Additional try statement in catch statement - code smell?

Situation:

My application need to process the first step in the business rules (the initial try-catch statement). If an certain error occurs when the process calls the helper method during the step, I need to switch to a second process in the catch statement. The back up process uses the same helper method. If an same error occurs during the second process, I need to stop the entire process and throw the exception.

Implementation:

I was going to insert another try-catch statement into the catch statement of the first try-catch statement.

//run initial process
try
{
    //initial information used in helper method
    string s1 = "value 1";

    //call helper method
    HelperMethod(s1);
}
catch(Exception e1)
{
    //backup information if first process generates an exception in the helper method
    string s2 = "value 2";

    //try catch statement for second process.
    try
    {
        HelperMethod(s2开发者_如何学C);
    }
    catch(Exception e2)
    {
        throw e2;
    }
}

What would be the correct design pattern to avoid code smells in this implementation?

I caused some confusion and left out that when the first process fails and switches to the second process, it will send different information to the helper method. I have updated the scenario to reflect the entire process.

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If the HelperMethod needs a second try, there is nothing directly wrong with this, but your code in the catch tries to do way too much, and it destroys the stacktrace from e2.

You only need:

try
{
    //call helper method
    HelperMethod();
}    
catch(Exception e1)
{
    // maybe log e1, it is getting lost here
    HelperMethod();
}

---

I wouldn't say it is bad, although I'd almost certainly refactor the second block of code into a second method, so keep it comprehensible. And probably catch something more specific than Exception. A second try is sometimes necessary, especially for things like Dispose() implementations that might themselves throw (WCF, I'm looking at you).

---

The general idea putting a try-catch inside the catch of a parent try-catch doesn't seem like a code-smell to me. I can think of other legitimate reasons for doing this - for instance, when cleaning up an operation that failed where you do not want to ever throw another error (such as if the clean-up operation also fails). Your implementation, however, raises two questions for me: 1) Wim's comment, and 2) do you really want to entirely disregard why the operation originally failed (the e1 Exception)? Whether the second process succeeds or fails, your code does nothing with the original exception.

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Generally speaking, this isn't a problem, and it isn't a code smell that I know of.

With that said, you may want to look at handling the error within your first helper method instead of just throwing it (and, thus, handling the call to the second helper method in there). That's only if it makes sense, but it is a possible change.

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Yes, a more general pattern is have the basic method include an overload that accepts an int attempt parameter, and then conditionally call itself recursively.

   private void MyMethod (parameterList)
   {  MyMethod(ParameterList, 0)l }

   private void MyMethod(ParameterList, int attempt)
   {
      try { HelperMethod(); }
      catch(SomeSpecificException)
      {
          if (attempt < MAXATTEMPTS)
              MyMethod(ParameterList, ++attempt);
          else throw;
      }
   }

---

It shouldn't be that bad. Just document clearly why you're doing it, and most DEFINITELY try catching a more specific Exception type.

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If you need some retry mechanism, which it looks like, you may want to explore different techniques, looping with delays etc.

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It would be a little clearer if you called a different function in the catch so that a reader doesn't think you're just retrying the same function, as is, over again. If there's state happening that's not being shown in your example, you should document it carefully, at a minimum.

You also shouldn't throw e2; like that: you should simply throw; if you're going to work with the exception you caught at all. If not, you shouldn't try/catch.

Where you do not reference e1, you should simply catch (Exception) or better still catch (YourSpecificException)

---

If you're doing this to try and recover from some sort of transient error, then you need to be careful about how you implement this.

For example, in an environment where you're using SQL Server Mirroring, it's possible that the server you're connected to may stop being the master mid-connection.

In that scenario, it may be valid for your application to try and reconnect, and re-execute any statements on the new master - rather than sending an error back to the caller immediately.

You need to be careful to ensure that the methods you're calling don't have their own automatic retry mechanism, and that your callers are aware there is an automatic retry built into your method. Failing to ensure this can result in scenarios where you cause a flood of retry attempts, overloading shared resources (such as Database servers).

You should also ensure you're catching exceptions specific to the transient error you're trying to retry. So, in the example I gave, SqlException, and then examining to see if the error was that the SQL connection failed because the host was no longer the master.

If you need to retry more than once, consider placing an 'automatic backoff' retry delay - the first failure is retried immediately, the second after a delay of (say) 1 second, then doubled up to a maximum of (say) 90 seconds. This should help prevent overloading resources.

I would also suggest restructuring your method so that you don't have an inner-try/catch.

For example:

bool helper_success = false; 
bool automatic_retry = false; 
//run initial process
try
{
    //call helper method
    HelperMethod();
    helper_success = true; 
}
catch(Exception e)
{
    // check if e is a transient exception. If so, set automatic_retry = true 
} 


if (automatic_retry)
{    //try catch statement for second process.
    try
    {
        HelperMethod();
    }
    catch(Exception e)
    {
        throw;
    }
}

---

Here's another pattern:

// set up state for first attempt
if(!HelperMethod(false)) {
        // set up state for second attempt
        HelperMethod(true);
        // no need to try catch since you're just throwing anyway
}

Here, HelperMethod is

bool HelperMethod(bool throwOnFailure)

and the return value indicates whether or not success occurred (i.e., false indicates failure and true indicates success). You could also do:

// could wrap in try/catch
HelperMethod(2, stateChanger);

where HelperMethod is

void HelperMethod(int numberOfTries, StateChanger[] stateChanger)

where numberOfTries indicates the number of times to try before throwing an exception and StateChanger[] is an array of delegates that will change the state for you between calls (i.e., stateChanger[0] is called before the first attempt, stateChanger[1] is called before the second attempt, etc.)

This last option indicates that you might have a smelly setup though. It looks like the class that is encapsulating this process is responsible for both keeping track of state (which employee to look up) as well as looking up the employee (HelperMethod). By SRP, these should be separate.

Of course, you need to a catch a more specific exception than you currently are (don't catch the base class Exception!) and you should just throw instead of throw e if you need to rethrow the exception after logging, cleanup, etc.

---

You could emulate C#'s TryParse method signatures:

class Program
{
    static void Main(string[] args)
    {
        Exception ex;
        Console.WriteLine("trying 'ex'");
        if (TryHelper("ex", out ex))
        {
            Console.WriteLine("'ex' worked");
        }
        else
        {
            Console.WriteLine("'ex' failed: " + ex.Message);
            Console.WriteLine("trying 'test'");
            if (TryHelper("test", out ex))
            {
                Console.WriteLine("'test' worked");
            }
            else
            {
                Console.WriteLine("'test' failed: " + ex.Message);
                throw ex;
            }
        }
    }

    private static bool TryHelper(string s, out Exception result)
    {
        try
        {
            HelperMethod(s);
            result = null;
            return true;
        }
        catch (Exception ex)
        {
            // log here to preserve stack trace
            result = ex;
            return false;
        }
    }

    private static void HelperMethod(string s)
    {
        if (s.Equals("ex"))
        {
            throw new Exception("s can be anything except 'ex'");
        }
    }

}

---

Another way is to flatten the try/catch blocks, useful if you're using some exception-happy API:

public void Foo()
{
  try
  {
    HelperMethod("value 1");
    return; // finished
  }
  catch (Exception e)
  {
     // possibly log exception
  }

  try
  {
    HelperMethod("value 2");
    return; // finished
  }
  catch (Exception e)
  {
     // possibly log exception
  }

  // ... more here if needed
}

---

An option for retry (that most people will probably flame) would be to use a goto. C# doesn't have filtered exceptions but this could be used in a similar manner.

const int MAX_RETRY = 3;

public static void DoWork()
{
    //Do Something
}

public static void DoWorkWithRetry()
{
    var @try = 0;
retry:
    try
    {
        DoWork();
    }
    catch (Exception)
    {
        @try++;
        if (@try < MAX_RETRY)
            goto retry;
        throw;
    }
}

---

In this case you know this "exception" probably will happen so I would prefer a simple approach an leave exceptions for the unknown events.

//run initial process
try
{
    //initial information used in helper method
    string s1 = "value 1";

    //call helper method
    if(!HelperMethod(s1))
    {
        //backup information if first process generates an exception in the helper method
        string s2 = "value 2";
        if(!HelperMethod(s2))
        {
          return ErrorOfSomeKind;
        }
    }
    return Ok;
}
catch(ApplicationException ex)
{
    throw;
}

---

I know that I've done the above nested try catch recently to handle decoding data where two third party libraries throw exceptions on failure to decode (Try json decode, then try base64 decode), but my preference is to have functions return a value which can be checked.

I generally only use the throwing of exceptions to exit early and notify something up the chain about the error if it's fatal to the process.

If a function is unable to provide a meaningful response, that is not typically a fatal problem (Unlike bad input data).

It seems like the main risk in nested try catch is that you also end up catching all the other (maybe important) exceptions that might occur.