Articolo
11/18/2011

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

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Applies an accumulator function over a sequence. The specified seed value is used as the initial accumulator value, and the specified function is used to select the result value.

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

Syntax

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

public static TResult Aggregate<TSource, TAccumulate, TResult>(
    this IQueryable<TSource> source,
    TAccumulate seed,
    Expression<Func<TAccumulate, TSource, TAccumulate>> func,
    Expression<Func<TAccumulate, TResult>> selector
)

Type Parameters

TSource
The type of the elements of source.

TAccumulate
The type of the accumulator value.

TResult
The type of the resulting value.

Parameters

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

seed
Type: TAccumulate
The initial accumulator value.

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

selector
Type: System.Linq.Expressions.Expression<Func<TAccumulate, TResult>>
A function to transform the final accumulator value into the result value.

Return Value

Type: TResult
The transformed 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 or selector 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 Aggregate<TSource, TAccumulate, TResult>(IQueryable<TSource>, TAccumulate, Expression<Func<TAccumulate, TSource, TAccumulate>>, Expression<Func<TAccumulate, TResult>>) method generates a MethodCallExpression that represents calling Aggregate<TSource, TAccumulate, TResult>(IQueryable<TSource>, TAccumulate, Expression<Func<TAccumulate, TSource, TAccumulate>>, Expression<Func<TAccumulate, TResult>>) 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, TAccumulate, TResult>(IQueryable<TSource>, TAccumulate, Expression<Func<TAccumulate, TSource, TAccumulate>>, Expression<Func<TAccumulate, TResult>>) 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 seed parameter is used as the seed value for the accumulated value, which corresponds to the first parameter in func. The final accumulated value is passed to selector to obtain the result value.

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, TAccumulate, TResult>(IQueryable<TSource>, TAccumulate, Expression<Func<TAccumulate, TSource, TAccumulate>>, Expression<Func<TAccumulate, TResult>>) to apply an accumulator function and a result selector.

      Dim fruits() As String = {"apple", "mango", "orange", "passionfruit", "grape"}

      ' Determine whether any string in the array is longer than "banana".
      Dim longestName As String = _
          fruits.AsQueryable().Aggregate( _
          "banana", _
          Function(longest, fruit) IIf(fruit.Length > longest.Length, fruit, longest), _
          Function(fruit) fruit.ToUpper() _
      )

      outputBlock.Text &= String.Format( _
          "The fruit with the longest name is {0}.", longestName) & vbCrLf

      ' This code produces the following output:
      '
      ' The fruit with the longest name is PASSIONFRUIT.

         string[] fruits = { "apple", "mango", "orange", "passionfruit", "grape" };

         // Determine whether any string in the array is longer than "banana".
         string longestName =
             fruits.AsQueryable().Aggregate(
             "banana",
             (longest, next) => next.Length > longest.Length ? next : longest,
            // Return the final result as an uppercase string.
             fruit => fruit.ToUpper()
             );

         outputBlock.Text += String.Format(
             "The fruit with the longest name is {0}.",
             longestName) + "\n";

         // This code produces the following output:
         //
         // The fruit with the longest name is PASSIONFRUIT.

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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