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

Returns elements from a sequence as long as a specified condition is true.

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

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

Type Parameters

TSource

The type of the elements of source.

Parameters

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

A sequence to return elements from.

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 the elements from the input sequence that occur before the element at which the test no longer passes.

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.

The TakeWhile(Of TSource)(IEnumerable(Of TSource), Func(Of TSource, Boolean)) method tests each element of source by using predicate and yields the element if the result is true. Enumeration stops when the predicate function returns false for an element or when source contains no more elements.

The TakeWhile and SkipWhile methods are functional complements. Given a sequence coll and a pure function p, concatenating the results of coll.TakeWhile(p) and coll.SkipWhile(p) yields the same sequence as coll.

In Visual Basic query expression syntax, a Take While clause translates to an invocation of TakeWhile.

The following code example demonstrates how to use TakeWhile(Of TSource)(IEnumerable(Of TSource), Func(Of TSource, Boolean)) to return elements from the start of a sequence as long as a condition is true.

        ' Create an array of strings. 
        Dim fruits() As String = _
            {"apple", "banana", "mango", "orange", "passionfruit", "grape"}

        ' Take strings from the array until one of 
        ' the strings matches "orange". 
        Dim query As IEnumerable(Of String) = _
            fruits.TakeWhile(Function(fruit) _
                                 String.Compare("orange", fruit, True) <> 0)

        ' 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 
        ' banana 
        ' mango

.NET Framework

Supported in: 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 Silverlight 8.1, Windows Phone Silverlight 8

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.

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