Use _emptyArray in List<T>.ToArray

[stephentoub]

I've only added this to List<T> in mscorlib as it's the only one of the collections with a ToArray that's exposed out (I've not touched legacy collections like ArrayList). The other changes for the other collections are in corefx in dotnet/corefx#5562.

cc: @jkotas, @ellismg
https://github.com/dotnet/corefx/issues/2363

[jaredpar]

Did you consider adding a method like the following:

T[] Allocate<T>(int size) 
{
#if FEATURE_CORCLR
  if (size == 0) { return Array.Empty<T>(); }
#endif
  return new T[size];
}

This could then become a drop in replacement for new T[size] in places where Array.Empty<T> was acceptable.

[stephentoub]

Interesting idea. Though in a case like this (and I expect others) you'd end up doing something like:

T[] array = Allocate(_size);
Array.Copy(_items, 0, array, 0, _size);
return array;

and you'd be doing a bit of extra work unnecessarily with the Array.Copy call.

[jaredpar]

Agree there would be some extra work here in copy. Feel like more Array.Empty<T> overall would get more use if we have a clean, simple way to have allocations return it automatically.

[justinvp]

Is dotnet/corefx#5562 going to be updated to return _emptyArray from List<T>.ToArray() instead of Array.Empty<T>() as well?

[stephentoub]

Yes, I just haven't had a chance yet.

[jkotas]

LGTM

[justinvp]

In dotnet/corefx#2363, @terrajobst says:

We bring this change to .NET Framework, but we’ll quirk it. Only customers targeting the latest version will get the new behavior.

I see it is within #if FEATURE_CORECLR here. Will there be another PR that adds the behavior to .NET Framework under an AppContext switch?

[stephentoub]

Will there be another PR that adds the behavior to .NET Framework under an AppContext switch?

This isn't the code for the full framework. I expect when it's ported back, a switch will be used.

[justinvp]

Ah, I was under the impression the code was shared -- didn't realize a port step would be necessary. Thanks!

[stephentoub]

Unfortunately. 😦 At least with the current state of things. It could change.