How to convert a List to List in null safe Dart?

Ideally you’d start with a List<String> in the first place. If you’re building your list like:

String? s = maybeNullString();
var list = <String?>[
  'foo',
  'bar',
  someCondition ? 'baz' : null,
  s,
];

then you instead can use collection-if to avoid inserting null elements:

String? s = maybeNullString();
var list = <String?>[
  'foo',
  'bar',
  if (someCondition) 'baz',
  if (s != null) s,
];

Dart 3 adds a nonNulls extension to filter out nulls:

var list = <String?>['foo', 'bar', null, 'baz', null];
var withoutNulls = list.nonNulls.toList();

Prior to Dart 3, an easy way to filter out null values from an Iterable<T?> and get an Iterable<T> result is to use .whereType<T>(). For example:

var list = <String?>['foo', 'bar', null, 'baz', null];
var withoutNulls = list.whereType<String>().toList();

Another approach is to use collection-for with collection-if:

var list = <String?>['foo', 'bar', null, 'baz', null];
var withoutNulls = <String>[
  for (var s in list)
    if (s != null) s
];

Finally, if you already know that your List doesn’t contain any null elements but just need to cast the elements to a non-nullable type, other options are to use List.from:

var list = <String?>['foo', 'bar', 'baz'];
var withoutNulls = List<String>.from(list);

or if you don’t want to create a new List, Iterable.cast:

var list = <String?>['foo', 'bar', 'baz'];
var withoutNulls = list.cast<String>();