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.

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
      outputBlock.Text &= output.ToString() & vbCrLf

      ' This code produces the following output:
      '
      ' apple
      ' banana
      ' mango



Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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