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

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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.

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function GroupBy(Of TSource, TKey, TElement, TResult) ( _
    source As IQueryable(Of TSource), _
    keySelector As Expression(Of Func(Of TSource, TKey)), _
    elementSelector As Expression(Of Func(Of TSource, TElement)), _
    resultSelector As Expression(Of Func(Of TKey, IEnumerable(Of TElement), TResult)) _
) As IQueryable(Of TResult)

public static IQueryable<TResult> GroupBy<TSource, TKey, TElement, TResult>(
    this IQueryable<TSource> source,
    Expression<Func<TSource, TKey>> keySelector,
    Expression<Func<TSource, TElement>> elementSelector,
    Expression<Func<TKey, IEnumerable<TElement>, TResult>> resultSelector
)

Type Parameters

  • TSource
    The type of the elements of source.
  • TKey
    The type of the key returned by the function represented in keySelector.
  • TResult
    The type of the result value returned by resultSelector.

Parameters

Return Value

Type: System.Linq.IQueryable<TResult>
An T:System.Linq.IQueryable`1 that has a type argument of TResult and where each element represents a projection over a group and its key.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IQueryable<TSource>. When you use instance method syntax to call this method, omit the first parameter.

Exceptions

Exception Condition
ArgumentNullException

source or keySelector or elementSelector or resultSelector is nulla null reference (Nothing in Visual Basic).

Remarks

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, TResult>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TSource, TElement>>, Expression<Func<TKey, IEnumerable<TElement>, TResult>>) method generates a MethodCallExpression that represents calling GroupBy<TSource, TKey, TElement, TResult>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TSource, TElement>>, Expression<Func<TKey, IEnumerable<TElement>, TResult>>) 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, TResult>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TSource, TElement>>, Expression<Func<TKey, IEnumerable<TElement>, TResult>>) depends on the implementation of the type of the source parameter. The expected behavior is that it groups the elements of source by key values that are obtained by invoking keySelector on each element. The elementSelector parameter is used to project the elements of each group, and the resultSelector parameter is used to obtain a result value from each group and its key.

Examples

The following code example demonstrates how to use GroupBy<TSource, TKey, TElement, TResult>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TSource, TElement>>, Expression<Func<TKey, IEnumerable<TElement>, TResult>>) to group the elements of a sequence and project a sequence of results of type TResult.

Structure Pet
   Public Name As String
   Public Age As Double
End Structure

Shared Sub GroupByEx4()
   ' 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.Age valuesby 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.
   Dim query = petsList.AsQueryable().GroupBy( _
       Function(pet) Math.Floor(pet.Age), _
       Function(pet) pet.Age, _
       Function(baseAge, ages) New With { _
           .Key = baseAge, _
           .Count = ages.Count(), _
           .Min = ages.Min(), _
           .Max = ages.Max() _
       })

   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 with this age: " & result.Count)
      output.AppendLine("Minimum age: " & result.Min)
      output.AppendLine("Maximum age: " & result.Max)
   Next

   ' Display the output.
   outputBlock.Text &= output.ToString() & vbCrLf

   ' This code produces the following output:

   '  Age group: 8
   '  Number of pets with this age: 1
   '  Minimum age: 8.3
   '  Maximum age: 8.3

   '  Age group: 4
   '  Number of pets with this age: 2
   '  Minimum age: 4.3
   '  Maximum age: 4.9

   '  Age group: 1
   '  Number of pets with this age: 1
   '  Minimum age: 1.5
   '  Maximum age: 1.5
End Sub

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.AsQueryable().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)
   {
      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
   */
}

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.