首先列出错误状态:无元素 [英] List firstWhere Bad state: No element

问题描述

我正在运行 list.firstWhere ，有时会抛出异常:

I am running list.firstWhere and this is sometimes throwing an exception:

Bad State: No element

When the exception was thrown, this was the stack:
#0      _ListBase&Object&ListMixin.firstWhere (dart:collection/list.dart:148:5)

我不明白这意味着什么，也无法通过查看

I do not understand what this means an also could not identify the issue by looking at the source.

我的 firstWhere 如下:

list.firstWhere((element) => a == b);

推荐答案

当没有匹配的元素时，即 a == b 对于<未指定code> list 和.

可选参数 orElse .

This will happen when there is no matching element, i.e. when a == b is never true for any of the elements in list and the optional parameter orElse is not specified.

您还可以指定 orElse 来处理这种情况:

You can also specify orElse to handle the situation:

list.firstWhere((a) => a == b, orElse: () => print('No matching element.'));

如果要在没有匹配项的情况下返回 null ，也可以使用 orElse 进行此操作:

If you want to return null instead when there is no match, you can also do that with orElse:

list.firstWhere((a) => a == b, orElse: () => null);

---

package:collection 还包含用于的便捷扩展方法> null 情况(在具有null安全性的情况下也应更好地工作):

---

package:collection also contains a convenience extension method for the null case (which should also work better with null safety):

import 'package:collection/collection.dart';

list.firstWhereOrNull((element) => element == other);

请参阅 firstWhereOrNull 更多信息.感谢 @EdwinLiu 指出这一点.

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