I want to go once through a loop but only if an exception is thrown go back through the loop. How would I write this in C#?
Thanks
This smells of bad design to me. The general rule is: exceptions should not be used for flow control. There are a number of reasons for this; namely, there are usually better/more reliable methods that can be used to check things before an exceptions is thrown, and also it decreases efficiency.
Nonetheless, just for the sake of argument, you could do something like the following:
while (true)
{
try
{
// do stuff here
}
catch (MyException)
{
continue;
}
// all is good
break;
}
Again - this is not the recommended way. I would be happy to suggest something better if you could provide a bit more context/examples/
What about the following where you can set a retry count:
int tryCount = 0;
while (tryCount < 3)
{
try
{
someReturn = SomeFunction(someParams);
}
catch (Exception)
{
tryCount++;
continue;
}
break;
}
That really depends on what you're doing, and the type of exception being thrown. Many exceptions aren't something that would be fixed by just trying again with the exact same inputs/data, and thus looping would just keep generating the exception ad infinitum.
Instead, you should check for relevant exceptions and then handle them in an appropriate manner for those particular exceptions.
You could use Polly
and then you just need to configure the Policy with your exceptions and retry count:
var retryPolicy = Policy
.Handle<IOException>(x => x.Message.Contains("already exist"))
.Or<FormatException>()
.Retry(3);
and you use like this:
retryPolicy.Execute(() =>
{
throw new FormatException();
});
Why not call a function that actually does the loop, and have a catch after it that would call the function again.
private void loop() {
for(...) {
}
}
some other method:
try {
loop();
} catch(Exception e) {
loop();
}
Something like:
bool done = false;
while( ! done )
{
try
{
DoSomething();
done = true;
} catch(Exception ex)
{
HandleException(ex);
}
}
As Noldorin said, it smells like a bad design. You're using exceptions to control the flow of the program. Better to have explicit checks for the conditions that will cause you to repeat the operation.
So I am using this simple stuff :D
bool exceptionthrow = false;
while (!exceptionthrow)
{
try
{
value = Convert.ToInt32(Console.ReadLine()); //example
exceptionthrow = true;
}
catch (Exception)
{
exceptionthrow = false;
continue;
}
}
Hope it helps :)
Related
I have this extension method that allows me to retry an operation if there is an exception, a typical use is trying to write to a file, but for some reason I can't so I retry a bit later...
The extension looks like:
public static void WithRetry<T>(this Action action, int timeToWait = 500, int timesToRetry = 3) where T : Exception
{
int retryCount = 0;
bool successful = false;
do
{
try
{
action();
successful = true;
}
catch (T)
{
retryCount++;
Thread.Sleep(timeToWait);
if (retryCount == timesToRetry) throw;
}
catch (Exception)
{
throw;
}
} while (retryCount < timesToRetry && !successful);
}
Visual studio tells me that I'm swallowing an exception in the first catch block, is this bad?
Thanks.
The warning is exactly what you are trying to achieve. You are swallowing the exceptions (timesToRetry-1) times. On the last try only you are actually throwing the exception. Until then all the exceptions will be swallowed and lost. Since this is the behavior you are trying to achieve. There is no harm in suppressing the message.
But as #HimBromBeere stated remove the catch(Exception) block. Also you can try logging the exception on each re-try because you will loose this data. What if different kind of exception is thrown each time. There is no way to be sure.
The warning is correct, you swallow exceptions. If you retry 10 times you will never know what went wrong the first 9 times, you only get exception number 10.
Maybe that's what you want. Personally, I would put all the occurring exceptions into an AggregateException and throw that when you hit your retry count.
Maybe like this:
public static void WithRetry<T>(this Action action, int timeToWait = 500, int timesToRetry = 3) where T : Exception
{
var exceptions = new List<Exception>();
for (int tryIndex = 0; tryIndex < timesToRetry; tryIndex++)
{
try
{
action();
return;
}
catch (T t)
{
exceptions.Add(t);
}
Thread.Sleep(timeToWait);
}
throw new AggregateException(exceptions);
}
This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
C# cleanest way to write retry logic?
i having a function contains web service call to the server which fails sometime (unable to connect remote server error) due to some disturbance in network. The code is in try catch block. i want to rerun the web service call within try block so that the web call will be done successfully.
const int MaxRetries = 5;
for(int i = 0; i < MaxRetries; i++)
{
try
{
// do stuff
break; // jump out of for loop if everything succeeded
}
catch(Exception)
{
Thread.Sleep(100); // optional delay here
}
}
bool success = false;
int retry = 0;
while (!success && retry<3)
{
try{
// web service calls
success = true;
} catch(Exception) {
retry ++;
}
}
public void Connect()
{
Connect(1);
}
private void Connect(int num)
{
if (num > 3)
throw new Exception("Maximum number of attempts reached");
try
{
// do stuff
}
catch
{
Connect(num++);
}
}
You can put a loop around the try catch block like this:
bool repeat = true
while( repeat){
repeat = false;
try
{
...
}
catch( Exception )
{
repeat = true;
}
}
I think you have your answer here. I just wanted to add a couple of suggestions based on my abundant experience with this problem.
If you add logging to the catch block, you can ascertain how often the web service call fails, and how many attempts were made in all. (Maybe put a toggle in web.config to turn this logging off once the issue subsides.)
That information may prove useful in discussions with system administrators if, for example, the web service provider is within your organization, such as on an intranet.
In addition, if you find that the calls are still failing too often, you could introduce a delay in the catch, so that the retry is not immediate. You might only want to do that on the final attempt. Sometimes it is worth the wait for the user, who doesn't want to lose all the data they have just entered.
And finally, depending on the situation, you could add a Retry button to the UI, so that the user could keep trying. The user could choose to wait five minutes for the network problem to clear itself up, and click Retry.
Wrap the try/catch in a while loop. Set a flag on success to exit the while (or just break out). Make sure you have some sort of retry limit so it won't keep going forever.
while (true)
{
try
{
// call webservice
// handle results
break;
}
catch (TemporaryException e)
{
// do any logging you wish
continue;
}
catch (FatalException e)
{
// do any logging you wish
break;
}
}
If you want to limit the retries, change the termination condition on the while loop.
void Method()
{
do
{
try
{
DoStuff();
return;
}
catch (Exception e)
{
// Do Something about exception.
}
}
while (true);
}
If you find yourself wanting to do this frequently in your code, you might consider implementing a reusable class that encapsulates the "re-try when an error is encountered" logic. This way, you can ensure that the behavior is standardized throughout your code base, instead of repeated each time.
There's an excellent example available on Dan Gartner's blog:
public class Retrier<TResult>
{
public TResult Try(Func<TResult> func, int maxRetries)
{
return TryWithDelay(func, maxRetries, 0);
}
public TResult TryWithDelay(Func<TResult> func, int maxRetries, int delayInMilliseconds)
{
TResult returnValue = default(TResult);
int numTries = 0;
bool succeeded = false;
while (numTries < maxRetries)
{
try
{
returnValue = func();
succeeded = true;
}
catch (Exception)
{
//todo: figure out what to do here
}
finally
{
numTries++;
}
if (succeeded)
return returnValue;
System.Threading.Thread.Sleep(delayInMilliseconds);
}
return default(TResult);
}
}
Well, the easiest would be to copy the code to the catch-block, right?
Another approach could look like:
private void YourMethodThatTriesToCallWebService()
{
//Don't catch errors
}
public void TryToCallWebService(int numTries)
{
bool failed = true;
for(int i = 0; i < numTries && failed; i++)
{
try{
YourMethodThatTriesToCallWebService();
failed = false;
}catch{
//do nothing
}
}
}
You should put the entire catch block into a while statement:
while(retryCount < MAX_RETRY && !success)
{
try
{
//do stuff , calling web service
success = true;
}
catch
{
retryCount++
success = false;
}
}
Is finally executed in this case? I wrote this code and couldn't decide if finally is really executed in this case or not. I would really like some explanations no matter what the answer is.
foreach(string s in allStrings)
{
try
{
//Error happens here
}
catch(Exception ex)
{
//Handle exception
break;
}
finally
{
//Clean up code
}
}
You've written 90% of the code that you'd need to answer this question for yourself.
Keep writing.
Yes. Finally blocks are always executed when control leaves a corresponding try or catch block. (Unless something super-special happens, like a runtime crash or the thread was aborted.)
Effectively, finally blocks are ALWAYS called. That's the reason they're named that way...
Based on Matt's answer. It is a total overkill indeed.
I suggest doing something like this, and making sure that the test passes.
MSTest is not the best library for testing, but it is the "Standard" :)
[TestClass()]
public class FinalClauseTester
{
private TestContext testContextInstance;
public TestContext TestContext
{
get
{
return testContextInstance;
}
set
{
testContextInstance = value;
}
}
[TestMethod]
[DeploymentItem(#"Something right goes here.")]
[DataSource("Something else goes here", "row", somethingOtherSetupCrap)]
public void TestFinalClause()
{
string[] allStrings = {"1", "2", "3", "4", "5"};
int yesCount = 0;
foreach(string s in allStrings)
{
try
{
//Error happens here
throw new Exception();
}
catch(Exception ex)
{
//Handle exception
if (yesCount == 3)
{
break;
}
}
finally
{
//Clean up code
yesCount++;
}
}
// And, at the end of this loop ...
Debug.Assert(yesCount = 3); // Or something like this.
}
}
Yes, it is. Finally blocks always are executed. See http://download.oracle.com/javase/tutorial/essential/exceptions/finally.html
Yes it tries the try, then catch catches the exception, and then finally is what is finally called for both normal execution and caught exceptions. I think there's a witticism hiding somewhere!
In Python, there is this useful exception handling code:
try:
# Code that could raise an exception
except Exception:
# Exception handling
else:
# Code to execute if the try block DID NOT fail
I think it's useful to be able to separate the code that could raise and exception from your normal code. In Python, this was possible as shown above, however I can't find anything like it in C#.
Assuming the feature or one like it doesn't exist, is it standard practice to put normal code in the try block or after the catch block?
The reason I ask is because I have the following code:
if (!IsReadOnly)
{
T newobj;
try
{
newobj = DataPortal.Update<T>(this);
List<string> keys = new List<string>(BasicProperties.Keys);
foreach (string key in keys)
{
BasicProperties[key] = newobj.BasicProperties[key];
}
}
catch (DataPortalException)
{
// TODO: Implement DataPortal.Update<T>() recovery mechanism
}
}
Which requires the normal code to be in the try block because otherwise if an exception was raised and subsequently handled, newobj would be unassigned, but it feels quite unnatural to have this much code in the try block which is unrelated to the DataPortalException. What to do?
Thanks
I would prefer to see the rest of the code outside the try/catch so it is clear where the exception you are trying to catch is coming from and that you don't accidentally catch an exception that you weren't trying to catch.
I think the closest equivalent to the Python try/catch/else is to use a local boolean variable to remember whether or not an exception was thrown.
bool success;
try
{
foo();
success = true;
}
catch (MyException)
{
recover();
success = false;
}
if (success)
{
bar();
}
But if you are doing this, I'd ask why you don't either fully recover from the exception so that you can continue as if there had been success, or else fully abort by returning an error code or even just letting the exception propagate to the caller.
Barbaric solution: create an Else class derived from Exception, throw an instance of it at the end of the try block, and use catch (Else) {...} to handle the other stuff.
I feel so dirty.
This will might get downvoted but doesn't c# have goto(note I have almost no c# knowledge so I have no idea if this works).
what about something like
try
{
...
}
catch(Exception ex)
{
...
goto Jump_past_tryelse
}
...//Code to execute if the try block DID NOT fail
Jump_past_tryelse:
...
C# does not have such a concept, so you are just left with three options,
put the else code inside the try.
put the else code outside the try catch block, use a local variable to indicate success or failure, and an if block around your else code.
put the else code in the finally block, use a local variable to indicate success or failure, and an if block arount you else code.
Allow me to repeat an idea from a similar StackOverflow question. You cannot do this directly, but you can write a method that encapsulates the behavior you need. Look at the original question to see how to implement the method (if you're not familiar with lambda expressions and Func delegates). The usage could look like this:
TryExceptRaise(() => {
// code that can throw exception
}, (Exception e) => {
// code to run in case of an exception
return (...);
}, () => {
// code to run if there is no exception
return (...);
});
Just put your "else" block before the catch. Then, it will only execute if code execution reaches that point:
try
{
fee();
fi();
foe();
fum();
/// put your "else" stuff here.
/// It will only be executed if fee-fi-foe-fum did not fail.
}
catch(Exception e)
{
// handle exception
}
Given that, I fail to see the use of try..catch...else unless there's something vital missing from the OP's description.
With C# version 7, you could use local functions to emulate this behaviour:
Example 1: (since C# version 7)
void Main()
{
void checkedCode()
{
try
{
foo();
}
catch (Exception ex)
{
recover();
return;
}
// ElseCode here
}
checkedCode();
}
If you prefer lambda syntax, you could also declare a run method
void Run(Action r) { r(); }
which only needs to be there once in your code, and then use the pattern for anonymous methods as follows
Example 2: (older C# versions and C# version 7)
Run(() => {
try
{
foo();
}
catch (Exception)
{
recover();
return;
}
// ElseCode here
});
whereever you need to enclose code in a safe context.
Try it in DotNetFiddle
Notes:
In both examples a function context is created so that we can use return; to exit on error.
You can find a similar pattern like the one used in Example 2 in JavaScript: Self-invoking anonymous functions (e.g. JQuery uses them). Because in C# you cannot self-invoke, the helper method Run is used.
Since Run does not have to be a local function, Example 2 works with older C# versions as well
You could do something like this:
if (!IsReadOnly)
{
T newobj = null;
try
{
newobj = DataPortal.Update<T>(this);
}
catch (DataPortalException)
{
// TODO: Implement DataPortal.Update<T>() recovery mechanism
}
if (newobj != null)
{
List<string> keys = new List<string>(BasicProperties.Keys);
foreach (string key in keys)
{
BasicProperties[key] = newobj.BasicProperties[key];
}
}
}
that would be the empty statement like hits
try
{
somethingThatCanThrow();
}
catch(Exception ex)
{
LogException(ex);
return;
}
ContinueFlow();
if (!IsReadOnly)
{
T newobj;
bool Done;
try
{
newobj = DataPortal.Update<T>(this);
List<string> keys = new List<string>(BasicProperties.Keys);
foreach (string key in keys)
{
BasicProperties[key] = newobj.BasicProperties[key];
}
Done = true;
}
catch (DataPortalException)
{
// TODO: Implement DataPortal.Update<T>() recovery mechanism
Done = false;
}
finally
{
if (newobj != null && Done == false)
{
List<string> keys = new List<string>(BasicProperties.Keys);
foreach (string key in keys)
{
BasicProperties[key] = newobj.BasicProperties[key];
}
}
}
}
Working on a project where a sequential set of methods must be run every x seconds. Right now I have the methods contained within another "parent method", and just sequentially call them right after another.
class DoTheseThings()
{
DoThis();
NowDoThat();
NowDoThis();
MoreWork();
AndImSpent();
}
Each method must run successfully without throwing an exception before the next step can be done. So now I wrapped each of those methods with a while and try..catch, then in the catch execute that method again.
while( !hadError )
{
try
{
DoThis();
}
catch(Exception doThisException )
{
hadError = true;
}
}
This seems smelly and not very dry. Is there a better way to do this so I'm not wrapping any new functionality in the same methods. Isn't some kind of Delegate collection the proper way to implement this?
Is there a more "proper" solution?
Action[] work=new Action[]{new Action(DoThis), new Action(NowDoThat),
new Action(NowDoThis), new Action(MoreWork), new Action(AndImSpent)};
int current =0;
while(current!=work.Length)
{
try
{
work[current]();
current++;
}
catch(Exception ex)
{
// log the error or whatever
// maybe sleep a while to not kill the processors if a successful execution depends on time elapsed
}
}
Isn't some kind of Delegate collection the proper way to implement this?
Delegate is a possible way to solve this problem.
Just create a delegate something like:
public delegate void WorkDelegate();
and put them in arraylist which you can iterate over.
I have a personal religious belief that you shouldn't catch System.Exception, or more accurately, you should only catch the exceptions you know how to handle.
That being said, I am going to assume that each one of the methods that you are calling are doing something different, and could result in different exceptions being thrown. Which means you would likely need to have different handlers for each method.
If you follow my religion as well, and the second statement is true, then you are not repeating code unnecessarily. Unless you have other requirements, my recommendations to improve your code would be:
1) Put the try-catch in each method, not around each method call.
2) Have the catches within each method catch ONLY the exceptions you know about.
http://blogs.msdn.com/fxcop/archive/2006/06/14/631923.aspx
http://blogs.msdn.com/oldnewthing/archive/2005/01/14/352949.aspx
http://www.joelonsoftware.com/articles/Wrong.html
HTH ...
your example seems ok.. its a dry one but will do the job well!! actually if this methods execute db access.. you can use transaction to ensure integrity...
if your dealing with shared variables for multi threader programs.. it is cleaner to use synchronization.. the most important thing in coding is that you write the proper code... that has less bugs.. and will do the task correctly..
public void DoTheseThings()
{
SafelyDoEach( new Action[]{
DoThis,
NowDoThat,
NowDoThis,
MoreWork,
AndImSpent
})
}
public void SafelyDoEach( params Action[] actions )
{
try
{
foreach( var a in actions )
a();
}
catch( Exception doThisException )
{
// blindly swallowing every exception like this is a terrible idea
// you should really only be swallowing a specific MyAbortedException type
return;
}
}
What would be the reason that an error was occuring?
If this were a resource issue, such as access to something like a connection or object, then you might want to look at using monitors, semaphores, or just locking.
lock (resource)
{
Dosomething(resource);
}
This way if a previous method is accessing the resource, then you can wait until it releases the resource to continue.
Ideally, you shouldn't have to run a loop to execute something each time it fails. It is failing at all, you would want to know about the issue and fix it. Having a loop to always just keep trying is not the right way to go here.
I'd do what Ovidiu Pacurar suggests, only I'd use a foreach loop and leave dealing with array indexes up to the compiler.
Simple delegate approach:
Action<Action> tryForever = (action) => {
bool success;
do {
try {
action();
success = true;
} catch (Exception) {
// should probably log or something here...
}
} while (!success);
};
void DoEverything() {
tryForever(DoThis);
tryForever(NowDoThat);
tryForever(NowDoThis);
tryForever(MoreWork);
tryForever(AndImSpent);
}
Stack approach:
void DoEverything() {
Stack<Action> thingsToDo = new Stack<Action>(
new Action[] {
DoThis, NowDoThat, NowDoThis, MoreWork, AndImSpent
}
);
Action action;
while ((action = thingsToDo.Pop()) != null) {
bool success;
do {
try {
action();
success = true;
} catch (Exception) {
}
} while (!success);
}