Flutter项目运行报错：List<dynamic>无法转为List<int>求助

Hey there! I’ve run into this exact type mismatch issue before when working with JSON in Flutter—let’s get this sorted for you.

The Root Cause #

Looking at the line where the error hits:

genreIds = jsonMap["genre_ids"];

The problem is that when Dart parses JSON, arrays are always returned as List<dynamic> by default. But your genreIds variable is declared as List<int>, and Dart won’t automatically cast this dynamic list to a list of integers for you. That’s why you’re seeing the subtype error.

Quick Fixes #

You have two straightforward ways to fix this line:

1. Use cast<int>()

This is the simplest approach if you’re confident that all elements in genre_ids are integers (which they should be, since this is coming from TMDB’s API):

genreIds = (jsonMap["genre_ids"] as List).cast<int>();

2. Use List<int>.from()

This method creates a new list and explicitly converts each element to an int—great if you want a bit more control (like handling potential non-int values, though that’s unlikely here):

genreIds = List<int>.from(jsonMap["genre_ids"]);

Updated Code Snippet #

Here’s what the corrected constructor line should look like (using the first fix as an example):

MediaItem._internalFromJson(Map jsonMap, {MediaType type: MediaType.movie}) 
    : type = type, 
      id = jsonMap["id"].toInt(), 
      voteAverage = jsonMap["vote_average"].toDouble(), 
      title = jsonMap[(type == MediaType.movie ? "title" : "name")], 
      posterPath = jsonMap["poster_path"] ?? "", 
      backdropPath = jsonMap["backdrop_path"] ?? "", 
      overview = jsonMap["overview"], 
      releaseDate = jsonMap[(type == MediaType.movie ? "release_date" : "first_air_date")], 
      genreIds = (jsonMap["genre_ids"] as List).cast<int>();

Why This Works #

Both methods tell Dart exactly how to convert the dynamic list from JSON into a strongly-typed list of integers, resolving the subtype mismatch that was causing your error.

Give either fix a try, and your project should run without that error!

内容的提问来源于stack exchange，提问作者Magesh Pandian