Статья
11/18/2011

Queryable.Aggregate<TSource> Method (IQueryable<TSource>, Expression<Func<TSource, TSource, TSource>>)

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Applies an accumulator function over a sequence.

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

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function Aggregate(Of TSource) ( _
    source As IQueryable(Of TSource), _
    func As Expression(Of Func(Of TSource, TSource, TSource)) _
) As TSource

public static TSource Aggregate<TSource>(
    this IQueryable<TSource> source,
    Expression<Func<TSource, TSource, TSource>> func
)

Type Parameters

TSource
The type of the elements of source.

Parameters

source
Type: System.Linq.IQueryable<TSource>
A sequence to aggregate over.

func
Type: System.Linq.Expressions.Expression<Func<TSource, TSource, TSource>>
An accumulator function to apply to each element.

Return Value

Type: TSource
The final accumulator value.

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 func is nulla null reference (Nothing in Visual Basic).
InvalidOperationException	source contains no elements.

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 Aggregate<TSource>(IQueryable<TSource>, Expression<Func<TSource, TSource, TSource>>) method generates a MethodCallExpression that represents calling Aggregate<TSource>(IQueryable<TSource>, Expression<Func<TSource, TSource, TSource>>) itself as a constructed generic method. It then passes the MethodCallExpression to the Execute<TResult>(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 Aggregate<TSource>(IQueryable<TSource>, Expression<Func<TSource, TSource, TSource>>) depends on the implementation of the type of the source parameter. The expected behavior is that the specified function, func, is applied to each value in the source sequence and the accumulated value is returned. The first value in source is used as the seed value for the accumulated value, which corresponds to the first parameter in func.

To simplify common aggregation operations, the set of standard query operators also includes two counting methods, Count and LongCount, and four numeric aggregation methods, namely Max, Min, Sum, and Average.

Examples

The following code example demonstrates how to use Aggregate<TSource>(IQueryable<TSource>, Expression<Func<TSource, TSource, TSource>>) to build a sentence from an array of strings.

      Dim sentence As String = "the quick brown fox jumps over the lazy dog"

      ' Split the string into individual words.
      Dim words() As String = sentence.Split(" "c)

      ' Use Aggregate() to prepend each word to the beginning of the 
      ' new sentence to reverse the word order.
      Dim reversed As String = _
          words.AsQueryable().Aggregate( _
              Function(workingSentence, nextWord) nextWord & " " & workingSentence _
          )

      outputBlock.Text &= reversed & vbCrLf

      ' This code produces the following output:
      '
      ' dog lazy the over jumps fox brown quick the

         string sentence = "the quick brown fox jumps over the lazy dog";

         // Split the string into individual words.
         string[] words = sentence.Split(' ');

         // Use Aggregate() to prepend each word to the beginning of the 
         // new sentence to reverse the word order.
         string reversed =
             words.AsQueryable().Aggregate(
             (workingSentence, next) => next + " " + workingSentence
             );

         outputBlock.Text += reversed + "\n";

         // This code produces the following output:
         //
         // dog lazy the over jumps fox brown quick the

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.

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