C# 中的 false 运算符有什么用? [英] What's the false operator in C# good for?

问题描述

C# 中有两个奇怪的运算符:

There are two weird operators in C#:

• 真正的运算符
• 假运算符

如果我理解正确，这些运算符可以用于我想使用的类型而不是布尔表达式，并且我不想提供到 bool 的隐式转换.

If I understand this right these operators can be used in types which I want to use instead of a boolean expression and where I don't want to provide an implicit conversion to bool.

假设我有以下课程:

    public class MyType
    {
        public readonly int Value;

        public MyType(int value)
        {
            Value = value;
        }

        public static bool operator true (MyType mt)
        {
            return  mt.Value > 0;
        }

        public static bool operator false (MyType mt)
        {
            return  mt.Value < 0;
        }

    }

所以我可以写下面的代码:

So I can write the following code:

    MyType mTrue = new MyType(100);
    MyType mFalse = new MyType(-100);
    MyType mDontKnow = new MyType(0);

    if (mTrue)
    {
         // Do something.
    }

    while (mFalse)
    {
        // Do something else.
    }

    do
    {
        // Another code comes here.
    } while (mDontKnow)

但是对于上面的所有示例，仅执行 true 运算符.那么 C# 中的 false 运算符有什么用呢?

However for all the examples above only the true operator is executed. So what's the false operator in C# good for?

注意:可以在此处找到更多示例、此处 和 这里.

推荐答案

您可以使用它来覆盖 && 和 || 运算符.

You can use it to override the && and || operators.

&& 和 || 操作符不能被覆盖，但是如果你覆盖了 |，&、true 和 false 编译器会以完全正确的方式调用 | 和 &你写||和&&.

The && and || operators can't be overridden, but if you override |, &, true and false in exactly the right way the compiler will call | and & when you write || and &&.

例如看这段代码(来自http://ayende.com/blog/1574/nhibernate-criteria-api-operator-overloading - 我在那里发现了这个技巧；存档版本 @BiggsTRC):

For example, look at this code (from http://ayende.com/blog/1574/nhibernate-criteria-api-operator-overloading - where I found out about this trick; archived version by @BiggsTRC):

public static AbstractCriterion operator &(AbstractCriterion lhs, AbstractCriterion rhs)
{
       return new AndExpression(lhs, rhs);
}

public static AbstractCriterion operator |(AbstractCriterion lhs, AbstractCriterion rhs)
{
       return new OrExpression(lhs, rhs);
}

public static bool operator false(AbstractCriterion criteria)
{
       return false;
}
public static bool operator true(AbstractCriterion criteria)
{
       return false;
}

这显然是一种副作用，而不是预期的使用方式，但它很有用.

This is obviously a side effect and not the way it's intended to be used, but it is useful.

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