为什么我不能对结构使用 as 关键字? [英] Why can't I use the as keyword for a struct?
问题描述
我定义了以下结构:
public struct Call
{
public SourceFile caller;
public SourceFile callee;
public Call(SourceFile caller, SourceFile callee)
{
this.caller = caller;
this.callee = callee;
}
}
后来，我把它赋给了另一个对象的 Tag 属性:
Later, I assign it to the Tag property of another object:
line.Tag = new Call(sf1, sf2);
但是当我尝试像这样检索 Tag 属性时，
But when I try to retrieve the Tag property like so,
Call call = line.Tag as Call;
Visual Studio 给出以下编译时错误:
Visual Studio gives the following compile-time error:
必须在引用类型或可为空类型
The operator as must be used within a reference type or nullable type
这是什么意思?我该如何解决?
What is the meaning of that? And how can I solve it?
推荐答案
结构体是值类型，因此不能与 as 运算符一起使用.如果转换失败，as 运算符必须能够分配一个 null 值.这仅适用于引用类型或可为空值类型.
A struct is a value type, so it cannot be used with the as operator. The as operator must be able to assign a value of null if the cast fails. This is only possible with a reference type or a nullable value type.
有几种方法可以解决这个问题，但最好的办法是将 Call 类型从结构更改为类.这将从本质上将您的类型从值类型更改为引用类型，这允许 as 运算符在转换失败时分配 null 值.
There are a couple ways to solve this, but your best bet is to change your Call type from a struct to a class. This will essentially change your type from a value type to a reference type, which allows the as operator to assign a value of null if the cast fails.
有关值类型与引用类型的更多信息，这篇是一篇不错的文章.另外，看看MSDN:
For more information on value types vs. reference types, this is a decent article. Also, have a look on MSDN:
这篇关于为什么我不能对结构使用 as 关键字?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
