为什么我不能用作为关键字的结构？ [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，我分配到另一个对象的Tag属性：
Later, I assign it to the Tag property of another object:
line.Tag = new Call(sf1, sf2);
但是，当我尝试检索Tag属性像这样，
But when I try to retrieve the Tag property like so,
Call call = line.Tag as Call;
Visual Studio中提供了以下编译时错误：
Visual Studio gives the following compile-time error:
运营商如必须在一个使用 引用类型或可空类型
The operator as must be used within a reference type or nullable type
什么是是，什么意思？我怎么能解决呢？
What is the meaning of that? And how can I solve it?
推荐答案
一个结构是值类型，所以它不能与作为运营商。该为运营商必须能够如果转换失败分配空值。这是唯一可能与引用类型或可空类型。
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，但最好的办法是改变你的呼叫从结构类类型。这将主要从值类型为引用类型，它允许为运营商，如果转换失败分配null值改变你的类型。
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详细信息，请这是一个体面的文章。此外，有MSDN上看看：
For more information on value types vs. reference types, this is a decent article. Also, have a look on MSDN:
- value types
- reference types
- as-operator
- nullable types.
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