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Use _emptyArray in List<T>.ToArray #2758

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Jan 21, 2016
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Use _emptyArray in List<T>.ToArray #2758

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9 changes: 8 additions & 1 deletion src/mscorlib/src/System/Collections/Generic/List.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -1001,12 +1001,19 @@ public void Sort(Comparison<T> comparison) {
}
}

// ToArray returns a new Object array containing the contents of the List.
// ToArray returns an array containing the contents of the List.
// This requires copying the List, which is an O(n) operation.
public T[] ToArray() {
Contract.Ensures(Contract.Result<T[]>() != null);
Contract.Ensures(Contract.Result<T[]>().Length == Count);

#if FEATURE_CORECLR
if (_size == 0)
{
return _emptyArray;
}
#endif

T[] array = new T[_size];
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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.

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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.

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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.

Array.Copy(_items, 0, array, 0, _size);
return array;
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