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

 

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)

<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

Parameters

source
Type: System.Linq.IQueryable(Of TSource)

A sequence to aggregate over.

seed
Type: TAccumulate

The initial accumulator value.

func
Type: System.Linq.Expressions.Expression(Of Func(Of TAccumulate, TSource, TAccumulate))

An accumulator function to invoke on each element.

selector
Type: System.Linq.Expressions.Expression(Of Func(Of TAccumulate, TResult))

A function to transform the final accumulator value into the result value.

Return Value

Type: TResult

The transformed final accumulator value.

Type Parameters

TSource

The type of the elements of source.

TAccumulate

The type of the accumulator value.

TResult

The type of the resulting value.

Exception Condition
ArgumentNullException

source or func or selector is null.

This method has at least one parameter of type Expression(Of TDelegate) whose type argument is one of the Func(Of T, TResult) types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression(Of TDelegate).

The Aggregate(Of TSource, TAccumulate, TResult)(IQueryable(Of TSource), TAccumulate, Expression(Of Func(Of TAccumulate, TSource, TAccumulate)), Expression(Of Func(Of TAccumulate, TResult))) method generates a MethodCallExpression that represents calling Aggregate(Of TSource, TAccumulate, TResult)(IQueryable(Of TSource), TAccumulate, Expression(Of Func(Of TAccumulate, TSource, TAccumulate)), Expression(Of Func(Of TAccumulate, TResult))) itself as a constructed generic method. It then passes the MethodCallExpression to the Execute(Of 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(Of TSource, TAccumulate, TResult)(IQueryable(Of TSource), TAccumulate, Expression(Of Func(Of TAccumulate, TSource, TAccumulate)), Expression(Of Func(Of 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(Of TSource) and LongCount(Of TSource), and four numeric aggregation methods, namely Max(Of TSource), Min(Of TSource), Sum, and Average.

The following code example demonstrates how to use Aggregate(Of TSource, TAccumulate, TResult)(IQueryable(Of TSource), TAccumulate, Expression(Of Func(Of TAccumulate, TSource, TAccumulate)), Expression(Of Func(Of 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(ByVal longest, ByVal fruit) IIf(fruit.Length > longest.Length, fruit, longest), _
    Function(ByVal fruit) fruit.ToUpper() _
)

MsgBox(String.Format( _
    "The fruit with the longest name is {0}.", longestName) _
)

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

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.1
Windows Phone
Available since 8.1
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