Method AggregateBy

AggregateBy<TSource, TKey, TAccumulate>(IEnumerable<TSource>, Func<TSource, TKey>, TAccumulate, Func<TAccumulate, TSource, TAccumulate>, IEqualityComparer<TKey>?)

Applies a key-generating function to each element of a sequence and returns an aggregate value for each key. An additional argument specifies a comparer to use for testing equivalence of keys.

Declaration

public static IEnumerable<KeyValuePair<TKey, TAccumulate>> AggregateBy<TSource, TKey, TAccumulate>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func, IEqualityComparer<TKey>? comparer = null) where TKey : notnull

Parameters

Type	Name	Description
IEnumerable<TSource>	source	Source sequence.
Func<TSource, TKey>	keySelector	Function that transforms each item of source sequence into a key to be compared against the others.
TAccumulate	seed	The initial accumulator value for each key-group.
Func<TAccumulate, TSource, TAccumulate>	func	An accumulator function to be invoked on each element. The accumulator value is tracked separately for each key.
IEqualityComparer<TKey>	comparer	The equality comparer to use to determine whether or not keys are equal. If null, the default equality comparer for TKey is used.

Returns

Type	Description
IEnumerable<KeyValuePair<TKey, TAccumulate>>	A sequence of unique keys and their accumulated value.

Type Parameters

Name	Description
TSource	Type of the elements of the source sequence.
TKey	Type of the projected element.
TAccumulate	Type of the accumulator value.

Remarks

This method is implemented by using deferred execution. The operator will be executed in it's entirety immediately when the sequence is first enumerated.

Examples

The following code example demonstrates how to aggregate values by key in a sequence, using AggregateBy.

var sequence = Enumerable.Range(1, 19);
            
// Aggregate elements in a sequence grouped by key
var result = sequence
    .AggregateBy(
        x => x % 3,
        0,
        (acc, e) => acc + e);

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

// This code produces the following output:
// [[1, 70], [2, 57], [0, 63]]

Exceptions

Type	Condition
ArgumentNullException	source, keySelector, or func is null.

AggregateBy<TSource, TKey, TAccumulate>(IEnumerable<TSource>, Func<TSource, TKey>, Func<TKey, TAccumulate>, Func<TAccumulate, TSource, TAccumulate>, IEqualityComparer<TKey>?)

Applies a key-generating function to each element of a sequence and returns an aggregate value for each key. An additional argument specifies a comparer to use for testing equivalence of keys.

Declaration

public static IEnumerable<KeyValuePair<TKey, TAccumulate>> AggregateBy<TSource, TKey, TAccumulate>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TKey, TAccumulate> seedSelector, Func<TAccumulate, TSource, TAccumulate> func, IEqualityComparer<TKey>? comparer = null) where TKey : notnull

Parameters

Type	Name	Description
IEnumerable<TSource>	source	Source sequence.
Func<TSource, TKey>	keySelector	Function that transforms each item of source sequence into a key to be compared against the others.
Func<TKey, TAccumulate>	seedSelector	A function that returns the initial seed for each key.
Func<TAccumulate, TSource, TAccumulate>	func	An accumulator function to be invoked on each element. The accumulator value is tracked separately for each key.
IEqualityComparer<TKey>	comparer	The equality comparer to use to determine whether or not keys are equal. If null, the default equality comparer for TKey is used.

Returns

Type	Description
IEnumerable<KeyValuePair<TKey, TAccumulate>>	A sequence of unique keys and their accumulated value.

Type Parameters

Name	Description
TSource	Type of the elements of the source sequence.
TKey	Type of the projected element.
TAccumulate	Type of the accumulator value.

Remarks

This method is implemented by using deferred execution. The operator will be executed in it's entirety immediately when the sequence is first enumerated.

Examples

The following code example demonstrates how to aggregate values by key in a sequence, using AggregateBy.

var sequence = Enumerable.Range(1, 19);
            
// Aggregate elements in a sequence grouped by key
var result = sequence
    .AggregateBy(
        x => x % 3,
        k => k * 1_000,
        (acc, e) => acc + e);

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

// This code produces the following output:
// [[1, 1070], [2, 2057], [0, 63]]

Exceptions

Type	Condition
ArgumentNullException	source, seedSelector, keySelector, or func is null.