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);

Program.cs

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)); // 1090 — total sales
Console.WriteLine(products.Average(p => p.Price)); // 116.666... — average price
Console.WriteLine(products.Max(p => p.Price)); // 150 — highest price

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}");

Run the following command:

dotnet run
6
485
80.8333333333333
95
60
4
1090
116.666666666667
150
Apple
Pass: 4 students, average 88.8
Fail: 2 students, average 65.0

Common Mistake: Calling Average() on an Empty Sequence Throws an Exception

Calling Average() on an empty sequence throws an InvalidOperationException. If the sequence may be empty, check with Any() first or use DefaultIfEmpty() to insert a fallback value.

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

// NG: calling Average() on an empty sequence
List<int> empty = new List<int>();
// double avg = empty.Average(); // InvalidOperationException: Sequence contains no elements.

The same logic can also be written as:

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

// OK: check with Any() before calling Average()
List<int> empty = new List<int>();
double avg1 = empty.Any() ? empty.Average() : 0.0;
Console.WriteLine(avg1); // 0

// OK: insert a default value with DefaultIfEmpty() before averaging
double avg2 = empty.DefaultIfEmpty(0).Average();
Console.WriteLine(avg2); // 0

Run the following command:

dotnet run
0
0

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().