The default ‘List’ constructor isn’t available when null safety is enabled. Try using a list literal, ‘List.filled’ or ‘List.generate’

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Short answer:

Instead of the pre-null-safety operations

var foo = List<int>();  // Now error
var bar = List<int>(n); // Now error
var baz = List<int>(0); // Now error

use the following:

var foo = <int>[];           // Always the recommended way.
var bar = List.filled(1, 0); // Not filled with `null`s.
var baz = List<int>.empty();

Long answer:

The List constructor had two uses:

  • new List() to create an empty growable list, equivalent to [].
  • new List(n) to create a fixed-length list of length n filled with null values

With null safety, the second use was unsound most of the time, and there was no good way to fix it. It’s possible to force a type argument to be non-nullable, but List<T>(4) only works when T is nullable. There is no way to enforce that.

So, the List(n) mode needed to go (replaced by List.filled(n, value) which forces you to provide a fill-value).
That left List(), which doesn’t really carry its own weight. You can just use [] instead (and you should!), so it was decided to remove the constructor entirely – all uses of it was either unsafe or useless.
(Also, it was a weird constructor already, because if we wanted to properly make it null safe, it would have an optional parameter with a non-nullable type and no default value.)

By removing it completely, it makes it possible to, potentially, introduce a new List constructor in the future, perhaps as a shorter alias for List.filled. One can hope.

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