[英]What's the difference between casting using (Object as TClass) and TClass(Object)
Got an issue where MyObj.classnameis(TMyClass.classname)
is true and TMyClass(MyObj)
works but (MyObj as TMyclass).doSomething
throws a conversion error. 有一个问题, MyObj.classnameis(TMyClass.classname)
为真, TMyClass(MyObj)
工作,但(MyObj as TMyclass).doSomething
引发转换错误。
I don't really want any help with that junk, although if you want to put it in the comments that'd be super. 我真的不希望任何有关垃圾的帮助,不过如果你想把它放在那些超级的评论中。 I just would like to know what the difference between Obj as Class
and Class(Obj)
is. 我只是想知道Obj as Class
和Class(Obj)
之间的区别是什么。
An as-cast checks the actual object type to make sure the cast is valid, and raises an exception if it's not. as-cast检查实际的对象类型以确保转换有效,如果不是则引发异常。 A "hard cast" ( TMyClass(MyObj)
style) does not check, it just tells the compiler to assume the cast is valid. “硬强制转换”( TMyClass(MyObj)
样式)不检查,它只是告诉编译器假设TMyClass(MyObj)
是有效的。
If you've got a situation where ClassNameIs returns true but the as-cast fails, that means you have two different classes in two different units with the same name, and the as-cast is trying to cast to the wrong one. 如果你遇到ClassNameIs返回true但是as-cast失败的情况,那意味着你有两个不同的类,它们在两个不同的单元中具有相同的名称,并且as-cast试图转换为错误的类。 This also means that your hard-cast is casting to the wrong one, which could potentially lead to memory corruption. 这也意味着您的强制转换是错误的,这可能会导致内存损坏。
Run a full project search for "TMyclass =" to see where your multiple declarations are, and either rename one of the classes or use a full definition (obj as MyUnit.TMyClass) so the compiler will know which class you're trying to cast to. 运行完整项目搜索“TMyclass =”以查看多个声明的位置,并重命名其中一个类或使用完整定义(obj作为MyUnit.TMyClass),以便编译器知道您要尝试转换哪个类至。
Addition to Mason's post: Instead of a searching through your code, you could also call a method like this one at the location that gives you problem. 除了Mason的帖子之外:您还可以在提供问题的位置调用类似这样的方法,而不是搜索您的代码。
function GetClassInheritance(Obj : TObject) : string;
var ClassRef : TClass;
begin
Result := '';
ClassRef := obj.ClassType;
Result := ClassRef.ClassName;
ClassRef := ClassRef.ClassParent;
while assigned(ClassRef) do
begin
Result := ClassRef.ClassName + '.' + Result;
ClassRef := ClassRef.ClassParent;
end;
Result := '(' + obj.ClassType.UnitName + ')' + Result
end;
That will return you a string formated as (UnitName)TObject.TPersistent.TComponent.... I don't remember seeing "ClassType.UnitName" in older version of delphi, so that part might not work with them, but the rest should. 这将返回一个格式化为(UnitName)TObject.TPersistent.TComponent的字符串....我不记得在旧版本的delphi中看到“ClassType.UnitName”,因此该部分可能无法使用它们,但其余的应该。
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