What is _TypeError and how to handle/test it?-CodePudding

When trying to assign a value of a wrong type to a variable in Dart, you get a _TypeError.

Example:

void main() {
  dynamic x = 1;
  String y = x;
}

Output: type 'int' is not a subtype of type 'String'

What exactly is a _TypeError? I can't find the documentation. I can't catch it (specifically) or expect it in a unit test.

Catching

The following is the only way I could catch it so far, but I don't want to catch 'em all. I want to use on ... catch(e)., but on _TypeError catch(e) doesn't work, because _TypeError is undefined.

void main() {
  dynamic x = 1;
  try {
    String y = x;
  } catch (e) {
    print('catched: '   e.runtimeType);
    print(e.toString());
  }
}

Output:

catched: _TypeError
type 'int' is not a subtype of type 'String'

Testing

How to expect it in a unit test? I expected this to work, but it doesn't:

test('throws a _TypeError', () {
  dynamic x = 1;
  String x;
  expect(() => x = y, throwsException);
};

Output:

Expected: throws <Instance of 'Exception'>
  Actual: <Closure: () => dynamic>
   Which: threw _TypeError:<type 'int' is not a subtype of type 'String'>

CodePudding user response：

_TypeError is an internal dart class used instead of TypeError, so in most cases, you can just use TypeError instead:

dynamic x = 1;
  try {
    String y = x;
  } on TypeError catch (e) {
    print('caught: '   e.runtimeType);
    print(e.toString());
  }

Testing

Sadly, I don't know of any way to test for TypeError, I don't believe they made a matcher for it, but I could be wrong, but I guess you could always test before the cast itself

test('throws a _TypeError', () {
  dynamic y = 1;
  String x;
  expect(y, isInstanceOf<String>());
};

if the above test fails, so will x = y;