Method CountDown

CountDown<TSource>(IAsyncEnumerable<TSource>, int)

Provides a countdown counter for a given count of elements at the tail of the sequence where zero always represents the last element, one represents the second-last element, two represents the third-last element and so on.

Declaration

public static IAsyncEnumerable<(TSource item, int? count)> CountDown<TSource>(this IAsyncEnumerable<TSource> source, int count)

Parameters

Type	Name	Description
IAsyncEnumerable<TSource>	source	The source sequence.
int	count	Count of tail elements of source to count down.

Returns

Type	Description
IAsyncEnumerable<(TSource item, int? count)>	A sequence of tuples of the element and it's count from the end of the sequence.

Type Parameters

Name	Description
TSource	The type of elements of source

Remarks

This method uses deferred execution semantics and streams its results. At most, count elements of the source sequence may be buffered at any one time unless source is a collection or a list.

This operator executes immediately.

Exceptions

Type	Condition
ArgumentNullException	source is null.
ArgumentOutOfRangeException	count is 0 or negative.

CountDown<TSource, TResult>(IAsyncEnumerable<TSource>, int, Func<TSource, int?, TResult>)

Provides a countdown counter for a given count of elements at the tail of the sequence where zero always represents the last element, one represents the second-last element, two represents the third-last element and so on.

Declaration

public static IAsyncEnumerable<TResult> CountDown<TSource, TResult>(this IAsyncEnumerable<TSource> source, int count, Func<TSource, int?, TResult> resultSelector)

Parameters

Type	Name	Description
IAsyncEnumerable<TSource>	source	The source sequence.
int	count	Count of tail elements of source to count down.
Func<TSource, int?, TResult>	resultSelector	A function that receives the element and the current countdown value for the element and which returns those mapped to a result returned in the resulting sequence. For elements before the last count, the countdown value is null.

Returns

Type	Description
IAsyncEnumerable<TResult>	A sequence of results returned by resultSelector.

Type Parameters

Name	Description
TSource	The type of elements of source
TResult	The type of elements of the resulting sequence.

Remarks

This method uses deferred execution semantics and streams its results. At most, count elements of the source sequence may be buffered at any one time unless source is a collection or a list.

This operator executes immediately.

Exceptions

Type	Condition
ArgumentNullException	source or resultSelector is null.
ArgumentOutOfRangeException	count is 0 or negative.