Method TagFirstLast

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TagFirstLast<TSource>(IEnumerable<TSource>)

Returns a sequence of tuples containing the element and flags indicating whether the element is the first and/or last of the sequence.

Declaration
public static IEnumerable<(TSource item, bool isFirst, bool isLast)> TagFirstLast<TSource>(this IEnumerable<TSource> source)
Parameters
Type Name Description
IEnumerable<TSource> source

The source sequence.
Returns
Type Description
IEnumerable<(TSource item, bool isFirst, bool isLast)>

Returns the resulting sequence.
Type Parameters
Name Description
TSource

The type of the elements of source.
Remarks

This operator uses deferred execution and streams its results.

Examples

The following code example demonstrates how tag elements of a sequence with information on whether it is the first or last element of the sequence using TagFirstLast.

var sequence = Enumerable.Range(1, 4);

// Replace a value in a sequence
var result = sequence
    .TagFirstLast();

Console.WriteLine(
    "[" +
    string.Join(", ", result) +
    "]");

// This code produces the following output:
// [(1, True, False), (2, False, False), (3, False, False), (4, False, True)]
Exceptions
Type Condition
ArgumentNullException

source is null.
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TagFirstLast<TSource, TResult>(IEnumerable<TSource>, Func<TSource, bool, bool, TResult>)

Returns a sequence resulting from applying a function to each element in the source sequence with additional parameters indicating whether the element is the first and/or last of the sequence.

Declaration
public static IEnumerable<TResult> TagFirstLast<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, bool, bool, TResult> resultSelector)
Parameters
Type Name Description
IEnumerable<TSource> source

The source sequence.
Func<TSource, bool, bool, TResult> resultSelector

A function that determines how to project the each element along with its first or last tag.
Returns
Type Description
IEnumerable<TResult>

Returns the resulting sequence.
Type Parameters
Name Description
TSource

The type of the elements of source.
TResult

The type of the element of the returned sequence.
Remarks

This operator uses deferred execution and streams its results.

Examples

The following code example demonstrates how tag elements of a sequence with information on whether it is the first or last element of the sequence using TagFirstLast.

var sequence = Enumerable.Range(1, 4);

// Replace a value in a sequence
var result = sequence
    .TagFirstLast(
        (item, first, last) => new 
        { 
            Item = item, 
            IsEdge = first || last, 
        });

Console.WriteLine(
    "[" +
    string.Join(", ", result) +
    "]");

// This code produces the following output:
// [{ Item = 1, IsEdge = True }, { Item = 2, IsEdge = False }, { Item = 3, IsEdge = False }, { Item = 4, IsEdge = True }]
Exceptions
Type Condition
ArgumentNullException

source or resultSelector is null.