Repeating a function in C# until it no longer throws an exception

Question

I've got a class that calls a SOAP interface, and gets an array of data back. However, if this request times out, it throws an exception. This is good. However, I want my program to attempt to make this call again. If it times out, I'd like it to keep making this call until it succeeds. How can I accomplish this?

For example:

try
{
   salesOrdersArray = MagServ.salesOrderList(sessID, filter);
}
catch
{
   ?? What Goes Here to FORCE the above line of code to rerun until it succeeds.
}

You could simply call the method again from the catch block, *BUT* - what happens if the service is down for an extended period of time? Do you want the method to run recursively for 24 hours? I would recommend limiting it to a set number of retries. — Tim, Oct 27 '11 at 18:40
Out of all of these answers, not one person tells you to catch a more specific exception. What if your try block contains objects in an invalid state to get the `salesOrderList`? Retrying will never succeed. — Marc, Oct 27 '11 at 18:43

score 18 · Accepted Answer · answered Oct 27 '11 at 18:41

18

You just need to loop forever:

while (true)
{
    try
    {
        salesOrdersArray = MagServ.salesOrderList(sessID, filter);
        break; // Exit the loop. Could return from the method, depending
               // on what it does...
    }
    catch
    {
        // Log, I suspect...
    }
}

Note that you should almost certainly not actually loop forever. You should almost certainly have a maximum number of attempts, and probably only catch specific exceptions. Catching all exceptions forever could be appalling... imagine if salesOrderList (unconventional method name, btw) throws ArgumentNullException because you've got a bug and filter is null... do you really want to tie up 100% of your CPU forever?

answered Oct 27 '11 at 18:41

Jon Skeet

1,421,763
867
9,128
9,194

1

Couldn't that (potentially) tie up system resources and/or make the calling app unresponsive? Doesn't sound like the best solution to me... – Tim Oct 27 '11 at 18:43
1

@Tim: I was adding the paragraph below - basically the code answers the question directly, but the paragraph explains that it's probably not a good idea :) – Jon Skeet Oct 27 '11 at 18:43
+1 for specific exceptions, I was typing my comment on the question during your edit. – Marc Oct 27 '11 at 18:43
I know, I do catch specific exceptions, SOAP errors, etc. Forgot the joys of break, thanks. – psyklopz Oct 27 '11 at 18:47
@psyklopz: You specify the exception type in the catch block: `catch (SoapException)` etc. You can specify multiple different exception types in multiple catch blocks. – Jon Skeet Oct 27 '11 at 18:50
When I do this sort of thing, I put in a `Sleep` of appropriate duration so that it doesn't use 100% of the CPU waiting for some service to come back online. I might even include a limited exponential backoff (like TCP) so that it recovers quickly from minor network glitches but tries once every few minutes if the service is down for an extended period. – Gabe Oct 28 '11 at 19:06
You can also get current time before and then wait till the diff b/w then and now is > 5 sec and then break out (or however long you want) – Sellorio Feb 14 '13 at 01:30

Miguel Angelo · Answer 2 · 2011-10-27T18:57:14.590

You must place the try/catch block inside a loop construct. If you wish not to consume 100% of your processor place a Thread.Sleep in the catch block, so everytime an exception occurs, it will wait some time, freeing the processor to do other things.

// iterate 100 times... not forever!
for (int i = 0; i < 100; i++)
{
    try {
        // do your work here;

        break; // break the loop if everything is fine
    } catch {
        Thread.Sleep(1000);
    }
}

You could also specify exception type, so that only the timeout exception is handled, and other kinds of exceptions pass-through.

// iterate 100 times... not forever!
for (int i = 0; i < 100; i++)
{
    try {
        // do your work here;

        break; // break the loop if everything is fine
    } catch (TimeOutException) {
        Thread.Sleep(1000);
    }
}

Note that, TimeOutException should be replaced by the real name of the exception... I don't know if that is the real name.

Also adjust the sleep time, given in millisecs and the amount of repeats, in the case I presented, 100 repeats of 1000ms yields a maximum wait of 1 minute and 40 seconds, plus the operation time itself.

score 4 · Answer 3 · answered Oct 27 '11 at 18:40

4

If you can't change the timeout, the below should work. salesOrdersArray should be initialized to null.

while(salesOrdersArray == null)
{
    try
    {
       salesOrdersArray = MagServ.salesOrderList(sessID, filter);
    }
    catch
    {
       // Log failure
    }
}

answered Oct 27 '11 at 18:40

Matthew Flaschen

278,309
50
514
539

+1: I like this a bit better than Jon's answer. I think it makes the end condition of the loop more obvious. – Brian Oct 27 '11 at 21:16

score 2 · Answer 4 · answered Oct 27 '11 at 18:40

It its not gernally a good idead to use exceptions as control flow, but this will do what you requested.

bool Caught = true;
while (Caught)
try
{
    salesOrdersArray = MagServ.salesOrderList(sessID, filter);
    Caught = false;
}
catch
{
    Caught = true;
}

score 1 · Answer 5 · answered Oct 27 '11 at 18:51

I will use a transactional queue (MSMQ) to store the service call. A loop will dequeue messages and call the service in a TransactionScope, if the call fails the message appear to be still in the queue. An ov erall timeout can be specified by adding a time to expire in the message. This solution is good if you really want a reliable solution since I guessed that calling that operation is critical.

score 0 · Answer 6 · edited Jul 31 '12 at 17:13

0

bool repeat = true;
while (repeat)
{
    try
    {
       salesOrdersArray = MagServ.salesOrderList(sessID, filter);
       repeat = false;
    }
    catch
    {
    }
}

edited Jul 31 '12 at 17:13

Adi Lester

24,731
12
95
110

answered Oct 27 '11 at 18:40

Albin Sunnanbo

46,430
8
69
108

score 0 · Answer 7 · answered Oct 27 '11 at 18:41

0

Try

bool failed = false;
do {
 try
 {
  salesOrdersArray = MagServ.salesOrderList(sessID, filter);
 }
 catch
 {
  failed = true;
 }
} while(failed);

The behavior you are after might cause an endless loop if this never succeeds though...

answered Oct 27 '11 at 18:41

Jonatan Hedborg

4,382
21
30

score 0 · Answer 8 · answered Oct 27 '11 at 18:41

0

Try something like this:

var failed = true;
while (failed)
{
  try 
  {
    salesOrdersArray = MagServ.salesOrderList(sessID, filter); 
    failed = false;
  }
  catch
  {
  }
}

Edit: Wow! Great minds think alike! :)

answered Oct 27 '11 at 18:41

Rob H

1,840
16
25

score 0 · Answer 9 · answered Oct 27 '11 at 18:41

Although I would NOT recommend you to do this for an infinite number of times, you could make a separate function out of that one sentence:

void GoConnect()
{
    try
    {
        salesOrdersArray = MagServ.salesOrderList(sessID, filter);
    }
    catch
    {
        GoConnect();
    }
}

petebowden · Answer 10 · 2011-10-27T18:50:45.187

while(salesOrdersArray == null){

  try
  {
    salesOrdersArray = MagServ.salesOrderList(sessID, filter);
  }
  catch(salesOrderException e)
  {
     log(e.message);
  }
}

This will run forever, and is using exceptions as a loop which is slow. Is there a way you can modify your function that it returns null, instead of throwing an exception? If you're expecting that this call will fail regularly, don't use a try/catch block.

Renato Gama · Answer 11 · 2011-10-28T18:56:19.633

0

I follow this pattern in order to solve this problem:

    public void Send(String data, Int32 attemptNumber)
    {
        try
        {
            yourCodeHere(data);
        }
        catch (WebException ex)
        {
            if (attemptNumber > 0)
                Send(data, --attemptNumber);
            else
                throw new AttemptNumberExceededException("Attempt number exceeded!", ex);
        }
        catch (Exception ex)
        {
            //Log pourpose code goes here!
            throw;
        }
    }

Trying forever seems not to be a good idea as you may end up having an infinite process. If you think you need many attempts to achieve your goal just set huge number here.

I personally think its wise to wait some milliseconds, or seconds after eac attempt Thread.Sleep(1000); before callig Send(data); --- you could for example, use the attempNumber variable to increse or decrease this waiting time if you think its wise for your scenario.

edited Oct 28 '11 at 18:56

answered Oct 27 '11 at 18:44

Renato Gama

16,431
12
58
92

why shouldnt I? My point is the same as Jon Skeet, below the code he wrote – Renato Gama Oct 28 '11 at 02:41
2

Don't `throw ex;` http://stackoverflow.com/questions/178456/what-is-the-proper-way-to-re-throw-an-exception-in-c http://stackoverflow.com/questions/22623/net-throwing-exceptions-best-practices – asawyer Oct 28 '11 at 05:31
thanks @asawyer didn't know about that so I edited my answer! is that ok now? – Renato Gama Oct 28 '11 at 07:32
Two things: 1- You almost never want to catch the base exception class, and if you do, it's pretty much only for logging purposes, and then `throw`ing again. 2 - Never throw the base `Exception` class. You are forcing consuming code into the bad practice of catching `Exception`. Use a more appropriate subclass, or create your own. – asawyer Oct 28 '11 at 11:52
it makes real sense...! never thought this way... sorry for boring you, what about now? – Renato Gama Oct 28 '11 at 18:58
Just take a little time to read up on best practices, and you'll be in business in no time. Heres a couple good links: http://blogs.msdn.com/b/ericlippert/archive/2008/09/10/vexing-exceptions.aspx http://www.codeproject.com/KB/architecture/exceptionbestpractices.aspx – asawyer Oct 28 '11 at 19:01

Repeating a function in C# until it no longer throws an exception

11 Answers11

Linked