Falling through switch statement (works sometimes?

问题:

I have a switch statement such as the one below:

switch (condition)
{
    case 0:
    case 1:
        // Do Something
        break;
    case 2:
        // Do Something
    case 3:
        // Do Something
        break;
}

I get a compile error telling me that Control cannot fall through from one case label ('case 2:') to another

Well... Yes you can. Because you are doing it from case 0: through to case 1:.

And in fact if I remove my case 2: and it's associated task, the code compiles and will fall through from case 0: into case1:.

So what is happening here and how can I get my case statements to fall through AND execute some intermediate code?

回答1:

There is a difference between stacking labels and fall-through.

C# supports the former:

case 0:
case 1:
    break;

but not fall-through:

case 2:
    // Do Something
case 3:
    // Do Something
    break;

If you want fall-through, you need to be explicit:

case 2:
    // Do Something
    goto case 3;
case 3:
    // Do Something
    break;

The reasoning is apparent, implicit fall-through can lead to unclean code, especially if you have more than one or two lines, and it isn't clear how the control flows anymore. By forcing the explicit fall-through, you can easily follow the flow.

Reference: msdn

回答2:

Quoting MSDN:

"C# does not support an implicit fall through from one case label to another. The one exception is if a case statement has no code."

basically it is not legal to put statements inside the case and not include a break.

case 1:
case 2:
//do stuff
break;

is legal

but:

case 1:
//do stuff without a break
case 2:
//do stuff
break; 

is not.

http://msdn.microsoft.com/en-us/library/06tc147t(v=vs.80).aspx

回答3:

You are not falling from case 0 to case 1 since they share the same code block. This is the same as writing case 1 before case 0.

回答4:

In C#, you cannot fall through a label to another implicitly except if there is no specific code for the first label. You can have

case 1:
case 2:
    // Do Something
break;

but not

case 1:
    // Do Something
case 2:
    // Do Something
break;

See msdn for a more in-depth explanation.

If you wish to fall through explicitly, you can by using the goto instruction. It is also one of the rare case where using goto isn't a bad practice.

case 1:
    // Do Something
    goto case 2;
case 2:
    // Do Something
break;

回答5:

Code can "fall through" in C# only when there is no code between the case statements. The code example infers there is code between case 2 and case 3.

回答6:

This is not allowed

switch (condition)
{
    case 0:
        // Do Something
    case 1:
        // Do Something
        break;
}

This is allowed

    switch (condition)
    {
        case 0:
        case 1:
            // Do Something
            break;
    }

回答7:

The problem is that you're doing something in case 2 and then trying to fall through, and that's not supported. You're going from 0 to 1 without any additionaly activity.