是否可以有只能分配一次的字段? [英] Is it possible to have fields that are assignable only once?

问题描述

我需要一个可以从任何我想要的地方分配到的字段，但应该只能分配一次(因此应忽略后续分配).我该怎么做?

I need a field that can be assigned to from where ever I want, but it should be possible to assign it only once (so subsequent assignments should be ignored). How can I do this?

推荐答案

那时那将不是只读字段.初始化真正的只读字段的唯一选项是字段初始值设定项和构造函数.

That would not be a readonly field then. Your only options for initializing real readonly fields are field initializer and constructor.

然而，您可以使用属性实现一种只读功能.将您的字段设为属性.实现一个冻结实例"方法，该方法翻转一个标志，说明不允许对只读部分进行更多更新.让你的二传手检查这个标志.

You could however implement a kind of readonly functionality using properties. Make your field as properties. Implement a "freeze instance" method that flipped a flag stating that no more updates to the readonly parts are allowed. Have your setters check this flag.

请记住，您正在放弃编译时检查以进行运行时检查.编译器会告诉您是否尝试从声明/构造函数以外的任何地方为只读字段赋值.使用下面的代码，您会得到一个异常(或者您可以忽略更新 - 两者都不是最佳的 IMO).

Keep in mind that you're giving up a compile time check for a runtime check. The compiler will tell you if you try to assign a value to a readonly field from anywhere but the declaration/constructor. With the code below you'll get an exception (or you could ignore the update - neither of which are optimal IMO).

为了避免重复检查，您可以将只读功能封装在一个类中.

to avoid repeating the check you can encapsulate the readonly feature in a class.

修订后的实现可能如下所示:

Revised implementation could look something like this:

class ReadOnlyField<T> {
    public T Value {
        get { return _Value; }
        set { 
            if (Frozen) throw new InvalidOperationException();
            _Value = value;
        }
    }
    private T _Value;

    private bool Frozen;

    public void Freeze() {
        Frozen = true;
    }
}


class Foo {
    public readonly ReadOnlyField<int> FakeReadOnly = new ReadOnlyField<int>();

    // forward to allow freeze of multiple fields
    public void Freeze() {
        FakeReadOnly.Freeze();
    }
}

然后你的代码可以做类似的事情

Then your code can do something like

        var f = new Foo();

        f.FakeReadOnly.Value = 42;

        f.Freeze();

        f.FakeReadOnly.Value = 1337;

最后一条语句会抛出异常.

The last statement will throw an exception.

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