This documentation is archived and is not being maintained.

Enumerable.TakeWhile(Of TSource) Method (IEnumerable(Of TSource), Func(Of TSource, Int32, Boolean))

Returns elements from a sequence as long as a specified condition is true. The 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 TakeWhile(Of TSource) ( _
	source As IEnumerable(Of TSource), _
	predicate As Func(Of TSource, Integer, Boolean) _
) As IEnumerable(Of TSource)

Type Parameters


The type of the elements of source.


Type: System.Collections.Generic.IEnumerable(Of TSource)
The sequence to return elements from.
Type: System.Func(Of 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(Of TSource)
An IEnumerable(Of T) that contains 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).


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, Int32, 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 first argument of predicate represents the element to test. The second argument represents the zero-based index of the element within source.

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, Int32, Boolean)) to return elements from the start of a sequence as long as a condition that uses the element's index is true.

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

        ' Take strings from the array until one
        ' of the string's lengths is greater than or
        ' equal to the string item's index in the array.
        Dim query As IEnumerable(Of String) = _
            fruits.TakeWhile(Function(fruit, index) _
                                 fruit.Length >= index)

        ' Display the results.
        Dim output As New System.Text.StringBuilder
        For Each fruit As String In query

        ' This code produces the following output:
        ' apple
        ' passionfruit
        ' banana
        ' mango
        ' orange
        ' blueberry

.NET Framework

Supported in: 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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