Method TakeUntil

TakeUntil<TSource>(IAsyncEnumerable<TSource>, Func<TSource, bool>)

Returns items from the input sequence until the given predicate returns true when applied to the current source item; that item will be the last returned.

Declaration

public static IAsyncEnumerable<TSource> TakeUntil<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate)

Parameters

Type	Name	Description
IAsyncEnumerable<TSource>	source	Source sequence
Func<TSource, bool>	predicate	Predicate used to determine when to stop yielding results from the source.

Returns

Type	Description
IAsyncEnumerable<TSource>	Items from the source sequence, until the predicate returns true when applied to the item.

Type Parameters

Name	Description
TSource	Type of the source sequence

Remarks

TakeUntil differs from Enumerable.TakeWhile in two respects. Firstly, the sense of the predicate is reversed: it is expected that the predicate will return false to start with, and then return true - for example, when trying to find a matching item in a sequence.

Secondly, TakeUntil yields the element which causes the predicate to return true. For example, in a sequence

{ 1, 2, 3, 4, 5 }

and with a predicate of

x => x == 3

, the result would be

{ 1, 2, 3 }

.

TakeUntil is as lazy as possible: it will not iterate over the source sequence until it has to, it won't iterate further than it has to, and it won't evaluate the predicate until it has to. (This means that an item may be returned which would actually cause the predicate to throw an exception if it were evaluated, so long as no more items of data are requested.)

Exceptions

Type	Condition
ArgumentNullException	source or predicate is null