C＃的switch-case字符串开始 [英] C# Switch-case string starting with

问题描述

有没有什么办法，使在switch语句中的case条件，你说，如果一个字符串开头的东西吗？

Is there any way to make a case condition in a switch statement where you say if a string begins with something?

前

Switch (mystring)
{
   case("abc")://String begins with abc (abcd or abc1 or abcz or abc.. or abc will fall in this condition).
      //Do Something
      break;
   default:
      break;
}

更新
其他字符串即可不同的长度。

UPDATE Other strings can be different length.

ABC ..

abczyv

dcs2。

QWERTY

作为... ... K

as...k

推荐答案

如果您知道条件的长度，你会关心都将是相同的长度，那么你可以：

If you knew that the length of conditions you would care about would all be the same length then you could:

switch(mystring.substring(0, Math.Min(3, mystring.Length))
{
  case "abc":
    //do something
    break;
  case "xyz":
    //do something else
    break;
  default:
    //do a different thing
    break;
}

的

Math.Min（3 ，mystring.Length）是否有这样的不足3个字符的字符串不会扔在子字符串操作的异常。

The Math.Min(3, mystring.Length) is there so that a string of less than 3 characters won't throw an exception on the sub-string operation.

有此技术的延伸，以匹配如一堆双字符字符串和一堆3字符的字符串，其中一些双字符比较匹配，然后其次是3个字元比较。除非你非常多的这样的字符串不过，它很快变得比简单的if-else链为运行代码，谁能够维护它的人。

There are extensions of this technique to match e.g. a bunch of 2-char strings and a bunch of 3-char strings, where some 2-char comparisons matching are then followed by 3-char comparisons. Unless you've a very large number of such strings though, it quickly becomes less efficient than simple if-else chaining for both the running code and the person who has to maintain it.

编辑：增加了，因为你现在已经表示，他们将不同的长度。你可以做我所提到的检查第一个X字符，然后下一Ÿ字符等的格局，但除非有一个模式，大部分的字符串的长度相同，这将是一种效率低下和可怕的维护（经典案例过早pessimisation）。

Added since you've now stated they will be of different lengths. You could do the pattern I mentioned of checking the first X chars and then the next Y chars and so on, but unless there's a pattern where most of the strings are the same length this will be both inefficient and horrible to maintain (a classic case of premature pessimisation).

Command模式在另一个答复中提到，所以我不会放弃的那个细节，就是在那里你映射字符串模式为ID，但他们的选择。

The command pattern is mentioned in another answer, so I won't give details of that, as is that where you map string patterns to IDs, but they are option.

我不会改变从的if-else链命令或映射模式，以获得高效开关的有时有超过若-else，因为你失去更多的比较为命令或取得ID模式。我想，虽然这样做，如果它使代码更清晰。

I would not change from if-else chains to command or mapping patterns to gain the efficiency switch sometimes has over if-else, as you lose more in the comparisons for the command or obtaining the ID pattern. I would though do so if it made code clearer.

中的if-else的可以工作得很好，无论是与字符串比较或使用正则表达式A链（后者，如果你有比较比前缀匹配，从而更为复杂，这可能会更简单，更快，我提到REG-EX的仅仅是因为他们有时会用这种模式的更一般的情况下）工作。

A chain of if-else's can work pretty well, either with string comparisons or with regular expressions (the latter if you have comparisons more complicated than the prefix-matches so far, which would probably be simpler and faster, I'm mentioning reg-ex's just because they do sometimes work well with more general cases of this sort of pattern).

如果你去，如果-别人的，尽量考虑哪些案件要经常发生，让那些不常见的情况下，之前的那些测试发生（当然，如果开头ABCD是看它都必须检查前案与美国广播公司）开始。

If you go for if-elses, try to consider which cases are going to happen most often, and make those tests happen before those for less-common cases (though of course if "starts with abcd" is a case to look for it would have to be checked before "starts with abc").

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