List, Clear

Should you clear a List to reuse it, or should you just allocate a new List? We test whether the Clear() method helps improve code that uses List.

It seems likely that clearing and reusing an existing list would result in better performance. This hypothesis can be tested.

Example

Here we create a List, and call Add() twice to append 2 elements to the List. The List has 2 elements at that point. We call Clear(), and then it has 0 elements.

using System;
using System.Collections.Generic;

List<int> ids = new List<int>();
ids.Add(10);
ids.Add(11);
Console.WriteLine("COUNT: {0}", ids.Count);
// Do not assign anything to clear.
ids.Clear();
Console.WriteLine("COUNT: {0}", ids.Count);
COUNT: 2
COUNT: 0

Benchmark, clear

All code that uses a List must first allocate it, or clear an existing List to reuse that one. Consider this benchmark program. The program tests 2 methods.

using System;
using System.Collections.Generic;
using System.Diagnostics;

const int _max = 1000000;
// ... New list.
var list = new List<int>(100);

// Version 1: clear 1 list and reuse it many times.
var s1 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    list.Clear();
    for (int x = 0; x < 100; x++)
    {
        list.Add(x);
    }
}
s1.Stop();
// Version 2: create a new list on each use.
var s2 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    list = new List<int>(100);
    for (int x = 0; x < 100; x++)
    {
        list.Add(x);
    }
}
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"));
156.06 ns    List Clear
182.58 ns    new List

Notes, results

In the results, calling Clear() on the List was faster. The Array.Clear method has undergone performance optimizations.

We tested the List Clear method. This method clears the List's contents and adjusts the Count to zero after it is called.