Method IndexBy

IndexBy<TSource, TKey>(IEnumerable<TSource>, Func<TSource, TKey>)

Applies a key-generating function to each element of a sequence and returns a sequence that contains the elements of the original sequence as well its key and index inside the group of its key.

Declaration

public static IEnumerable<(int index, TSource item)> IndexBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)

Parameters

Type	Name	Description
IEnumerable<TSource>	source	Source sequence.
Func<TSource, TKey>	keySelector	Function that projects the key given an element in the source sequence.

Returns

Type	Description
IEnumerable<(int Index, TSource Item)>	A sequence of elements paired with their index within the key-group. The index is the key and the element is the value of the pair.

Type Parameters

Name	Description
TSource	Type of the source sequence elements.
TKey	Type of the projected key.

Remarks

This operator uses deferred execution and streams its results.

Examples

The following code example demonstrates how to return a sub-sequence index with each element using IndexBy.

var sequence = new[]
{
    new { Key = 1, Value = 123, },
    new { Key = 2, Value = 987, },
    new { Key = 3, Value = 789, },
    new { Key = 1, Value = 123, },
    new { Key = 1, Value = 781, },
};

// Group adjacent items
var result = sequence
    .IndexBy(
        x => x.Key);

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

// This code produces the following output:
// [ (0, { Key = 1, Value = 123 }), (0, { Key = 2, Value = 987 }), (0, { Key = 3, Value = 789 }), (1, { Key = 1, Value = 123 }), (2, { Key = 1, Value = 781 }) ]

Exceptions

Type	Condition
ArgumentNullException	source or keySelector is null.

IndexBy<TSource, TKey>(IEnumerable<TSource>, Func<TSource, TKey>, IEqualityComparer<TKey>?)

Applies a key-generating function to each element of a sequence and returns a sequence that contains the elements of the original sequence as well its key and index inside the group of its key. An additional parameter specifies a comparer to use for testing the equivalence of keys.

Declaration

public static IEnumerable<(int index, TSource item)> IndexBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? comparer)

Parameters

Type	Name	Description
IEnumerable<TSource>	source	Source sequence.
Func<TSource, TKey>	keySelector	Function that projects the key given an element in the source sequence.
IEqualityComparer<TKey>	comparer	The equality comparer to use to determine whether or not keys are equal. If null, the default equality comparer for TSource is used.

Returns

Type	Description
IEnumerable<(int Index, TSource Item)>	A sequence of elements paired with their index within the key-group. The index is the key and the element is the value of the pair.

Type Parameters

Name	Description
TSource	Type of the source sequence elements.
TKey	Type of the projected key.

Remarks

This operator uses deferred execution and streams its results.

Examples

The following code example demonstrates how to return a sub-sequence index with each element using IndexBy.

var sequence = new[]
{
    new { Key = "jan", Value = 123, },
    new { Key = "feb", Value = 987, },
    new { Key = "mar", Value = 789, },
    new { Key = "Jan", Value = 123, },
    new { Key = "JAN", Value = 781, },
};

// Group adjacent items
var result = sequence
    .IndexBy(
        x => x.Key,
        StringComparer.OrdinalIgnoreCase);

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

// This code produces the following output:
// [ (0, { Key = jan, Value = 123 }), (0, { Key = feb, Value = 987 }), (0, { Key = mar, Value = 789 }), (1, { Key = Jan, Value = 123 }), (2, { Key = JAN, Value = 781 }) ]

Exceptions

Type	Condition
ArgumentNullException	source or keySelector is null.