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Enumerable.Where<TSource> Method (IEnumerable<TSource>, Func<TSource, Int32, Boolean>)

 

Filters a sequence of values based on a predicate. Each element's index is used in the logic of the predicate function.

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

<ExtensionAttribute>
Public Shared Function Where(Of TSource) (
	source As IEnumerable(Of TSource),
	predicate As Func(Of TSource, Integer, Boolean)
) As IEnumerable(Of TSource)

Parameters

source
Type: System.Collections.Generic.IEnumerable<TSource>

An IEnumerable<T> to filter.

predicate
Type: System.Func<TSource, Int32, Boolean>

A function to test each source element for a condition; the second parameter of the function represents the index of the source element.

Return Value

Type: System.Collections.Generic.IEnumerable<TSource>

An IEnumerable<T> that contains elements from the input sequence that satisfy the condition.

Type Parameters

TSource

The type of the elements of source.

Exception Condition
ArgumentNullException

source or predicate is null.

This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

The first argument of predicate represents the element to test. The second argument represents the zero-based index of the element within source.

The following code example demonstrates how to use Where<TSource>(IEnumerable<TSource>, Func<TSource, Int32, Boolean>) to filter a sequence based on a predicate that involves the index of each element.

' Create an array of integers.
Dim numbers() As Integer = {0, 30, 20, 15, 90, 85, 40, 75}

' Restrict the results to those numbers whose
' values are less than or equal to their index times 10.
Dim query As IEnumerable(Of Integer) = _
    numbers.Where(Function(number, index) number <= index * 10)

' Display the results.
Dim output As New System.Text.StringBuilder
For Each number As Integer In query
    output.AppendLine(number)
Next
MsgBox(output.ToString())

' This code produces the following output:
'
' 0
' 20
' 15
' 40

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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