为什么我不能在结构中使用as关键字？

I defined the following struct:

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;
    }
}

Later, I assign it to the Tag property of another object:

稍后，我将其分配给另一个对象的Tag属性：

line.Tag = new Call(sf1, sf2);

But when I try to retrieve the Tag property like so,

但是当我尝试像这样检索Tag属性时，

Call call = line.Tag as Call;

Visual Studio gives the following compile-time error:

Visual Studio提供以下编译时错误：

The operator as must be used within a reference type or nullable type

必须在引用类型或可空类型中使用运算符

What is the meaning of that? And how can I solve it?

那是什么意思？我该如何解决？

6 个解决方案

#1

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.

struct是值类型，因此不能与as运算符一起使用。如果转换失败，as运算符必须能够赋值null。这仅适用于引用类型或可空值类型。

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。这实际上会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指定null值。

For more information on value types vs. reference types, this is a decent article. Also, have a look on MSDN:

有关值类型与引用类型的更多信息，这是一篇不错的文章。另外，看看MSDN：

value types
价值类型
reference types
参考类型
as-operator
作为运营商
nullable types.
可空类型。

#2

Some of the existing answers aren't quite right. You can't use non-nullable types with as, because the result of as is the null value of the type if the first operand isn't actually of an appropriate type.

一些现有的答案并不完全正确。您不能将非可空类型与as一起使用，因为如果第一个操作数实际上不是合适的类型，则as的结果是该类型的空值。

However, you can use as with value types... if they're nullable:

但是，您可以使用值类型...如果它们可以为空：

int a = 10;
object o = a;

int? x = o as int?; // x is a Nullable<int> with value 10
long? y = o as long?; // y is a Nullable<long> with the null value

So you could use:

所以你可以使用：

Call? call = line.Tag as Call?;

Then you can use it as:

然后你可以用它作为：

if (call != null)
{
    // Do stuff with call.Value
}

Two caveats though:

但有两点需要注意：

In my experience this is slower than just using is followed by a cast
根据我的经验，这比使用后跟一个演员要慢
You should seriously reconsider your current Call type:
- It's exposing public fields, which is generally poor encapsulation
- 它暴露了公共领域，这通常是很糟糕的封装
- It's a mutable value type, which is almost certainly a mistake
- 这是一个可变的值类型，几乎肯定是一个错误
你应该认真重新考虑你当前的Call类型：它暴露了公共字段，这通常是不好的封装它是一个可变的值类型，这几乎肯定是一个错误

I would strongly suggest you make it a class instead - at which point this problem goes away anyway.

我强烈建议你把它变成一个班级 - 无论如何这个问题都会消失。

Another thought: if the tag should always be a Call, then it's better to cast it:

另一个想法：如果标签应该始终是一个Call，那么最好将其转换为：

Call call = (Call) line.Tag;

That way, if the data doesn't match your expectation (i.e. there's some bug such that the Tag isn't a Call) then you get to find out about it early, rather than after you've potentially done some other work. Note that this cast will behave differently depending on whether Call is a struct or a class, if Tag is null - you can cast a null value to a variable of a reference type (or a nullable value type), but not to a non-nullable value type.

这样，如果数据与您的期望不符（即，有一些错误，使得标签不是一个呼叫），那么您可以尽早找到它，而不是在您可能完成其他工作之后。请注意，根据Call是一个结构还是一个类，如果Tag为null，这个转换将表现不同 - 您可以将null值转换为引用类型的变量（或可空值类型），但不能转换为非可空值类型。

#3

From the C# Spec

来自C＃规范

§7.10.11 The as operator is used to explicitly convert a value to a given reference type or nullable type. Unlike a cast expression (§7.7.6), the as operator never throws an exception. Instead, if the indicated conversion is not possible, the resulting value is null.

§7.10.11as运算符用于将值显式转换为给定的引用类型或可空类型。与强制转换表达式（第7.7.6节）不同，as运算符从不抛出异常。相反，如果无法指示转换，则结果值为空。

References and nullable types can be null. Stucts are value types so they can't be null.

引用和可空类型可以为null。 Stucts是值类型，因此它们不能为null。

#4

Call? call = line.Tag as Call?;

#5

It's a limitation of C#. If the type were a reference type, then if the cast failed it would simply return 'null', but since it's a value type, it doesn't know what to return when the cast fails.

这是C＃的限制。如果类型是引用类型，那么如果转换失败，它将只返回'null'，但由于它是一个值类型，它不知道在转换失败时返回什么。

You must replace your use of as with two: 'is' and 'as'

您必须将以下两种用法替换为：'is'和'as'

if (line.Tag is Call) {
  call = (Call)line.Tag;
} else {
  // Do whatever you would do if as returned null.
}

#6

-1

What is the meaning - As stated, structures are value types.

意义是什么 - 如上所述，结构是价值类型。

How can I solve it - Change it to

我怎样才能解决它 - 改成它

Call call = line.Tag;

#1