Adding clones to a list is slow

Question

I'm trying to speed up a loop, which is cloning 2_500_000 objects. The clone itself is taking 800 ms for the entire loop but when I'm adding them to a list, it's taking 3 seconds..

List<T> list = new List<T>();

Stopwatch sw = new Stopwatch();
sw.Start();

foreach(T entity in listSource)
{
    T entityCloned = GetEntityClone(entity); // Taking 800ms totally

    if (entityCloned != null)
        list.Add(entityCloned);
}

sw.Stop();

Could you please help me to find out why those Adds are taking so much time?

Answer 1

I saved some time (around 33%) by using array instead of list:

MyObject class defitinion:

public class MyObject
{
    public int Id { get; set; }
    public bool Flag { get; set; }

    public static MyObject GetEntityClone(MyObject obj)
    {
        if (obj == null) return null;

        var newObj = new MyObject()
        {
            Id = obj.Id,
            Flag = obj.Flag
        };

        return newObj;
    }
}

Code:

var sourceList = new List<MyObject>();

// let's mock the source data, every 27th element will be null
for (int i = 0; i < 2500000; ++i)
{
    if (i % 27 != 0)
        sourceList.Add(new MyObject { Id = i, Flag = (i % 2 == 0) });
}

var destArray = new MyObject[2500000];

Stopwatch sw = new Stopwatch();
sw.Start();
Console.WriteLine(sw.ElapsedMilliseconds);

var currentElement = 0;
for (int i = 0; i < sourceList.Count; ++i)
{
    MyObject entityCloned = MyObject.GetEntityClone(sourceList[i]);

    if (entityCloned != null)
        destArray[currentElement++] = entityCloned;
}

var result = new MyObject[currentElement];
Array.Copy(destArray, 0, result, 0, currentElement);
sw.Stop();
Console.WriteLine(sw.ElapsedMilliseconds);