: " & result.Count)
 output.AppendLine("Minimum age: " & result.Min)
 output.AppendLine("Maximum age: " & result.Max)
 Next
 ' Display the output.
 outputBlock.Text &= output.ToString() & vbCrLf
 End Sub
 ' 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

 class Pet
 {
 public string Name { get; set; }
 public double Age { get; set; }
 }
 public static void GroupByEx3()
 {
 // 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 objects by the Math.Floor of their 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),
 (age, pets) => new
 {
 Key = age,
 Count = pets.Count(),
 Min = pets.Min(pet => pet.Age),
 Max = pets.Max(pet => pet.Age)
 });
 // Iterate over each anonymous type.
 foreach (var result in query)
 {
 outputBlock.Text += "\nAge group: " + result.Key + "\n";
 outputBlock.Text += "Number of pets in this age group: " + result.Count + "\n";
 outputBlock.Text += "Minimum age: " + result.Min + "\n";
 outputBlock.Text += "Maximum age: " + result.Max + "\n";
 }
 /* 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
 */
 }

Enumerable Class

GroupBy Overload

System.Linq Namespace