Array indexer signature returns object - does it box?

Question

I stumbled on the fact that the indexer this[int index] { get; } works differently for an array of structs than it does for a List of structs. Namely, that the indexer in the case of an T[] returns a reference to the element within the array whereas the indexer in the case of a List<T> returns a copy of the element.

This is a very big semantic and performance difference, and I'm glad that T[] allows us to work around the performance limitation of List<T>.

However, I'm puzzled by the actual implementation. The code for Array in the .net reference sources reads thus:

Object IList.this[int index] {
    get { return GetValue(index); }
    set { SetValue(value, index); }
}

Where GetValue is defined as follows:

public unsafe Object GetValue(int index)
{
    if (Rank != 1)
       throw new ArgumentException(Environment.GetResourceString("Arg_Need1DArray"));
    Contract.EndContractBlock();
    TypedReference elemref = new TypedReference();
    InternalGetReference(&elemref, 1, &index);
    return TypedReference.InternalToObject(&elemref);
}

The return type of the indexer is Object, implying that boxing will take place.

So my question is, can I be certain that no boxing will occur when I access an element of a T[] where T is a struct?

I assume that the compiler and/or CLR treat Array specially, and don't actually bother with the signature of the indexer. Is this correct? Is there a fuller discussion of this somewhere?

IList.SomeMethod is an implicit intefrace implementation. It's a way of telling "this method exists if object is classified as IList". It will only be used if you specifically request it, e.g. in case of indexer ((IList)someArray)[x]. Accessing array indexer directly (e.g. someArray[x]) is another thing, you won't find sources for any method because there is none. — Sinatr
– Sinatr, Commented Sep 7, 2016 at 15:33

Jon Skeet · Accepted Answer · 2016-09-07 15:08:49Z

9

Namely, that the indexer in the case of an T[] returns a reference to the element within the array

Not really. It's more that there isn't an indexer for arrays - instead an element-access expression represents an array access instead of an element access (sections 7.6.6.1 and 7.6.6.2 of the C# 5 spec respectively).

There's a very significant difference between these two - in particular, an array access is classified as a variable, whereas an indexer access is classified as a value.

It's very similar to the difference between a property and a field - they both have the same syntax for access, but a property invokes a function member and returns a value, whereas a field access just yields the variable.

So my question is, can I be certain that no boxing will occur when I access an element of a T[] where T is a struct?

If you're accessing it as a T[], sure. The indexer you looked at is only used when you're viewing the array as an IList. So if you use:

IList array = new int[2];
object x = array[0];

then yes, that'll box the value... but if you write

int[] array = new int[2];
int x = array[0];

then that won't, and it won't be accessing that indexer code or the GetValue method at all.

answered Sep 7, 2016 at 15:08

Jon Skeet

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4 Comments

Jacob Krall Over a year ago

Another way to look at it: System.Array is like a base type of all arrays. The implementation of IList.this[int index] in this base type exists so arrays will implement System.Collections.IList, not for regular element accesses.

bright Over a year ago

Great, thanks. A follow up question - is there anything in Array.cs that is executed when i call array[index]? Or is that all implemented directly by the CLR?

Jon Skeet Over a year ago

@bright: All in the CLR, as far as I'm aware. There's IL specifically for array access, although I can't remember the op names offhand.

bright Over a year ago

@JonSkeet, found it - ldelem.<type>. Thanks.

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