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.NET Framework Class Library

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)

Type Parameters

TSource

The type of the elements of source.

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.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable<TSource>. When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).
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

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.