问题
When I create a switch statement in VS2008 C# like this (contrived):
switch (state) {
case '1':
state = '2';
case '2':
state = '1';
}
it complains that I'm not allowed to drop through:
Control cannot fall through from one case label ('case '1' (0x31):') to another
If you're not allowed to drop through, then what is the purpose of the break statement at all? Why didn't the language designers just leave it out and automatically jump to the end of the switch statement instead of forcing us to put in an unnecessary construct?
回答1:
From the horse's mouth (MSDN) Why is the C# switch statement designed to not allow fall-through, but still require a break?.
Quoting the salient bits, this is why they don't allow fall-through:
This implicit fall-through behavior is often used to reduce the amount of code needed and often isn't an issue the first time that code is written. However, as code moves from the initial development phase into a maintenance phase, the code above can lead to subtle errors that are very hard to debug. These errors result from the very common mistake of the developer adding a case, yet forgetting to put a break at the end of the block.
In C#, the switch statement requires that explicit flow control occur at the end of a case, either a break, goto, return, or throw. If the developer desires fall-through semantics, it can be achieved by an explicit goto at the end of a case statement.
And this is why it's not automatic:
As a result of the C# rules requiring explicit flow-control to occur at the end of a case block (most usually a break), many people question why the behavior simply wasn't changed such that fall-through didn't occur. That is, don't make break required, simply change the semantics of switch to not have fall-through for cases. The reason this wasn't done was so that developers who were very used to C++ wouldn't have a hard time understanding what a switch statement was doing.
回答2:
Basically to make it more familiar to C/C++/Java developers. Personally I think it was a mistake, but that's the reasoning.
I would have preferred a forced block:
case '1':
{
}
Aside from anything else, that would have avoided the weird variable scoping situations for switch/case. You could still have multiple case labels, of course:
case '0':
case '1':
{
}
It might also be nice to be able to list multiple cases more simply:
case '0', '1':
{
}
Oh, and a slight nit-pick about your description of the existing language: you don't have to have a break. It's just that the end of the case has to be unreachable. You can also have throw, goto or return. There may be others that I've missed, too :)
回答3:
You are allowed to drop through, but you have to do so explicitly with the goto keyword:
switch (state) {
case '1':
state = '2';
goto case '2';
case '2':
state = '1';
break;
}
You can break or gotoin C#, but what you cannot do is not state which you want, because that's a potential source of hard-to-spot bugs.
It's a lot easier to spot that your code says goto when you wanted break (or vice versa) than it is to spot that you forgot to add either.
It might sound stupid, but many a tired two-hour search for the cause of a C++ bug ends in a sudden realisation that you forgot to add a break and your code is falling through all the time. C# avoids that by forcing you to state what you want.
回答4:
If you don't have any code in case 1, you are allowed to fall through, so you can say that "all of these cases share this piece of code"
来源：https://stackoverflow.com/questions/3108888/why-does-c-sharp-have-break-if-its-not-optional