ToList with Capacity?

user_1291717 0
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When we do .ToList() for an IEnumerable, the list can potentially reallocate while scanning the IEnumerable because it doesn't know the size upfront. If the size is known, is there a simple way to avoid the performance penalty? Something to the effect of initializing a List with the required capacity and then copying the IEnumerable into it? Ideally something as simple as .ToList(capacity) (which doesn't exist).

c#
1 Answers

In cases when the capacity is part of IEnumerable<T> that is also an ICollection<T>, the library will allocate at the correct capacity.

Here is a reference implementation of List<T>(IEnumerable<T> source), which is invoked when you call ToList():

public List(IEnumerable<T> collection) {
    if (collection==null)
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.collection);
    Contract.EndContractBlock();

    ICollection<T> c = collection as ICollection<T>;
    if( c != null) {
        int count = c.Count;
        if (count == 0) {
            _items = _emptyArray;
        } else {
            _items = new T[count];
            c.CopyTo(_items, 0);
            _size = count;
        }
    } else {                
        _size = 0;
        _items = _emptyArray;
        // This enumerable could be empty.  Let Add allocate a new array, if needed.
        // Note it will also go to _defaultCapacity first, not 1, then 2, etc.

        using(IEnumerator<T> en = collection.GetEnumerator()) {
            while(en.MoveNext()) {
                Add(en.Current);                                    
            }
        }
    }
}

Note how the constructor behaves when collection implements ICollection<T>: rather than iterating the content and calling Add for each item, it allocates the internal _items array, and copies the content into it without reallocations.

In situations when the capacity is not embedded in class implementing IEnumerable<T>, you can easily define one yourself, using a combination of standard methods:

public static class ToListExtension {

    public static List<T> ToList<T>(this IEnumerable<T> source, int capacity) 
    {
        var res = new List<T>(capacity);
        res.AddRange(source);
        return res;
    }

}
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