Difference between List.from() and as List in Dart

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Let’s examine some examples:

var intList = <int>[1, 2, 3];
var dynamicList = intList as List<dynamic>; // Works.
var intList2 = dynamicList as List<int>; // Works.

But:

var dynamicList = <dynamic>[1, 2, 3];
var intList = dynamicList as List<int>; // Fails at runtime.

What’s the difference?

In the first example, intList has a static type of List<int>, and the actual runtime type of the object is also List<int>. dynamicList has a static type of List<dynamic> but has an actual runtime type of List<int> (it’s the same object as intList). Since the object was originally a List<int>, it is impossible for the List object to hold anything but int elements, and casting back to List<int> is safe.

In the second example, dynamicList has a static type of List<dynamic> and an actual runtime type of List<dynamic>. Since the object is initially constructed as a List<dynamic>, non-int elements could have been added to it, and casting to List<int> is not necessarily safe. You therefore instead must create a new List (or List-like) object that casts each element individually, which is exactly what List.from and List.cast do.

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