Returns all distinct elements of the given source, where distinctiveness is determined by a transformation and an equality comparer for the transformed type.

In this page

• Definition
• Overloads
• DistinctBy(Func<TSource, TKey> transformer)
• DistinctBy(Func<TSource, TKey> transformer, IEqualityComparer? comparer)

Returns all distinct elements of the given source, where distinctiveness is determined by a transformation and the default equality comparer for the transformed type.

public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> transformer);

TSource
The type of the elements of the original sequences.

TKey
The type of the projected element.

source IEnumerable <TSource>
The source sequence.

transformer Func <TSource, TKey>
A transform function to apply to each element of the sequence.

IEnumerable<TResult>
A sequence consisting of distinct elements from the source, comparing them by the specified key transformation.

ArgumentNullException
source is null

-or-

transformer is null

The following code example demonstrates how to use DistinctBy(Func<TSource, TKey> transformer) to determine the distinct elements of a sequence.

string[] source = { "cat", "dog", "pig", "duck", "squid", "bird", "lamb", "frog", "whale" };

var distinct = source.DistinctBy(word => word.Length);
//=> [ "cat", "duck", "squid" ]

This operation uses deferred execution and streams the results, although a set of already-seen keys is retained. If a key is seen multiple times, only the first element with that key is returned.

Returns all distinct elements of the given source, where distinctiveness is determined by a transformation and the specified comparer for the transformed type.

public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> transformer, IEqualityComparer<TKey>? comparer);

TSource
The type of the elements of the original sequences.

TKey
The type of the projected element.

source IEnumerable <TSource>
The source sequence.

transformer Func <TSource, TKey>
A transform function to apply to each element of the sequence.

comparerIEqualityComparer
An equality comparer to compare values.

IEnumerable<TResult>
A sequence consisting of distinct elements from the source, comparing them by the specified key transformation.

ArgumentNullException
source is null

-or-

transformer is null

he following code example demonstrates how to use DistinctBy(Func<TSource, TKey> transformer, IEqualityComparer? comparer) to determine the distinct elements of a sequence. Distinctiveness is determined by the provided equality comparer.

string[] source = { "dog", "DOG", "duck", "duck", "whale" };
var distinct = source.DistinctBy(word => word, StringComparer.OrdinalIgnoreCase);
//=> [ "dog", "duck", "whale" ]

This operation uses deferred execution and streams the results, although a set of already-seen keys is retained. If a key is seen multiple times, only the first element with that key is returned.