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

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

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

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.
resultSelector
Type: System.Linq.Expressions.Expression<Func<TKey, IEnumerable<TSource>, TResult>>
A function to create a result value from each group.

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.

ExceptionCondition
ArgumentNullException

source or keySelector or resultSelector 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, TResult>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TKey, IEnumerable<TSource>, TResult>>) method generates a MethodCallExpression that represents calling GroupBy<TSource, TKey, TResult>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TKey, IEnumerable<TSource>, 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, TResult>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TKey, IEnumerable<TSource>, TResult>>) 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 resultSelector parameter is used to obtain a result value from each group and its key.

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


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.AsQueryable().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
   */
}


Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1

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

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