Dart: Casting Lists or Maps throws error when they are empty

Question

I need to cast a List and a Map of unknown types to specific types. If the List or Map are not empty all is well. But if they are ever empty I get a cast error.

When list is empty:

final list = [];
final result = list as List<Map<String, dynamic>?>;

Output

type 'List<dynamic>' is not a subtype of type 'List<Map<String, dynamic>?>' in type cast

When Map is empty:

final map = {};
final result = map as Map<String, dynamic>;

Output

type '_Map<dynamic, dynamic>' is not a subtype of type 'Map<String, dynamic>' in type cast

For context: I'm fetching data from firebase. The DocumentSnaphot.data() of the firebase package returns an Object?. Therefore I have no control over what type the data is. Hence the casting...

Have you tried checking whether it is empty and do the casting only if it is not empty, otherwise assign a specific value? — Peter Koltai, Aug 23 '23 at 19:52
Yes that would work if I had just a List or just a Map. But here they're nested. What if the list only contained multiple empty Maps? I would have to loop through the list to check and cast each individual map. Seems rather unnecessary... — JakesMD, Aug 23 '23 at 20:48
"If the List or Map are not empty all is well." That seems very unlikely. You cannot cast `List`s or `Map`s that way with `as`; `as` does not perform any runtime transformation on the object and would fail for non-empty collections too. For example, `['foo'] as `List` will also fail. If you want to cast the *elements* of a `List`, then you should use `List.cast`/`Map.cast`. Also see https://stackoverflow.com/a/70963189/ — jamesdlin, Aug 24 '23 at 02:53
Or if you just want your empty `List`s and `Map`s to have the correct type in the first place, you can do: `final list = ?>[{}];` — jamesdlin, Aug 24 '23 at 02:55

offworldwelcome · Answer 1 · 2023-08-23T20:05:04.817

You can use pattern matching to respond to the different types returned by Firebase.

Here's an example:

void parseData(Object? data) {
  switch (data) {
    /// Handle empty list.
    case []:
      log("Received empty list.");
    /// Matches a list of integers.
    case List<int> list:
      log("Received integer list: $list");
    /// Matches when it's a list of maps whose key is of type String, but value
    /// can be any type (dynamic).
    case List<Map<String, dynamic>> list:
      log("Received list of maps: $list");
    /// Handle unexpected data type.
    default:
      log("Unknown data type: ${data.runtimeType}");
  }
}

You can then use the parseData function to parse your Firebase data:

Object? invalid = 1;

Object? dataList = [1, 2, 3];

Object? dataListOfMap = [
  {
    "key": [1, 2, 3],
  },
];

parseData(invalid);       /// 'Unknown data type: int'.
parseData(dataList);      /// 'Received integer list: [1, 2, 3]'.
parseData(dataListOfMap); /// 'Received list of maps: [{key: [1, 2, 3]}]'.

Dart also provides the is operator to first check if some reference is of a certain type:

if (myInstance is SomeType) ...

JakesMD · Answer 2 · 2023-08-24T06:12:29.830

0

For casting lists you should use List<T>.from(list as List), List.cast or List.castFrom.

For casting maps you should use Map<T>.from(map as Map) or Map.castFrom.

class FakeDocumentSnapshot {
  Object? data() {
    return {
      'list': [{}]
    };
  }
}

final snapshot = FakeDocumentSnapshot();

final data = Map<String, dynamic>.from((snapshot.data() ?? {}) as Map);

final list = (List.from(data['list'] ?? [])..removeWhere((e) => e == null));

final result = list
    .map((x) => Map<String, dynamic>.from(x as Map))
    .toList();

(I answered my own question)

edited Aug 24 '23 at 06:12

answered Aug 24 '23 at 06:00

JakesMD

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This answer is misleading. You should **not** use List.from to cast anything. That creates a copy of the list. Also, `List.castFrom` (and by extension `List.cast`), will still crash if one of the elements of the list is not castable to the type `T`. You still need to verify the proper types before casting, which was your original issue because internally, the `cast` and `castFrom` methods do the `as T` statement which caused you issues. – offworldwelcome Aug 24 '23 at 15:15

Dart: Casting Lists or Maps throws error when they are empty

2 Answers2