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FluidMechanics Potential Flows
Asked: 2024-09-03 15:32:26 +0800 CST

重载 true 和 false，重载 false 似乎从未被使用过

  • 772

在《C#12 In a Nutshell》（第 259 页，第重载 true 和 false 章）一书中，他们为我们提供了这个例子来说明如何重载 true 和 false 运算符。

SqlBoolean a = SqlBoolean.Null;
if (a)
    Console.WriteLine ("True");
else if (!a)
    Console.WriteLine ("False");
else
    Console.WriteLine ("Null");

public struct SqlBoolean
{
    public static bool operator true (SqlBoolean x)
    {
        return  x.m_value == True.m_value;
    }

    public static bool operator false(SqlBoolean x)
    {
        return x.m_value == False.m_value;
    }

    public static SqlBoolean operator ! (SqlBoolean x)
    {
        if (x.m_value == Null.m_value) return Null;
        if (x.m_value == False.m_value) return True;
        return False;
    }

    public static readonly SqlBoolean Null = new SqlBoolean (0);
    public static readonly SqlBoolean False = new SqlBoolean (1);
    public static readonly SqlBoolean True = new SqlBoolean (2);

    SqlBoolean (byte value) { m_value = value; }
    byte m_value;
}

我试图了解何时false调用过载，并且根据我的理解（并通过使用调试器进行确认），似乎从未调用过过载。false那么为什么过载会有用呢？由于我们只使用while (a)或while (!a)等...过载true和!还不够吗？

c#

2 个回答

  1. Best Answer

    C# 标准有这样的说明：

    注意：尽管true和false未在表达式中明确使用（因此未包含在§12.4.2 中的优先级表中），但它们被视为运算符，因为它们在多个表达式上下文中被调用：布尔表达式（§12.24）以及涉及条件（§12.18）和条件逻辑运算符（§12.14）的表达式。结束注释

    通过这些链接，您会发现该false运算符用于条件逻辑运算符：

    操作x && y的计算方式为T.false(x) ? x : T.&(x, y)，其中是中声明T.false(x)的 的调用，是所选 的调用。换句话说，首先计算 ，然后对结果调用 以确定 是否肯定为假。然后，如果肯定为假，则操作的结果为先前为 计算的值。否则，计算 ，然后对先前为 计算的值和 计算的值调用所选以产生操作的结果。operator falseTT.&(x, y)operator &xoperator falsexxxyoperator &xy

    下面是一些测试代码，展示了运算符的各种使用时间：

    LoggingBoolean t = new LoggingBoolean(true);
LoggingBoolean f = new LoggingBoolean(false);

Console.WriteLine("Boolean expression");
if (t)
{
    Console.WriteLine("In true body");
}

if (f)
{
    Console.WriteLine("In false body");
}

Console.WriteLine("Conditional operator");

Console.WriteLine(t ? "true branch" : "false branch");
Console.WriteLine(f ? "true branch" : "false branch");

Console.WriteLine("Conditional && logic");
Console.WriteLine(t && t);
Console.WriteLine(t && f);
Console.WriteLine(f && t);
Console.WriteLine(f && f);

Console.WriteLine("Conditional || logic");
Console.WriteLine(t || t);
Console.WriteLine(t || f);
Console.WriteLine(f || t);
Console.WriteLine(f || f);

internal struct LoggingBoolean
{
    public bool Value { get; }

    public LoggingBoolean(bool value) => Value = value;

    public static bool operator false(LoggingBoolean x)
    {
        Console.WriteLine("false operator called");
        return !x.Value;
    }

    public static bool operator true(LoggingBoolean x)
    {
        Console.WriteLine("true operator called");
        return x.Value;
    }

    public static LoggingBoolean operator &(LoggingBoolean x, LoggingBoolean y) =>
        new LoggingBoolean(x.Value && y.Value);

    public static LoggingBoolean operator |(LoggingBoolean x, LoggingBoolean y) =>
        new LoggingBoolean(x.Value || y.Value);

    public override string ToString() => $"LoggingBoolean({Value})";
}

    输出：

    Boolean expression
true operator called
In true body
true operator called
Conditional operator
true operator called
true branch
true operator called
false branch
Conditional && logic
false operator called
LoggingBoolean(True)
false operator called
LoggingBoolean(False)
false operator called
LoggingBoolean(False)
false operator called
LoggingBoolean(False)
Conditional || logic
true operator called
LoggingBoolean(True)
true operator called
LoggingBoolean(True)
true operator called
LoggingBoolean(True)
true operator called
LoggingBoolean(False)
    • 5

  2. 当涉及短路时，False 运算符将发挥作用 - 使用&&运算符，如果第一个表达式失败，我们可以忽略第二个表达式。

    在这种情况下，将调用 false 运算符，请看示例：

    // Need to define this operator in order to leverage short circuiting with &&,
// it will be analogical with ||
public static SqlBoolean operator &(SqlBoolean left, SqlBoolean right)
{
    // Implement appropriately, this is just example
    return True;
}

    然后测试：

    var shortCircuitAnd = a && a;
    • 3

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