Aggregation Methods

Method

Description

SQL equivalents

Count, LongCount

Returns the number of elements in the input sequence, optionally satisfying a predicate

COUNT( )

Min, Max

Returns the smallest or largest element in the sequence

MIN( ), MAX( )

Sum, Average

Calculates a numeric sum or average over elements in the sequence

SUM( ), AVG ()

Aggregate

Performs a custom aggregation

Exception thrown

Count and LongCount

Argument

Type

Source sequence

IEnumerable<TSource>

Predicate (optional)

TSource => bool

Count simply enumerates over a sequence, returning the number of items:

	int fullCount = new int[] { 5, 6, 7}.Count();    // 3

The internal implementation of Enumerable.Count tests the input sequence to see whether it happens to implement ICollection<T>. If it does, it simply calls ICollection<T>.Count. Otherwise, it enumerates over every item, incrementing a counter.

You can optionally supply a predicate:

	int digitCount =
	  "pa55w0rd".Count (c => char.IsDigit (c));   // 3

LongCount does the same job as Count, but returns a 64-bit integer, allowing for sequences of greater than 2 billion elements.

Min and Max

Argument

Type

Source sequence

IEnumerable<TSource>

Result selector (optional)

TSource => TResult

Min and Max return the smallest or largest element from a sequence:

	int[] numbers = { 28, 32, 14 };
	int smallest = numbers.Min();    // 14;
	int largest  = numbers.Max();    // 32;

If you include a selector expression, each element is first projected:

	int smallest = numbers.Max (n => n % 10);   // 8;

A selector ...

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