英文:
Why _Map<String, String> is not in [Map<String, String>, bool]
问题
我需要进行运行时类型检查。
void main() {final type = [Map<String, String>, bool];final mapTest = {"A": "B"};if (type.contains(mapTest.runtimeType)) {print("是的");} else {print("不, ${mapTest.runtimeType} 不在 $type 中");}bool boobool = true;if (type.contains(boobool.runtimeType)) {print("是的, ${boobool.runtimeType} 在 $type 中");} else {print("不, ${mapTest.runtimeType} 不在 $type 中");}}
mapTest 应该通过测试。但是它没有。有什么问题？
英文:
I need to do runtime type check.
void main() {final type = [Map<String, String>, bool];final mapTest = {"A": "B"};if (type.contains(mapTest.runtimeType)) {print("yes");} else {print("no, ${mapTest.runtimeType} is not in $type");}bool boobool = true;if (type.contains(boobool.runtimeType)) {print("yes, ${boobool.runtimeType} is in $type");} else {print("no, ${mapTest.runtimeType} is not in $type");}}
mapTest should pass the test. But it doesn't. What's wrong ?
no, _Map<String, String> is not in {Map<String, String>, bool}yes, bool is in {Map<String, String>, bool}
答案1
得分: 1
-
Map<K, V>是一个抽象类型。任何对象都不可能具有Map<K, V>的runtimeType；任何对象都将是某个具体类型的实例。 -
Type对象通常仅在精确相同的Type时相等；它不会检查一个Type是否表示另一个的子类型。
因此，mapTest.runtimeType 不可能与 Map<K, V> 相等。
您也将无法使用 is；mapTest is type[0] 是不合法的，因为 is 要求右操作数在静态上是已知的。
最好的做法是，在这种情况下，您可以检查静态类型而不是运行时类型：
/// 在所有非`dynamic`对象上提供 [staticType] 获取器。extension StaticTypeExtension<T> on T {/// 返回此对象的静态类型。////// 这可用于报告对象的静态类型（由于推断或处理泛型时可能不太明显），它可能与 [Object.runtimeType] 不同。Type get staticType => T;}void main() {final type = [Map<String, String>, bool];final mapTest = {"A": "B"};if (type.contains(mapTest.staticType)) {print("yes");} else {print("no, ${mapTest.staticType} is not in $type");}bool boobool = true;if (type.contains(boobool.staticType)) {print("yes, ${boobool.staticType} is in $type");} else {print("no, ${mapTest.staticType} is not in $type");}}
这将打印：
yesyes, bool is in [Map<String, String>, bool]
话虽如此，最终您想要做的事情可能相当值得质疑。依赖于特定的 Type 值通常不是一个好主意，因为如前所述，子类型不会自动处理。
英文:
Map<K, V>is an abstract type. It is impossible for any object to have aruntimeTypeofMap<K, V>; any object will be an instance of some concrete type.Typeobjects usually are equal only for the exact sameType; it does not check if oneTyperepresents a subtype of the other.
Consequently, it is impossible for mapTest.runtimeType to ever compare equal to Map<K, V>.
You also will not be able to use is; mapTest is type[0] is not legal since is requires that the right operand be statically known.
At best, for this case, you could check for static types instead of runtime types:
/// Provides a [staticType] getter on all non-`dynamic` objects.extension StaticTypeExtension<T> on T {/// Returns the static type of this object.////// This can be used to report the static type of an object (which might not/// always be obvious due to inference or when dealing with generics), which/// might be different from [Object.runtimeType].Type get staticType => T;}void main() {final type = [Map<String, String>, bool];final mapTest = {"A": "B"};if (type.contains(mapTest.staticType)) {print("yes");} else {print("no, ${mapTest.staticType} is not in $type");}bool boobool = true;if (type.contains(boobool.staticType)) {print("yes, ${boobool.staticType} is in $type");} else {print("no, ${mapTest.staticType} is not in $type");}}
which prints:
yesyes, bool is in [Map<String, String>, bool]
That said, whatever it is you ultimately want to do with this is probably pretty questionable. It's usually not a good idea to rely on specific Type values because, as mentioned, subtypes won't be automatically handled.
答案2
得分: 0
因为 mapTest 的运行时类型是 _InternalLinkedHashMap<String, String>，与 Map<String, String> 不同，所以未通过类型检查。
为了通过类型检查，你可以考虑将 mapTest 显式转换为 Map<String, String>。
final type = [Map<String, String>, bool];final mapTest = {"A": "B"};if (type.contains(mapTest.runtimeType) || type.contains(mapTest.runtimeType.toString())) {print("yes");} else if (mapTest is Map<String, String> && type.contains(Map<String, String>)) {print("yes, ${mapTest.runtimeType} is in $type");} else {print("no, ${mapTest.runtimeType} is not in $type");}
英文:
It is because mapTest's runtime type is _InternalLinkedHashMap<String, String>, which is different from Map<String, String> so it fails the type check.
To make the type check pass, you can consider converting the mapTest to Map<String,String explicitly.
final type = [Map<String, String>, bool];final mapTest = {"A": "B"};if (type.contains(mapTest.runtimeType) || type.contains(mapTest.runtimeType.toString())) {print("yes");} else if (mapTest is Map<String, String> && type.contains(Map<String, String>)) {print("yes, ${mapTest.runtimeType} is in $type");} else {print("no, ${mapTest.runtimeType} is not in $type");}
答案3
得分: 0
Dart通过使用`_Map`类继承的`Map`类的构造函数创建`Map`，并且您直接在`type`中提到了`Map<String, String>`的类型。因此，我建议您采用在变量`type`中创建`runtimeType`的方式。```dartvoid main() {final type = [{"A": "B"} .runtimeType, bool];final mapTest = {"A": "B"} as Map<String, String>;if (type.contains(mapTest.runtimeType)) {print("yes");} else {print("no, ${mapTest.runtimeType} is not in $type");}bool boobool = true;if (type.contains(boobool.runtimeType)) {print("yes, ${boobool.runtimeType} is in $type");} else {print("no, ${mapTest.runtimeType} is not in $type");}}
英文:
Dart create a Map by using constructor of _Map which inherited by class Map, and you directly mentioned type of Map<String, String> in in type.
So I suggest you to go with the way of creating a runtimeType in a variable type.
void main() {final type = [{"A": "B"} .runtimeType, bool];final mapTest = {"A": "B"} as Map<String, String>;if (type.contains(mapTest.runtimeType)) {print("yes");} else {print("no, ${mapTest.runtimeType} is not in $type");}bool boobool = true;if (type.contains(boobool.runtimeType)) {print("yes, ${boobool.runtimeType} is in $type");} else {print("no, ${mapTest.runtimeType} is not in $type");}}
答案4
得分: 0
将以下内容翻译为中文：
各种答案，但我找到了最简单的解决方案。
将
final type = [Map<String, String>, bool];
改为
final type = [Map<String, String>().runtimeType, bool];
似乎实现了预期目的。
是的，_Map<String, String> 在 [_Map<String, String>, bool, List<String>] 中。
英文:
Various answers but I found the simplest solution.
Change
final type = [Map<String, String>, bool];
to
final type = [Map<String, String>().runtimeType, bool];
It seems to fulfill the purpose
yes, _Map<String, String> is in [_Map<String, String>, bool, List<String>]
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