Split a List into smaller lists of N size

Question

I am attempting to split a list into a series of smaller lists.

My Problem: My function to split lists doesn\'t split them into lists of the correct

Answer 1

Based on Dimitry Pavlov answere I would remove .ToList(). And also avoid the anonymous class. Instead I like to use a struct which does not require a heap memory allocation. (A ValueTuple would also do job.)

public static IEnumerable> ChunkBy(this IEnumerable source, int chunkSize)
{
    if (source is null)
    {
        throw new ArgumentNullException(nameof(source));
    }
    if (chunkSize <= 0)
    {
        throw new ArgumentOutOfRangeException(nameof(chunkSize), chunkSize, "The argument must be greater than zero.");
    }

    return source
        .Select((x, i) => new ChunkedValue(x, i / chunkSize))
        .GroupBy(cv => cv.ChunkIndex)
        .Select(g => g.Select(cv => cv.Value));
} 

[StructLayout(LayoutKind.Auto)]
[DebuggerDisplay("{" + nameof(ChunkedValue.ChunkIndex) + "}: {" + nameof(ChunkedValue.Value) + "}")]
private struct ChunkedValue
{
    public ChunkedValue(T value, int chunkIndex)
    {
        this.ChunkIndex = chunkIndex;
        this.Value = value;
    }

    public int ChunkIndex { get; }

    public T Value { get; }
}

This can be used like the following which only iterates over the collection once and also does not allocate any significant memory.

int chunkSize = 30;
foreach (var chunk in collection.ChunkBy(chunkSize))
{
    foreach (var item in chunk)
    {
        // your code for item here.
    }
}

If a concrete list is actually needed then I would do it like this:

int chunkSize = 30;
var chunkList = new List>();
foreach (var chunk in collection.ChunkBy(chunkSize))
{
    // create a list with the correct capacity to be able to contain one chunk
    // to avoid the resizing (additional memory allocation and memory copy) within the List.
    var list = new List(chunkSize);
    list.AddRange(chunk);
    chunkList.Add(list);
}