Why does the CLR re-use empty strings, but not empty arrays?

Question

I notice that

Console.WriteLine((object) new string(' ', 0) == (object) new string(' ', 0));

prints true, which indicates that the CLR keeps the empty string around and re-uses the same instance. (It prints false for any other number than 0.)

However, the same is not true for arrays:

Console.WriteLine(new int[0] == new int[0]);   // False

Now, if we look at the implementation of Enumerable.Empty<T>(), we find that it caches and re-uses empty arrays:

public static IEnumerable<TResult> Empty<TResult>()
{
    return EmptyEnumerable<TResult>.Instance;
}

[...]

public static IEnumerable<TElement> Instance
{
    get
    {
        if (EmptyEnumerable<TElement>.instance == null)
            EmptyEnumerable<TElement>.instance = new TElement[0];
        return EmptyEnumerable<TElement>.instance;
    }
}

So the framework team felt that keeping an empty array around for every type is worth it. The CLR could, if it wanted to, go a small step further and do this natively so it applies not only to calls to Enumerable.Empty<T>() but also new T[0]. If the optimisation in Enumerable.Empty<T>() is worth it, surely this would be even more worth it?

Why does the CLR not do this? Is there something I’m missing?

Answer 1

Strings may use interning, that makes them a different story (from all other kind of objects).

Arrays are essentially just objects. Re-using instances where that is not clear from the syntax or context isn't without side effects or risks.

static int[] empty = new int[0];
...
   lock (empty) { ... }

If some other code locked on another (they thought) empty int[] you might have a deadlock that is very hard to find.

Other scenarios include using arrays as the key in a Dictionary, or anywhere else their identity matters. The framework can't just go around changing the rules.