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

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

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

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

Type Parameters

TSource

The type of the elements of source.

Parameters

source
Type: System.Collections.Generic.IEnumerable<TSource>

A sequence to return elements from.

predicate
Type: System.Func<TSource, Boolean>

A function to test each element for a condition.

Return Value

Type: System.Collections.Generic.IEnumerable<TSource>
An IEnumerable<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<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 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<TSource>(IEnumerable<TSource>, Func<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<TSource>(IEnumerable<TSource>, Func<TSource, Boolean>) to return elements from the start of a sequence as long as a condition is true.

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

            IEnumerable<string> query =
                fruits.TakeWhile(fruit => String.Compare("orange", fruit, true) != 0);

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

            /*
             This code produces the following output:

             apple
             banana
             mango
            */

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Portable Class Library

Supported in: Portable Class Library
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