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Enumerable.Where(Of TSource) Method (IEnumerable(Of TSource), Func(Of TSource, Boolean))

Filters a sequence of values based on a predicate.

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

'Declaration
<ExtensionAttribute> _
Public Shared Function Where(Of TSource) ( _
	source As IEnumerable(Of TSource), _
	predicate As Func(Of TSource, Boolean) _
) As IEnumerable(Of TSource)
'Usage
Dim source As IEnumerable(Of TSource)
Dim predicate As Func(Of TSource, Boolean)
Dim returnValue As IEnumerable(Of TSource)

returnValue = source.Where(predicate)

Type Parameters

TSource

The type of the elements of source.

Parameters

source
Type: System.Collections.Generic.IEnumerable(Of TSource)

An IEnumerable(Of T) to filter.

predicate
Type: System.Func(Of TSource, Boolean)

A function to test each element for a condition.

Return Value

Type: System.Collections.Generic.IEnumerable(Of TSource)
An IEnumerable(Of 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(Of 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).

ExceptionCondition
ArgumentNullException

source or predicate is Nothing.

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.

In query expression syntax, a where (Visual C#) or Where (Visual Basic) clause translates to an invocation of Where(Of TSource)(IEnumerable(Of TSource), Func(Of TSource, Boolean)).

The following code example demonstrates how to use Where(Of TSource)(IEnumerable(Of TSource), Func(Of TSource, Boolean)) to filter a sequence.

' Create a list of strings. 
Dim fruits As New List(Of String)(New String() _
                        {"apple", "passionfruit", "banana", "mango", _
                         "orange", "blueberry", "grape", "strawberry"})

' Restrict the results to those strings whose  
' length is less than six. 
Dim query As IEnumerable(Of String) = _
    fruits.Where(Function(fruit) fruit.Length < 6)

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

' This code produces the following output: 

' apple 
' mango 
' grape

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

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

.NET Framework

Supported in: 3.5

.NET Compact Framework

Supported in: 3.5

XNA Framework

Supported in: 3.0

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