How to convert a List<String?> to List<String> in null safe Dart?

Question

I have a dart list:

List<String?> vals;

I want to remove any null values and convert it to a List<String>. I've tried:

List<String> removeNulls(List<String?> list) {
  return list.where((c) => c != null).toList() as List<String>;
}

At run time I'm getting the following error:

List<String?>' is not a subtype of type 'List<String>?'

What is the correct way to resolve this?

Answer 1

• 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,
  ];
  

• 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', null, 'baz', null];
  var withoutNulls = List<String>.from(list.where((c) => c != null));
  

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

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

Answer 2

• Without creating a new List

  void main() {
    List<String?> list = ['a', null, 'b', 'c', null];
    list.removeWhere((e) => e == null); // <-- This is all you need.
    print(list); // [a, b, c]
  }
  

• Creating a new List

  First create a method, filter for example:

  List<String> filter(List<String?> input) {
    input.removeWhere((e) => e == null);
    return List<String>.from(input);
  }
  

  You can now use it:

  void main() {
    List<String?> list = ['a', null, 'b', 'c', null];
    List<String> filteredList = filter(list); // New list
    print(filteredList); // [a, b, c]
  }
  

To use retainWhere, replace the predicate in removeWhere with (e) => e != null