Cast “Map<String, List<dynamic>>” to “Map<String, List<String>>”

问题

I want to cast a Map<String, dynamic> to Map<String, List<String>>.

I have tried the following in Flutter:

Map<String, dynamic> dynamicMap = {
  'a': ['1', '2', '3'],
  'b': ['1', '2', '3'],
  'c': ['1', '2', '3'],
};
Map<String, List<String>> typedMap = dynamicMap;

But get the following error:

type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'Map<String, List<String>>'

I then tried:

Map<String, List<String>> typedMap = dynamicMap.cast<String, List<String>>();

which seems to work initally, but you get the same error when trying to use it, such as print(typedMap).

Thanks for the help.

EDIT

It's worth mentioning that the following:

Map<String, dynamic> dynamicMap = {
  'a': ['1', '2', '3'],
  'b': ['1', '2', '3'],
  'c': ['1', '2', '3'],
};
Map<String, List<String>> typedMap = dynamicMap.cast<String, List<String>>();

did in fact work (apologies for the mistake in the question).

However, when the data comes say from json like so:

String rawJson = """{
   "a": ["1", "2", "3"],
   "b": ["1", "2", "3"],
   "c": ["1", "2", "3"]
}""";
final Map<String, dynamic> dynamicMapFromJson = json.decode(rawJson);
Map<String, List<String>> typedMap = dynamicMapFromJson.cast<String, List<String>>();

The code no longer works and raised the exception mentioned.

The answers below helped solve this problem.

回答1:

Map<String, dynamic> dynamicMap = {
  'a': ['1', '2', '3'],
  'b': ['1', '2', '3'],
  'c': ['1', '2', '3'],
};
Map<String, List<String>> typedMap;
dynamicMap.forEach((key,value){
 List<String> list = List.castFrom(value);
 typedMap[key] = list
});

This is because a dynamic data type variable can be assigned with any of the data type at run time. But values of typedMap here has been already declared with data type of List<String> , nothing else can be assigned to it other than a List<String>. The error is because you're trying to asign a dynamic data type to a List<string> data type, a pseudo example

String s = dynamic(); //impossible
dynamic s = String(); //possible

回答2:

Map method can help you to convert in your desire cast.

Check out follow code.

I also demonstrated how to access all value of map as a list. i hope this will work for you.

Map<String, dynamic> dynamicMap = {
    'a': ['1', '2', '3'],
    'b': ['1', '2', '3'],
    'c': ['1', '2', '3'],
};
Map<String, List<String>> typedMap =
    dynamicMap.map((key, value) => MapEntry(key, value));

typedMap.forEach((key, value) {
    print(value[0]);
});

回答3:

I solution I ended up using and was most reliable was:

Map<String, List<String>> typedMap = dynamicMap.map(
  (key, value) => MapEntry(key, List.castFrom(value))
);

Thanks @Viren V Varasadiya and @iamyadunandan for the help

来源：https://stackoverflow.com/questions/61014768/cast-mapstring-listdynamic-to-mapstring-liststring