如何使C#Switch语句使用IgnoreCase [英] How to make C# Switch Statement use IgnoreCase

问题描述

如果我有一个switch-case语句，其中开关中的对象是字符串，是否可以进行ignoreCase比较?

If I have a switch-case statement where the object in the switch is string, is it possible to do an ignoreCase compare?

例如，我有

string s = "house";
switch (s)
{
  case "houSe": s = "window";
}

s 是否将获得"window"值?如何覆盖switch-case语句，以便它将使用ignoreCase比较字符串?

Will s get the value "window"? How do I override the switch-case statement so it will compare the strings using ignoreCase?

推荐答案

您似乎已经知道，对两个字符串进行小写并进行比较与进行忽略大小写比较不同.造成这种情况的原因很多.例如，Unicode标准允许带有变音符号的文本以多种方式编码.某些字符在单个代码点中同时包含基本字符和变音符号.这些字符也可以表示为基本字符，后跟组合变音符号.这两种表示形式对于所有目的都是相等的，并且.NET Framework中具有文化意识的字符串比较将正确地将它们识别为相等，无论使用CurrentCulture还是InvariantCulture(带有或不带有IgnoreCase).另一方面，按序比较将错误地认为它们是不平等的.

As you seem to be aware, lowercasing two strings and comparing them is not the same as doing an ignore-case comparison. There are lots of reasons for this. For example, the Unicode standard allows text with diacritics to be encoded multiple ways. Some characters includes both the base character and the diacritic in a single code point. These characters may also be represented as the base character followed by a combining diacritic character. These two representations are equal for all purposes, and the culture-aware string comparisons in the .NET Framework will correctly identify them as equal, with either the CurrentCulture or the InvariantCulture (with or without IgnoreCase). An ordinal comparison, on the other hand, will incorrectly regard them as unequal.

不幸的是， switch 除了序数比较外没有做任何其他事情.序数比较对于某些类型的应用程序来说很好，例如使用严格定义的代码解析ASCII文件，但是序数字符串比较在大多数其他用途中是错误的.

Unfortunately, switch doesn't do anything but an ordinal comparison. An ordinal comparison is fine for certain kinds of applications, like parsing an ASCII file with rigidly defined codes, but ordinal string comparison is wrong for most other uses.

过去我为了获得正确的行为所做的只是模拟我自己的switch语句.有很多方法可以做到这一点.一种方法是创建由成对的大小写字符串和委托组成的 List< T> .可以使用适当的字符串比较来搜索列表.找到匹配项后，即可调用关联的委托.

What I have done in the past to get the correct behavior is just mock up my own switch statement. There are lots of ways to do this. One way would be to create a List<T> of pairs of case strings and delegates. The list can be searched using the proper string comparison. When the match is found then the associated delegate may be invoked.

另一种选择是执行明显的 if 语句链.由于结构非常规则，因此通常情况听起来并不像听起来那样糟糕.

Another option is to do the obvious chain of if statements. This usually turns out to be not as bad as it sounds, since the structure is very regular.

对此的好处是，与字符串进行比较时，在模拟自己的开关功能方面并没有任何性能上的损失.系统不会像使用整数那样制作O(1)跳转表，因此无论如何一次都将比较每个字符串.

The great thing about this is that there isn't really any performance penalty in mocking up your own switch functionality when comparing against strings. The system isn't going to make a O(1) jump table the way it can with integers, so it's going to be comparing each string one at a time anyway.

如果要比较的情况很多，而性能是一个问题，则可以将上述的 List< T> 选项替换为排序的字典或哈希表.这样，性能可能会达到甚至超过switch语句选项.

If there are many cases to be compared, and performance is an issue, then the List<T> option described above could be replaced with a sorted dictionary or hash table. Then the performance may potentially match or exceed the switch statement option.

以下是代表列表的示例:

Here is an example of the list of delegates:

delegate void CustomSwitchDestination();
List<KeyValuePair<string, CustomSwitchDestination>> customSwitchList;
CustomSwitchDestination defaultSwitchDestination = new CustomSwitchDestination(NoMatchFound);
void CustomSwitch(string value)
{
    foreach (var switchOption in customSwitchList)
        if (switchOption.Key.Equals(value, StringComparison.InvariantCultureIgnoreCase))
        {
            switchOption.Value.Invoke();
            return;
        }
    defaultSwitchDestination.Invoke();
}

当然，您可能需要向CustomSwitchDestination委托中添加一些标准参数，以及可能的返回类型.而且，您会想起更好的名字！

Of course, you will probably want to add some standard parameters and possibly a return type to the CustomSwitchDestination delegate. And you'll want to make better names!

如果每种情况的行为都不适合以这种方式委托调用(例如，如果需要不同的参数)，那么您将陷入束缚的 if 陈述中.我也做了几次.

If the behavior of each of your cases is not amenable to delegate invocation in this manner, such as if differnt parameters are necessary, then you’re stuck with chained if statments. I’ve also done this a few times.

    if (s.Equals("house", StringComparison.InvariantCultureIgnoreCase))
    {
        s = "window";
    }
    else if (s.Equals("business", StringComparison.InvariantCultureIgnoreCase))
    {
        s = "really big window";
    }
    else if (s.Equals("school", StringComparison.InvariantCultureIgnoreCase))
    {
        s = "broken window";
    }

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