Restart a foreach loop in C#?

问题

How can I restart a foreach loop in C#??

For example:

Action a;
foreach(Constrain c in Constrains)
{
   if(!c.Allows(a))
   {
      a.Change();
      restart;
   }
}

restart here is like continue or break but it restarts the foreach from the begining It is like setting the counter of a for loop to 0 again..

Is that possible in C#?

Edit:I want to thank both Mehrdad Afshari and Mahesh Velaga for letting me discover a bug (index=0) in my current implementation, that would not have been discovered otherwise..

回答1:

Use the good old goto:

restart:
foreach(Constrain c in Constrains)
{
   if(!c.Allows(a))
   {
      a.Change();
      goto restart;
   }
}

If you're diagnosed with gotophobia 100% of the time for some reason (which is not a good thing without a reason), you can try using a flag instead:

bool restart;
do {
   restart = false;
   foreach(Constrain c in Constrains)
   {
      if(!c.Allows(a))
      {
         a.Change();
         restart = true;
         break;
      }
   }
} while (restart);

回答2:

Although a very old thread - none of the answers paid due attention to the semantics of that code:

• You have a chain of constraints on a
• If a breaks any of them, try another a and push that through the chain.

That is, a.Change() should be separated from the constraint checking loop, also adhering to the CQS principle:

while (!MeetsConstraints(a))
{
    a.Change();
}

bool MeetsConstraints(Thing a)
{
    return Constraints.All(c => c.Allows(a));
}

No goto, no ugly loops, just simple and clean. </self-back-slapping>

回答3:

One way you can do that is using for, as you have already mentioned:

restart here is like continue or break but it restarts the foreach from the begining It is like setting the counter of a for loop to 0 again

Action a;
for(var index = 0; index < Constratins.Count; index++)
{
   if(!Constraints[index].Allows(a))
   {
      a.Change();
      index = -1; // restart
   }
}

回答4:

void Main()
{
    IEnumerable<Constrain> cons;
    SomeObject a;

    while(!TryChangeList(cons, a)) { }
}

// the name tryChangeList reveals the intent that the list will be changed
private bool TryChangeList(IEnumerable<Constrain> constrains, SomeObject a)
{
    foreach(var con in constrains)
    {
        if(!c.Allows(a))
        {
            a.Change();
            return false;
        }
    }
    return true;
}

回答5:

for (var en = Constrains.GetEnumerator(); en.MoveNext(); )
{
    var c = en.Current;
    if (!c.Allows(a))
    {
        a.Change();
        en = Constrains.GetEnumerator();
    }
}

来源：https://stackoverflow.com/questions/4574102/restart-a-foreach-loop-in-c