错误 CS0133：将函数的结果分配给 C#.net 中的 const

发布于 2024-09-03 06:35:17 字数 680 浏览 7 评论 0原文

尝试清理范围并避免可能多次调用 RegisterWindowMessage。
目前有一个类与以下成员一起使用一次，

[DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
static extern int RegisterWindowMessage(string lpString);

private int m_message = RegisterWindowMessage("MY_MSG");

因为我们只有一个实例，这似乎没问题，但认为使用起来会更整洁。根据我对 C# 的基本理解，这应该调用 RegisterWindowMessage 并将结果分配给 int 并且不允许它更改。

private const int message = RegisterWindowMessage("MY_MSG");

但是尝试这样做会导致

error CS0133: The expression being assigned to 'someclass.messageEvent' must be constant

现在我很困惑，这是否意味着之前每次使用 m_message 时都会分配并调用该函数，是否还缺少其他内容？

原文

Trying to tidy up scope and avoid possible multiple calls to RegisterWindowMessage.
Currently have a class used once with the following member

[DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
static extern int RegisterWindowMessage(string lpString);

private int m_message = RegisterWindowMessage("MY_MSG");

As we only have one instance this seems ok, but think it would be more tidy to use. With my basic C# understanding this should call RegisterWindowMessage and assign the result to int and not allow it to change.

private const int message = RegisterWindowMessage("MY_MSG");

however attempting to do so leads to a

error CS0133: The expression being assigned to 'someclass.messageEvent' must be constant

so now I'm confused, does this mean the function was being assigned and called each time m_message was used previously, is there something else missing?

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过气美图社 2024-09-10 06:35:17

const 字段必须是编译时常量。如果您只是想要在初始赋值¹之后执行时不会更改的内容，请将其设置为只读：

private static readonly int Message = RegisterWindowMessage("MY_MSG");

请注意，我已将其设置为静态，const 是隐式的。这意味着 RegisterWindowMessage 只会为此 AppDomain 调用一次，这就是我认为您想要的。

编辑：汉斯是对的，你应该检查返回值。您可以在第一次使用它时或在初始化类型时执行此操作 - 通常类型初始值设定项抛出异常不是一个好主意，但您应该看看会产生什么影响。

¹ 严格来说，静态只读字段可以在声明中或静态构造函数中赋值；可以在声明或任何实例构造函数中分配实例只读字段。它可以被分配多次，这通常没有用，但偶尔会有用。

A const field has to be a compile-time constant. If you just want something which won't change at execution time after initial assignment¹, make it readonly:

private static readonly int Message = RegisterWindowMessage("MY_MSG");

Note that I've made this static, which const is implicitly. This means RegisterWindowMessage will only be called once for this AppDomain, which is what I think you want.

EDIT: Hans is right, you should check the return value. You could either do that when you first use it, or when the type is initialized - usually it's a bad idea for type initializers to throw exceptions, but you should see what the impact is.

¹ Strictly speaking, a static readonly field can be assigned in the declaration or in the static constructor; an instance readonly field can be assigned in the declaration or in any instance constructor. It can be assigned multiple times, which is usually not useful, but can be just occasionally.

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千寻… 2024-09-10 06:35:17

这里还有另一个考虑因素。 RegisterWindowMessage() 可能会失败，您确实需要检查这一点。当出现问题时使用它返回的 0 将非常难以诊断。

这就消除了直接在只读声明中对其进行初始化的情况。您可以改用静态构造函数。问题是异常消息将被隐藏在 InnerException 中。也许还可以，因为失败很少见。

最好的解决方案是延迟调用 API 的静态属性 getter：

private int m_message

public static int message {
  get {
    if (m_message == 0) {
      m_message = RegisterWindowMessage("blah");
      if (m_message == 0) throw new Win32Exception();
    }
    return m_message;
  }
}

如果可能从不同线程调用，请使用众所周知的锁定模式。

There is another consideration here. RegisterWindowMessage() can fail, you really need to check for that. Using the 0 it returns when something is wrong is going to be horribly difficult to diagnose otherwise.

That knocks initializing it directly in the readonly declaration out. You could use a static constructor instead. Problem with that is that the exception message will be buried in an InnerException. Kinda okay perhaps since failure will be rare.

The best solution is a static property getter that lazily calls the API:

private int m_message

public static int message {
  get {
    if (m_message == 0) {
      m_message = RegisterWindowMessage("blah");
      if (m_message == 0) throw new Win32Exception();
    }
    return m_message;
  }
}

Use well-known locking patterns if this might be called from different threads.

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终遇你 2024-09-10 06:35:17

要添加 Hans 给出的答案，您可以做得比简单地抛出空 Win32Exception 更好。这适用于任何使用 GetLastError 的 API 调用：

[DllImport("user32.dll", SetLastError = true)]
extern static int RegisterWindowMessage(string lpString);

if (m_message == 0)
    throw new Win32Exception(Marshal.GetLastWin32Error());

这将产生更具信息性的异常。

To add to the answer that Hans gave, you can do better than simply throwing an empty Win32Exception. This applies to any API call that uses GetLastError:

[DllImport("user32.dll", SetLastError = true)]
extern static int RegisterWindowMessage(string lpString);

if (m_message == 0)
    throw new Win32Exception(Marshal.GetLastWin32Error());

That will produce a more informative exception.

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