Search code examples
dartdart-null-safety

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

Why List() constructor is not accessible after Dart's null safety?

// Compile time error: 'List' is deprecated and shouldn't be used.
// The default 'List' constructor isn't available when null safety is enabled. 
// Try using a list literal, 'List.filled' or 'List.generate'.
List<int> foo = List();

However, you can still do:

List<int> foo = []; // No error

So, what's the difference between the two? Either both of them should show the error or none of them.

Solution

  • 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.