Count elements

Think of a C# array. It has many elements—but how can we get this number? An array's elements can be counted in several ways.

Collections usually store their count in a field—this influences the performance of counting. There are some complexities with the syntax and speed of counting.

Arrays

For an array, we need to use Length (not Count). In programs, an array's size is set differently than List or ArrayList. The size does not grow dynamically.

using System;

string[] roomItems = new string[3];
roomItems[0] = "cat";
roomItems[1] = "apple";
roomItems[2] = "frog";
// Count with Length property.
Console.WriteLine(roomItems.Length);
3

ArrayList

Here we get the number of elements in an ArrayList, which is a non-generic collection that stores its length in a property called Count.

using System;
using System.Collections;

ArrayList items = new ArrayList();
items.Add("One");
items.Add("Two");

// Count the elements in the ArrayList.
int result = items.Count;
Console.WriteLine(result);
2

List

Here we use List, a generic collection. We must specify its type in the angle brackets. These collections have superior performance and usually preferable.

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

List<int> list = new List<int>();
list.Add(1);
list.Add(2);

// Part 1: count with the Count property.
int result = list.Count;
Console.WriteLine(result);

// Part 2: count with the extension method.
result = list.Count();
Console.WriteLine(result);
2
2

Count extension

You can use the Count() extension method when you have an IEnumerable collection or are using LINQ statements. But be careful with this method.

using System;
using System.Linq;

string[] array = new string[]
{
    "One",
    "Two",
    "Three"
};

// An enumerable collection, not in memory yet.
var e = from s in array
        select s;

int c = e.Count(); // Extension method.
Console.WriteLine(c);
// Work is now done to get the count.
3

Benchmark, Count

Let us benchmark the Count() method. In my benchmark, where I took the Count of an array many times, the Count() extension performed worse.

using System;
using System.Diagnostics;
using System.Linq;

const int _max = 1000000;
var array = new int[100];
// Version 1: access Count() on array.
var s1 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    if (array.Count() != 100)
    {
        return;
    }
}
s1.Stop();
// Version 2: access Length on array.
var s2 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    if (array.Length != 100)
    {
        return;
    }
}
s2.Stop();
Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
32.49 ns    Count()
0.79 ns     Length

Hoist

In the C# language, the Length or Count property is often evaluated each time a for-loop is run. It can be faster to hoist the Count or Length check out of the loop.

using System;
using System.Collections.Generic;

var values = new List<int>() { 10, 20 };

// Loop over values, no hoisting.
for (int i = 0; i < values.Count; i++)
{
    Console.WriteLine($"VALUE: {values[i]}");
}

// Loop over values, with hoisting of max.
int max = values.Count;
for (int i = 0; i < max; i++)
{
    Console.WriteLine($"VALUE: {values[i]}");
}
VALUE: 10
VALUE: 20
VALUE: 10
VALUE: 20

Summary

We computed the element count of a List or array. And we saw some benchmarks of the extension method that also is named Count. You will need to use Count in most for-loops.