Method IndexBy

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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.
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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.