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

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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)

Syntax

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

public static IEnumerable<TSource> TakeWhile<TSource>(
    this IEnumerable<TSource> source,
    Func<TSource, int, bool> predicate
)

Type Parameters

  • TSource
    The type of the elements of source.

Parameters

  • predicate
    Type: System.Func<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<TSource>
An IEnumerable<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<TSource>. When you use instance method syntax to call this method, omit the first parameter.

Exceptions

Exception Condition
ArgumentNullException

source or predicate is nulla null reference (Nothing in Visual Basic).

Remarks

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<TSource>(IEnumerable<TSource>, Func<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.

Examples

The following code example demonstrates how to use TakeWhile<TSource>(IEnumerable<TSource>, Func<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
         output.AppendLine(fruit)
      Next
      outputBlock.Text &= output.ToString() & vbCrLf

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


      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)
      {
         outputBlock.Text += fruit + "\n";
      }

      /*
       This code produces the following output:

       apple
       passionfruit
       banana
       mango
       orange
       blueberry
      */

Version Information

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

Platforms

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