InternalList<T>: the low-level List<T>


For some of us who write low-level code that is intended to be compact and fast, the standard List<T> can seem like overkill. It requires two separate heap objects, and must perform extra checks that a plain array does not. For that reason, if you’re writing code that you want to be as fast as possible, you might consider using a T[] array instead of List<T>.

The downside to List<T>

From a performance perspective, there are two things that make a List<T> “worse” than a plain array: size, and speed.


A List<T> requires at least two heap allocations: one for the List<T> object itself, and another for the array backing the List<T>. Each .NET object has a two-word header, and List<T> has four fields:

private T[] _items;
private int _size;
private object _syncRoot; // not used unless you call SyncRoot
private int _version;

So List<T> requires six additional words of memory compared to an array (I mean “machine words” in the traditional sense, meaning an IntPtr-sized memory area, so the total is 24 extra bytes in a 32-bit process and probably 40 bytes in 64-bit, assuming the CLR fits _size and _version into a single word.)


List<T> must also perform extra range checks. For example, the indexer looks like this:

public T this[int index]
    get {
        if ((uint)index >= (uint)_size)
        return _items[index];
    set {
        if ((uint)index >= (uint)_size)
        _items[index] = value;

Note that the _items[index] operation implicitly contains a range check: the CLR performs a check equivalent to (uint)index < (uint)_items.Length before it actually reads from an array or writes to it. So there are actually two range checks here. The JIT usually does not know that _size < _items.Length so it cannot remove the second check based on the result of the first check.

The downside to T[]

Unfortunately, if you use a plain array instead, you can’t simply “add” or “remove” items, since an array has a single fixed size. Consequently if you decide that you really want a plain array but you need to add, remove, or (heaven forbid!) insert items, you’ll end up pretty much reimplementing List<T> by yourself. You’ll have an array variable and a count variable…

private T[] _array;
private int _count;

and then you might write a bunch of code to do the things that List<T> already does, like insertions:

static T[] Insert<T>(int index, T item, T[] array, ref int count)
    Debug.Assert((uint)index <= (uint)count);
    if (count == array.Length) {
        int newCap = array.Length * 2;
        array = CopyToNewArray(array, count, newCap);
    for (int i = count; i > index; i--)
        array[i] = array[i - 1];
    array[index] = item;
    return array;
static T[] CopyToNewArray<T>(T[] _array, int _count, int newCapacity)
    T[] a = new T[newCapacity];
    Array.Copy(_array, a, _count);

Of course, this road leads to madness. Luckily, you never need to write code like this: just use InternalList<T> instead!

Introducing InternalList

InternalList<T> is a drop-in substitute for List<T> defined like this:

public struct InternalList<T> : IListAndListSource<T>, 
              IListRangeMethods<T>, ICloneable<InternalList<T>>
    public static readonly T[] EmptyArray = new T[0];
    public static readonly InternalList<T> Empty = new InternalList<T>(0);

    private T[] _array;
    private int _count;

    public InternalList(int capacity) {...}
    public InternalList(T[] array, int count) { _array=array; _count=count; }
    public InternalList(IEnumerable<T> items) : this(items.GetEnumerator()) {}
    public InternalList(IEnumerator<T> items) {}

    public int Count {...}
    public int Capacity {...}
    public void Resize(int newSize, bool allowReduceCapacity = true) {...}
    public void Add(T item) {...}

To eliminate the extra memory required by List<T>, InternalList is a struct rather a class; and for maximum performance, it asserts rather than throwing an exception when an incorrect array index is used, so that Release builds (where Debug.Assert disappears) run as fast as possible.

InternalList also has an InternalArray property that provides access to the internal array. This actually allows you to work around certain pesky problems with the ordinary List<T>. For example, an ordinary List doesn't allow you to do this:

List<Point> pts;
// error CS1612: Cannot modify the return value of 'List<Point>.this[int]' because it is not a variable
pts[0].X = 5;

But if pts is an InternalList then you can write pts.InternalArray[0].X = 5;.

InternalList<T> has other things that List<T> doesn’t, such as a Resize() method (and an equivalent setter for Count), and a handy Last property to get or set the last item.

But it should be understood that InternalList is only meant for rare cases where you need better performance than List<T>. It does have major disadvantages:

  1. You must not write new InternalList<T>() because C# does not support struct default constructors and InternalList<T> requires non-null initialization; methods such as Add(), Insert() and Resize() assume _array is not null. The correct initialization is InternalList<T> list = InternalList<T>.Empty;

  2. Passing this structure by value is dangerous because changes to a copy of the structure may or may not be reflected in the original list. In particular the _count of the original list won’t change but the contents of _array may change. It’s best not to pass it around at all, but if you must pass it, pass it by reference. This also implies that an InternalList<T> should not be exposed by any public API, and storing InternalList<T> inside another collection (e.g. Dictionary<object, InternalList<T>> can be done but must be done carefully to avoid code that compiles but doesn’t work as intended.

Again, the fundamental problem is that when you pass InternalList by value, a copy of the _count and _array variables is made. Changes to those variables do not affect the other copies, but changes to the elements of _array do affect other copies (incidentally, this is similar to the mutation behavior of slices in D). If you want to return an internal list from a public API you can cast it to IList<T> or IReadOnlyList<T>, but be aware that future changes made to the InternalList by your code may not be seen properly by clients using the IList<T>, and vice versa.

Finally, alongside InternalList<T>, there is a static class InternalList that has some static methods (CopyToNewArray, Insert, RemoveAt, Move to help manage raw arrays. Most methods of InternalList<T> simply call methods of InternalList.


The main purpose of InternalList<T> is to save memory, but occasionally you’ll notice a speed difference.

I temporarily tweaked my list benchmarks from the List Trifecta series. Generally InternalList<T> performs identically to List<T> except that this linear scan for-loop can read the list 55% faster (i.e. 34% less time).

long sum = 0;
for (int c = 0; c < Cycles; c++) {
	sum = 0;
	for (int i = 0; i < list.Count; i++)
		sum += list[i];

Scan by index results

The IEnumerator is (of course) not faster but not notably slower. However, notice that scanning via IEnumerator takes a full 10 times as much time as a plain for loop (a foreach loop on InternalList<T> L may be much faster than a foreach loop on (IEnumerable<T>)L, but I didn’t benchmark that, sorry. I’m kinda busy.)

double avg = 0;
for (int c = 0; c < Cycles; c++)
	avg = list.Average(); // uses IEnumerator<long>


“Filling” is slightly slower if you don’t set Capacity in advance because InternalList<T> enlarges itself 23.9% more often in order to save a little memory. Note: the graph is almost meaningless because the timer resolution is too poor and because the most expensive part of Add is enlarging the internal array; List and InternalList do this at different sizes and there aren’t enough data points to definitively see what is going on.


The other results are pretty much the same as List<T>.

results results


InternalList<T> is part of Loyc.Essentials.dll, which is in the Loyc.Essentials NuGet package.

You can see the original source code here but it cannot be copied directly into another project since it references a couple of external things in Loyc.Essentials.dll. Therefore, I made this standalone version. Enjoy!