Efficiency of List creation from array

Question

I need to create a list starting from an array already created immediately before and will only be converted to the list. So I could harness the array to the list without making a copy, but the constructor makes a copy. I can even understand the motivation for this. However there are cases that I can guarantee that the array does not have and will have no reference to it other than where it was created from.

Is there any way to make this construction more efficient and use the array internally in the list? I know there are implications if I misuse it.

The most obvious example for this is to get the result of a string.Split(). If you need a list your only obvious way out would be to do this conversion. For now I'm not considering writing a method to split directly into a list.

Try : string.Split().ToList() so you do not create the array. — jdweng
– jdweng, Commented Jan 24, 2019 at 18:32
Yes, but it doesn't create a variable. So later in your code you don't have to create the list as a 2nd variable. — jdweng
– jdweng, Commented Jan 24, 2019 at 18:52
I think you cannot avoid the copy of an array. Look at stackoverflow.com/questions/54298050/…. — Johnny
– Johnny, Commented Jan 24, 2019 at 19:06

Johnny · Accepted Answer · 2019-01-25 07:22:47Z

2

As far as I know, there is no official way to do that but it is still possible using System.Reflection. By looking at the source code of the List<T>, .NET Framework 4.7.2, the two important properties are _items and _size. There is also _version but that one changes only when you modify List<T>. Modification are Add, AddRange, Remove, etc. but also Reverse and Sort. So let's assume this is the same operation as creating the list from IEnumerable<T> where _version stays zero.

public static class ListExtensions
{
    public static void SetUnderlyingArray<T>(this List<T> list, T[] array)
    {
        lock (list)
        {
            SetInternalArray(list, array);
            SetInternalArraySize(list, array.Length);
        }
    }

    private static void SetInternalArraySize<T>(this List<T> list, int size)
    {
        var prop = list.GetType().GetField(
            "_size", 
            BindingFlags.NonPublic | BindingFlags.Instance);
        prop.SetValue(list, size);
    }

    private static void SetInternalArray<T>(this List<T> list, T[] array)
    {
        var prop = list.GetType().GetField(
            "_items",
            BindingFlags.NonPublic | BindingFlags.Instance);
        prop.SetValue(list, array);
    }
}

and then set the underlying array

int[] array = Enumerable.Repeat(1, 1000000).ToArray();
List<int> list = new List<int>();

list.SetUnderlyingArray(array);

Note This solution is highly dependent on the details of the implementation and might be wrong if something change in the List<T> internals but it gives insight on how it could be accomplished.

edited Jan 25, 2019 at 7:22

answered Jan 24, 2019 at 20:36

Johnny

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2 Comments

AArteDoCodigo.com.br - Maniero Over a year ago

I will try. I don´t want this solution exactly because it is detail dependent. But probably this is the only option. If _items were protected it would be ease.

Johnny Over a year ago

@Maniero Exactly. I stated that dependency I am aware of that and I do understand your concern...

nloewen · Accepted Answer · 2019-01-24 20:51:54Z

1

If you don't specifically need a List<T> You could create a new class that implements IList<T> and doesn't make a copy.

answered Jan 24, 2019 at 20:51

nloewen

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3 Comments

AArteDoCodigo.com.br - Maniero Over a year ago

This is an option if everything fails, I thought about that, it-s a radical way, but it's a valid one.

nloewen Over a year ago

I'd say it's less radical than a reflection approach. There's no dependency on the implementation of List<T>

AArteDoCodigo.com.br - Maniero Over a year ago

Yep, reflection is radical too

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