Continue in nested while loops

问题:

In this code sample, is there any way to continue on the outer loop from the catch block?

while
{
   // outer loop

   while
   {
       // inner loop
       try
       {
           throw;
       }
       catch 
       {
           // how do I continue on the outer loop from here?
           continue;
       }
   }
}

回答1:

UPDATE: This question was inspiration for my article on this subject. Thanks for the great question!

---

"continue" and "break" are nothing more than a pleasant syntax for a "goto". Apparently by giving them cute names and restricting their usages to particular control structures, they no longer draw the ire of the "all gotos are all bad all the time" crowd.

If what you want to do is a continue-to-outer, you could simply define a label at the top of the outer loop and then "goto" that label. If you felt that doing so did not impede the comprehensibility of the code, then that might be the most expedient solution.

However, I would take this as an opportunity to consider whether your control flow would benefit from some refactoring. Whenever I have conditional "break" and "continue" in nested loops, I consider refactoring.

Consider:

successfulCandidate = null;
foreach(var candidate in candidates)
{
  foreach(var criterion in criteria)
  {
    if (!candidate.Meets(criterion)) // Edited.
    {  // TODO: no point in continuing checking criteria.
       // TODO: Somehow "continue" outer loop to check next candidate
    }
  }
  successfulCandidate = candidate;
  break;
}
if (successfulCandidate != null) // do something

Two refactoring techniques:

First, extract the inner loop to a method:

foreach(var candidate in candidates)
{
  if (MeetsCriteria(candidate, criteria))
  { 
      successfulCandidate = candidate;
      break;
  }
}

Second, can all the loops be eliminated? If you are looping because you are trying to search for something, then refactor it into a query.

var results = from candidate in candidates 
              where criteria.All(criterion=>candidate.Meets(criterion))
              select candidate;
var successfulCandidate = results.FirstOrDefault();
if (successfulCandidate != null)
{
  do something with the candidate
}

If there are no loops then there is no need to break or continue!

回答2:

    while
    {
       // outer loop

       while
       {
           // inner loop
           try
           {
               throw;
           }
           catch 
           {
               // how do I continue on the outer loop from here?
               goto REPEAT;
           }
       }
       // end of outer loop
REPEAT: 
       // some statement or ; 
    }

Problem solved. (what?? Why are you all giving me that dirty look?)

回答3:

You can use a break; statement.

while
{
   while
   {
       try
       {
           throw;
       }
       catch 
       {
           break;
       }
   }
}

Continue is used to jump back to the top of the current loop.

If you need to break out more levels than that you will either have to add some kind of 'if' or use the dreaded/not recommended 'goto'.

回答4:

Swap the try/catch structure with the inner while loop:

while {
  try {
    while {
      throw;
    }
  }
  catch {
    continue;
  }
}

回答5:

No.
I suggest, extracting the inner loop into a separate method.

while
{
   // outer loop
       try
       {
           myMethodWithWhileLoopThatThrowsException()
       }
       catch 
       {
           // how do I continue on the outer loop from here?
           continue;
       }
   }
}

回答6:

Use break in the inner loop.

回答7:

You just want to break from the inner which would continue the outer.

while
{
   // outer loop

   while
   {
       // inner loop
       try
       {
           throw;
       }
       catch 
       {
           // how do I continue on the outer loop from here?
           break;
       }
   }
}

回答8:

I think the best way to accomplish this would be to use the break statement. Break ends the current loop and continues execution from where it ends. In this case, it would end the inner loop and jump back into the outer while loop. This is what your code would look like:

while
{
   // outer loop

   while
   {
       // inner loop
       try
       {
           throw;
       }
       catch 
       {
           // break jumps to outer loop, ends inner loop immediately.
           break; //THIS IS THE BREAK
       }
   }
}

I believe that is what you were looking to be accomplished, correct? Thanks!

回答9:

using System;

namespace Examples
{

    public class Continue : Exception { }
    public class Break : Exception { }

    public class NestedLoop
    {
        static public void ContinueOnParentLoopLevel()
        {
            while(true)
            try {
               // outer loop

               while(true)
               {
                   // inner loop

                   try
                   {
                       throw new Exception("Bali mu mamata");
                   }
                   catch (Exception)
                   {
                       // how do I continue on the outer loop from here?

                       throw new Continue();
                   }
               }
            } catch (Continue) {
                   continue;
            }
        } 
    }

}

}

回答10:

Use an own exception type, e.g., MyException. Then:

while
{
   try {
   // outer loop
   while
   {
       // inner loop
       try
       {
           throw;
       }
       catch 
       {
           // how do I continue on the outer loop from here?
           throw MyException;
       }
   }
   } catch(MyException)
   { ; }
}

This will work for continuing and breaking out of several levels of nested while statements. Sorry for bad formatting ;)