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Which is faster: clear collection or instantiate new

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遇见更好的自我
遇见更好的自我 2020-12-01 06:26

I have some number of generic lists in my code, that have tens or hundreds elements. Sometimes I need to refill this lists with other objects, so question is: what will be f

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  • 孤街浪徒
    孤街浪徒 (楼主)
    2020-12-01 06:39

    I've run this test:

    private static void Main(string[] args)
{
    int defaultN = 1000;

    Stopwatch sw = new Stopwatch();

    while (true)
    {
        Console.WriteLine("Enter test elements number:");
        int n;
        if (!int.TryParse(Console.ReadLine(), out n)) n = defaultN;
        else defaultN = n;

        Console.WriteLine($"Test with {n} elements");

        List list = Enumerable.Repeat(new object(), n).ToList();
        sw.Start();
        Clear(list);
        sw.Stop();
        Console.WriteLine("Clear: {0} ms", sw.ElapsedTicks / 10000D);

        GC.Collect();
        GC.WaitForPendingFinalizers();

        List list2 = Enumerable.Repeat(new object(), n).ToList();
        sw.Restart();
        Reinitialize(list2);
        sw.Stop();
        Console.WriteLine("Reinitialize: {0} ms", sw.ElapsedTicks / 10000D);

        GC.Collect();
        GC.WaitForPendingFinalizers();

        List list3 = Enumerable.Repeat(new object(), n).ToList();
        sw.Restart();
        ReinitializeAndCollect(list3);
        sw.Stop();
        Console.WriteLine("ReinitializeAndCollect: {0} ms", sw.ElapsedTicks / 10000D);

        Console.WriteLine("===");
    }
}
private static List Clear(List list)
{
    list.Clear();
    return list;
}
private static List Reinitialize(List list) => new List();
private static List ReinitializeAndCollect(List list)
{
    list = new List();

    GC.Collect();
    GC.WaitForPendingFinalizers();

    return list;
}



    My conclusion based on a results of my ordinary core i3 processor:
    


    In case of thousands of elements - it is better to clear list. It is fast and memory efficient.
    


    If collection has more than 100 000 elements - reinitializing becomes more attractive. If after profiling you think that there is a bottleneck here, use it. Re-initialization will be very fast, but as third method test shows, future garbage collecting will be about as slow as just clearing the list.
    


    So short answer is: if you didn't profiled your application, use Clear. Reusing objects is good. If you did - you already know what to do. 
    
    
    
             
    
                                                        
    
            
            
    
              
                
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