Enumerable.Count() / Sum() / Average()

using System.Linq;

int    count   = source.Count();
int    count2  = source.Count(x => condition);
long   longCnt = source.LongCount();

int    sum     = source.Sum();
double avg     = source.Average();
T      max     = source.Max();
T      min     = source.Min();

// You can also pass a selector to aggregate transformed values.
int    sumKey  = source.Sum(x => x.Price);
double avgKey  = source.Average(x => x.Score);

using System;
using System.Collections.Generic;
using System.Linq;

List<int> scores = new List<int> { 80, 95, 70, 88, 60, 92 };

// Basic aggregation
Console.WriteLine(scores.Count());    // 6
Console.WriteLine(scores.Sum());      // 485
Console.WriteLine(scores.Average());  // 80.833...
Console.WriteLine(scores.Max());      // 95
Console.WriteLine(scores.Min());      // 60

// Conditional count (number of scores >= 80)
Console.WriteLine(scores.Count(s => s >= 80)); // 4

// Aggregate a list of objects using a selector.
var products = new List<(string Name, int Price, int Qty)>
{
    ("Apple",   150, 3),
    ("Orange",   80,  5),
    ("Banana",   120, 2),
};
Console.WriteLine(products.Sum(p => p.Price * p.Qty));    // 1210 — total sales
Console.WriteLine(products.Average(p => p.Price));        // 116.666... — average price
Console.WriteLine(products.Max(p => p.Price));            // 150 — highest price

// .NET 6 and later: use MaxBy to get the most expensive product object.
var mostExpensive = products.MaxBy(p => p.Price);
Console.WriteLine(mostExpensive.Name); // Apple

// Aggregate by group.
var grouped = scores.GroupBy(s => s >= 80 ? "Pass" : "Fail");
foreach (var g in grouped)
    Console.WriteLine($"{g.Key}: {g.Count()} students, average {g.Average():F1}");

Methods such as Sum() and Average() use immediate evaluation — they traverse the entire sequence as soon as they are called. Calling them one after another causes multiple traversals, so for large data sets it is more efficient to materialize the sequence into a list first.

Average() throws an InvalidOperationException if the sequence is empty. If the sequence may be empty, check with Any() beforehand or combine it with DefaultIfEmpty().

If you only need to check whether any element matches a condition, Enumerable.Any(predicate) is more efficient than Count(predicate) > 0. To aggregate by group, combine these methods with Enumerable.GroupBy().