有没有办法让 switch 使用 C# 8 switch 表达式返回字符串值？

Question

我有这段代码，其中开关的每个部分都向ModeMessage2 返回一个值。是否可以使用新的 C# switch 表达式（或任何其他代码优化）来优化 switch 的工作方式？

switch (Settings.Mode)
{
    case MO.Learn:
        ModeMessage2 =
            "Use this mode when you are first learning the phrases and their meanings.";
        if (Settings.Cc == CC.H)
        {
            Settings.Cc = CC.JLPT5;
            App.cardSetWithWordCount = null;
            App.DB.RemoveSelected();
        }
        break;
    case MO.Practice:
        ModeMessage2 =
             "Use this mode to help you memorize the phrases and their meanings.";
        if (Settings.Cc == CC.H)
        {
            Settings.Cc = CC.JLPT5;
            App.cardSetWithWordCount = null;
            App.DB.RemoveSelected();
        }
        break;
    case MO.Quiz:
        if (Settings.Cc == CC.H)
        {
            Settings.Cc = CC.JLPT5;
            App.cardSetWithWordCount = null;
            App.DB.RemoveSelected();
        }
        App.DB.UpdSet(SET.Adp, false);
        ModeMessage2 =
            "Use this mode to run a self marked test.";
        break;
}

Answer 1

您的代码类似于以下代码，但如果设置 ModeMessage2 或其他属性有副作用，那么事情发生的顺序可能很重要，在这种情况下，这在技术上并不是 100% 等效的。

IList<MO> specialModes = new[] { MO.Learn, MO.Practice, MO.Quiz };

if (specialModes.Contains(Settings.Mode) && Settings.Cc == CC.H) {
   Settings.Cc = CC.JLPT5;
   App.cardSetWithWordCount = null;
   App.DB.RemoveSelected();
}

ModeMessage2 = Settings.Mode switch {
   MO.Learn => "Use this mode when you are first learning the phrases and their meanings.",
   MO.Practice => "Use this mode to help you memorize the phrases and their meanings.",
   MO.Quiz => "Use this mode to run a self marked test.",
   _ => "Unknown mode value" // or throw
};

if (Settings.Mode == MO.Quiz)
   App.DB.UpdSet(SET.Adp, false);

Answer 2

var modeMessage2 = Settings.Mode switch
{
    MO.Learn => "Use this mode when you are first learning the phrases and their meanings.",
    MO.Practice => "Use this mode to help you memorize the phrases and their meanings.",
    MO.Quiz => "Use this mode to run a self marked test."
}

if (Settings.Cc == CC.H)
{
    Settings.Cc = CC.JLPT5;
    App.cardSetWithWordCount = null;
    App.DB.RemoveSelected();
}

if (Settings.Mode == MO.Quiz) {
    App.DB.UpdSet(SET.Adp, false);
}

Answer 3

你应该使用字典来做到这一点--

 Dictionary<TypeOf(Settings.Mode), string> map = new Dictionary<TypeOf(Settings.Mode), string>();

 map.Add(MO.Learn,"Use this mode when you are first learning the phrases and their meanings.");
 map.Add(MO.Practice,"Use this mode to help you memorize the phrases and their meanings.");
 map.Add(MO.Quiz,"Use this mode to run a self marked test.");


 ModeMessage2 = map[Settings.mode]);

这将比任何 switch 语句都快得多，并且更易于维护。

如果有意义，您也可以使用数组。

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以下评论者请注意：我做出以下假设，在某些情况下可能是错误的，但在一般情况下并非如此。 1) 代码的编写方式是“分配”在代码的整个生命周期内只发生一次——在这种情况下，如果地图被多次使用，您可以获得节省，以便在 N 次之后分配成本消失到 0。 2) 我们不知道密钥的类型，假设它是一个字符串的 cmets 正在做出一个可能不正确的假设。即便如此，字符串的任何“快速”比较都使用哈希，这与字典用来提高速度的方法相同。 3) 众所周知，编程中最慢的事情就是分支。字典（或数组映射）允许您没有任何分支，只是对内存位置的计算。