EnumerableTakeWhile Method
 

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)

public static IEnumerable<TSource> TakeWhile<TSource>(
	this IEnumerable<TSource> source,
	Func<TSource, int, bool> predicate
)
public:
generic<typename TSource>
[ExtensionAttribute]
static IEnumerable<TSource>^ TakeWhile(
	IEnumerable<TSource>^ source,
	Func<TSource, int, bool>^ predicate
)
static member TakeWhile<'TSource> : 
        source:IEnumerable<'TSource> *
        predicate:Func<'TSource, int, bool> -> IEnumerable<'TSource>
<ExtensionAttribute>
Public Shared Function TakeWhile(Of TSource) (
	source As IEnumerable(Of TSource),
	predicate As Func(Of TSource, Integer, Boolean)
) As IEnumerable(Of TSource)

Parameters

source
Type:

The sequence to return elements from.

predicate
Type:

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:

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

Type Parameters

TSource

The type of the elements of source.

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

string[] fruits = { "apple", "passionfruit", "banana", "mango", 
                      "orange", "blueberry", "grape", "strawberry" };

IEnumerable<string> query =
    fruits.TakeWhile((fruit, index) => fruit.Length >= index);

foreach (string fruit in query)
{
    Console.WriteLine(fruit);
}

/*
 This code produces the following output:

 apple
 passionfruit
 banana
 mango
 orange
 blueberry
*/
' 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
    output.AppendLine(fruit)
Next
MsgBox(output.ToString())

' This code produces the following output:
'
' apple
' passionfruit
' banana
' mango
' orange
' blueberry
Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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