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To avoid all standard-answers I could have Googled on, I will provide an example you all can attack at will.
C# and Java (and too many others) have with plenty of types some of ‘overflow’ behaviour I don’t like at all (e.g type.MaxValue + type.SmallestValue == type.MinValue for example : int.MaxValue + 1 == int.MinValue).
But, seen my vicious nature, I’ll add some insult to this injury by expanding this behaviour to, let’s say an Overridden DateTime type. (I know DateTime is sealed in .NET, but for the sake of this example, I’m using a pseudo language that is exactly like C#, except for the fact that DateTime isn’t sealed).
The overridden Add method:
/// <summary>
/// Increments this date with a timespan, but loops when
/// the maximum value for datetime is exceeded.
/// </summary>
/// <param name="ts">The timespan to (try to) add</param>
/// <returns>The Date, incremented with the given timespan.
/// If DateTime.MaxValue is exceeded, the sum wil 'overflow' and
/// continue from DateTime.MinValue.
/// </returns>
public DateTime override Add(TimeSpan ts)
{
try
{
return base.Add(ts);
}
catch (ArgumentOutOfRangeException nb)
{
// calculate how much the MaxValue is exceeded
// regular program flow
TimeSpan saldo = ts - (base.MaxValue - this);
return DateTime.MinValue.Add(saldo)
}
catch(Exception anyOther)
{
// 'real' exception handling.
}
}
Of course an if could solve this just as easy, but the fact remains that I just fail to see why you couldn’t use exceptions (logically that is, I can see that when performance is an issue that in certain cases exceptions should be avoided).
I think in many cases they are more clear than if-structures and don’t break any contract the method is making.
IMHO the “Never use them for regular program flow” reaction everybody seems to have is not that well underbuild as the strength of that reaction can justify.
Or am I mistaken?
I've read other posts, dealing with all kind of special cases, but my point is there's nothing wrong with it if you are both:
Clear
Honour the contract of your method
Shoot me.
Have you ever tried to debug a program raising five exceptions per second in the normal course of operation ?
I have.
The program was quite complex (it was a distributed calculation server), and a slight modification at one side of the program could easily break something in a totally different place.
I wish I could just have launched the program and wait for exceptions to occur, but there were around 200 exceptions during the start-up in the normal course of operations
My point : if you use exceptions for normal situations, how do you locate unusual (ie exceptional) situations ?
Of course, there are other strong reasons not to use exceptions too much, especially performance-wise
Exceptions are basically non-local goto statements with all the consequences of the latter. Using exceptions for flow control violates a principle of least astonishment, make programs hard to read (remember that programs are written for programmers first).
Moreover, this is not what compiler vendors expect. They expect exceptions to be thrown rarely, and they usually let the throw code be quite inefficient. Throwing exceptions is one of the most expensive operations in .NET.
However, some languages (notably Python) use exceptions as flow-control constructs. For example, iterators raise a StopIteration exception if there are no further items. Even standard language constructs (such as for) rely on this.
My rule of thumb is:
If you can do anything to recover from an error, catch exceptions
If the error is a very common one (eg. user tried to log in with the wrong password), use returnvalues
If you can't do anything to recover from an error, leave it uncaught (Or catch it in your main-catcher to do some semi-graceful shutdown of the application)
The problem I see with exceptions is from a purely syntax point of view (I'm pretty sure the perfomance overhead is minimal). I don't like try-blocks all over the place.
Take this example:
try
{
DoSomeMethod(); //Can throw Exception1
DoSomeOtherMethod(); //Can throw Exception1 and Exception2
}
catch(Exception1)
{
//Okay something messed up, but is it SomeMethod or SomeOtherMethod?
}
.. Another example could be when you need to assign something to a handle using a factory, and that factory could throw an exception:
Class1 myInstance;
try
{
myInstance = Class1Factory.Build();
}
catch(SomeException)
{
// Couldn't instantiate class, do something else..
}
myInstance.BestMethodEver(); // Will throw a compile-time error, saying that myInstance is uninitalized, which it potentially is.. :(
Soo, personally, I think you should keep exceptions for rare error-conditions (out of memory etc.) and use returnvalues (valueclasses, structs or enums) to do your error checking instead.
Hope I understood your question correct :)
A first reaction to a lot of answers :
you're writing for the programmers and the principle of least astonishment
Of course! But an if just isnot more clear all the time.
It shouldn't be astonishing eg : divide (1/x) catch (divisionByZero) is more clear than any if to me (at Conrad and others) . The fact this kind of programming isn't expected is purely conventional, and indeed, still relevant. Maybe in my example an if would be clearer.
But DivisionByZero and FileNotFound for that matter are clearer than ifs.
Of course if it's less performant and needed a zillion time per sec, you should of course avoid it, but still i haven't read any good reason to avoid the overal design.
As far as the principle of least astonishment goes : there's a danger of circular reasoning here : suppose a whole community uses a bad design, this design will become expected! Therefore the principle cannot be a grail and should be concidered carefully.
exceptions for normal situations, how do you locate unusual (ie exceptional) situations ?
In many reactions sth. like this shines trough. Just catch them, no? Your method should be clear, well documented, and hounouring it's contract. I don't get that question I must admit.
Debugging on all exceptions : the same, that's just done sometimes because the design not to use exceptions is common. My question was : why is it common in the first place?
Before exceptions, in C, there were setjmp and longjmp that could be used to accomplish a similar unrolling of the stack frame.
Then the same construct was given a name: "Exception". And most of the answers rely on the meaning of this name to argue about its usage, claiming that exceptions are intended to be used in exceptional conditions. That was never the intent in the original longjmp. There were just situations where you needed to break control flow across many stack frames.
Exceptions are slightly more general in that you can use them within the same stack frame too. This raises analogies with goto that I believe are wrong. Gotos are a tightly coupled pair (and so are setjmp and longjmp). Exceptions follow a loosely coupled publish/subscribe that is much cleaner! Therefore using them within the same stack frame is hardly the same thing as using gotos.
The third source of confusion relates to whether they are checked or unchecked exceptions. Of course, unchecked exceptions seem particularly awful to use for control flow and perhaps a lot of other things.
Checked exceptions however are great for control flow, once you get over all the Victorian hangups and live a little.
My favorite usage is a sequence of throw new Success() in a long fragment of code that tries one thing after the other until it finds what it is looking for. Each thing -- each piece of logic -- may have arbritrary nesting so break's are out as also any kind of condition tests. The if-else pattern is brittle. If I edit out an else or mess up the syntax in some other way, then there is a hairy bug.
Using throw new Success() linearizes the code flow. I use locally defined Success classes -- checked of course -- so that if I forget to catch it the code won't compile. And I don't catch another method's Successes.
Sometimes my code checks for one thing after the other and only succeeds if everything is OK. In this case I have a similar linearization using throw new Failure().
Using a separate function messes with the natural level of compartmentalization. So the return solution is not optimal. I prefer to have a page or two of code in one place for cognitive reasons. I don't believe in ultra-finely divided code.
What JVMs or compilers do is less relevant to me unless there is a hotspot. I cannot believe there is any fundamental reason for compilers to not detect locally thrown and caught Exceptions and simply treat them as very efficient gotos at the machine code level.
As far as using them across functions for control flow -- i. e. for common cases rather than exceptional ones -- I cannot see how they would be less efficient than multiple break, condition tests, returns to wade through three stack frames as opposed to just restore the stack pointer.
I personally do not use the pattern across stack frames and I can see how it would require design sophistication to do so elegantly. But used sparingly it should be fine.
Lastly, regarding surprising virgin programmers, it is not a compelling reason. If you gently introduce them to the practice, they will learn to love it. I remember C++ used to surprise and scare the heck out of C programmers.
The standard anwser is that exceptions are not regular and should be used in exceptional cases.
One reason, which is important to me, is that when I read a try-catch control structure in a software I maintain or debug, I try to find out why the original coder used an exception handling instead of an if-else structure. And I expect to find a good answer.
Remember that you write code not only for the computer but also for other coders. There is a semantic associated to an exception handler that you cannot throw away just because the machine doesn't mind.
Josh Bloch deals with this topic extensively in Effective Java. His suggestions are illuminating and should apply to .NET as well (except for the details).
In particular, exceptions should be used for exceptional circumstances. The reasons for this are usability-related, mainly. For a given method to be maximally usable, its input and output conditions should be maximally constrained.
For example, the second method is easier to use than the first:
/**
* Adds two positive numbers.
*
* #param addend1 greater than zero
* #param addend2 greater than zero
* #throws AdditionException if addend1 or addend2 is less than or equal to zero
*/
int addPositiveNumbers(int addend1, int addend2) throws AdditionException{
if( addend1 <= 0 ){
throw new AdditionException("addend1 is <= 0");
}
else if( addend2 <= 0 ){
throw new AdditionException("addend2 is <= 0");
}
return addend1 + addend2;
}
/**
* Adds two positive numbers.
*
* #param addend1 greater than zero
* #param addend2 greater than zero
*/
public int addPositiveNumbers(int addend1, int addend2) {
if( addend1 <= 0 ){
throw new IllegalArgumentException("addend1 is <= 0");
}
else if( addend2 <= 0 ){
throw new IllegalArgumentException("addend2 is <= 0");
}
return addend1 + addend2;
}
In either case, you need to check to make sure that the caller is using your API appropriately. But in the second case, you require it (implicitly). The soft Exceptions will still be thrown if the user didn't read the javadoc, but:
You don't need to document it.
You don't need to test for it (depending upon how aggresive your
unit testing strategy is).
You don't require the caller to handle three use cases.
The ground-level point is that Exceptions should not be used as return codes, largely because you've complicated not only YOUR API, but the caller's API as well.
Doing the right thing comes at a cost, of course. The cost is that everyone needs to understand that they need to read and follow the documentation. Hopefully that is the case anyway.
How about performance? While load testing a .NET web app we topped out at 100 simulated users per web server until we fixed a commonly-occuring exception and that number increased to 500 users.
I think that you can use Exceptions for flow control. There is, however, a flipside of this technique. Creating Exceptions is a costly thing, because they have to create a stack trace. So if you want to use Exceptions more often than for just signalling an exceptional situation you have to make sure that building the stack traces doesn't negatively influence your performance.
The best way to cut down the cost of creating exceptions is to override the fillInStackTrace() method like this:
public Throwable fillInStackTrace() { return this; }
Such an exception will have no stacktraces filled in.
Here are best practices I described in my blog post:
Throw an exception to state an unexpected situation in your software.
Use return values for input validation.
If you know how to deal with exceptions a library throws, catch them at the lowest level possible.
If you have an unexpected exception, discard current operation completely. Don’t pretend you know how to deal with them.
I don't really see how you're controlling program flow in the code you cited. You'll never see another exception besides the ArgumentOutOfRange exception. (So your second catch clause will never be hit). All you're doing is using an extremely costly throw to mimic an if statement.
Also you aren't performing the more sinister of operations where you just throw an exception purely for it to be caught somewhere else to perform flow control. You're actually handling an exceptional case.
Apart from the reasons stated, one reason not to use exceptions for flow control is that it can greatly complicate the debugging process.
For example, when I'm trying to track down a bug in VS I'll typically turn on "break on all exceptions". If you're using exceptions for flow control then I'm going to be breaking in the debugger on a regular basis and will have to keep ignoring these non-exceptional exceptions until I get to the real problem. This is likely to drive someone mad!!
Lets assume you have a method that does some calculations. There are many input parameters it has to validate, then to return a number greater then 0.
Using return values to signal validation error, it's simple: if method returned a number lesser then 0, an error occured. How to tell then which parameter didn't validate?
I remember from my C days a lot of functions returned error codes like this:
-1 - x lesser then MinX
-2 - x greater then MaxX
-3 - y lesser then MinY
etc.
Is it really less readable then throwing and catching an exception?
Because the code is hard to read, you may have troubles debugging it, you will introduce new bugs when fixing bugs after a long time, it is more expensive in terms of resources and time, and it annoys you if you are debugging your code and the debugger halts on the occurence of every exception ;)
If you are using exception handlers for control flow, you are being too general and lazy. As someone else mentioned, you know something happened if you are handling processing in the handler, but what exactly? Essentially you are using the exception for an else statement, if you are using it for control flow.
If you don't know what possible state could occur, then you can use an exception handler for unexpected states, for example when you have to use a third-party library, or you have to catch everything in the UI to show a nice error message and log the exception.
However, if you do know what might go wrong, and you don't put an if statement or something to check for it, then you are just being lazy. Allowing the exception handler to be the catch-all for stuff you know could happen is lazy, and it will come back to haunt you later, because you will be trying to fix a situation in your exception handler based on a possibly false assumption.
If you put logic in your exception handler to determine what exactly happened, then you would be quite stupid for not putting that logic inside the try block.
Exception handlers are the last resort, for when you run out of ideas/ways to stop something from going wrong, or things are beyond your ability to control. Like, the server is down and times out and you can't prevent that exception from being thrown.
Finally, having all the checks done up front shows what you know or expect will occur and makes it explicit. Code should be clear in intent. What would you rather read?
You can use a hammer's claw to turn a screw, just like you can use exceptions for control flow. That doesn't mean it is the intended usage of the feature. The if statement expresses conditions, whose intended usage is controlling flow.
If you are using a feature in an unintended way while choosing to not use the feature designed for that purpose, there will be an associated cost. In this case, clarity and performance suffer for no real added value. What does using exceptions buy you over the widely-accepted if statement?
Said another way: just because you can doesn't mean you should.
As others have mentioned numerously, the principle of least astonishment will forbid that you use exceptions excessively for control flow only purposes. On the other hand, no rule is 100% correct, and there are always those cases where an exception is "just the right tool" - much like goto itself, by the way, which ships in the form of break and continue in languages like Java, which are often the perfect way to jump out of heavily nested loops, which aren't always avoidable.
The following blog post explains a rather complex but also rather interesting use-case for a non-local ControlFlowException:
http://blog.jooq.org/2013/04/28/rare-uses-of-a-controlflowexception
It explains how inside of jOOQ (a SQL abstraction library for Java), such exceptions are occasionally used to abort the SQL rendering process early when some "rare" condition is met.
Examples of such conditions are:
Too many bind values are encountered. Some databases do not support arbitrary numbers of bind values in their SQL statements (SQLite: 999, Ingres 10.1.0: 1024, Sybase ASE 15.5: 2000, SQL Server 2008: 2100). In those cases, jOOQ aborts the SQL rendering phase and re-renders the SQL statement with inlined bind values. Example:
// Pseudo-code attaching a "handler" that will
// abort query rendering once the maximum number
// of bind values was exceeded:
context.attachBindValueCounter();
String sql;
try {
// In most cases, this will succeed:
sql = query.render();
}
catch (ReRenderWithInlinedVariables e) {
sql = query.renderWithInlinedBindValues();
}
If we explicitly extracted the bind values from the query AST to count them every time, we'd waste valuable CPU cycles for those 99.9% of the queries that don't suffer from this problem.
Some logic is available only indirectly via an API that we want to execute only "partially". The UpdatableRecord.store() method generates an INSERT or UPDATE statement, depending on the Record's internal flags. From the "outside", we don't know what kind of logic is contained in store() (e.g. optimistic locking, event listener handling, etc.) so we don't want to repeat that logic when we store several records in a batch statement, where we'd like to have store() only generate the SQL statement, not actually execute it. Example:
// Pseudo-code attaching a "handler" that will
// prevent query execution and throw exceptions
// instead:
context.attachQueryCollector();
// Collect the SQL for every store operation
for (int i = 0; i < records.length; i++) {
try {
records[i].store();
}
// The attached handler will result in this
// exception being thrown rather than actually
// storing records to the database
catch (QueryCollectorException e) {
// The exception is thrown after the rendered
// SQL statement is available
queries.add(e.query());
}
}
If we had externalised the store() logic into "re-usable" API that can be customised to optionally not execute the SQL, we'd be looking into creating a rather hard to maintain, hardly re-usable API.
Conclusion
In essence, our usage of these non-local gotos is just along the lines of what [Mason Wheeler][5] said in his answer:
"I just encountered a situation that I cannot deal with properly at this point, because I don't have enough context to handle it, but the routine that called me (or something further up the call stack) ought to know how to handle it."
Both usages of ControlFlowExceptions were rather easy to implement compared to their alternatives, allowing us to reuse a wide range of logic without refactoring it out of the relevant internals.
But the feeling of this being a bit of a surprise to future maintainers remains. The code feels rather delicate and while it was the right choice in this case, we'd always prefer not to use exceptions for local control flow, where it is easy to avoid using ordinary branching through if - else.
Typically there is nothing wrong, per se, with handling an exception at a low level. An exception IS a valid message that provides a lot of detail for why an operation cannot be performed. And if you can handle it, you ought to.
In general if you know there is a high probability of failure that you can check for... you should do the check... i.e. if(obj != null) obj.method()
In your case, i'm not familiar enough with the C# library to know if date time has an easy way to check whether a timestamp is out of bounds. If it does, just call if(.isvalid(ts))
otherwise your code is basically fine.
So, basically it comes down to whichever way creates cleaner code... if the operation to guard against an expected exception is more complex than just handling the exception; than you have my permission to handle the exception instead of creating complex guards everywhere.
You might be interested in having a look at Common Lisp's condition system which is a sort of generalization of exceptions done right. Because you can unwind the stack or not in a controlled way, you get "restarts" as well, which are extremely handy.
This doesn't have anything much to do with best practices in other languages, but it shows you what can be done with some design thought in (roughly) the direction you are thinking of.
Of course there are still performance considerations if you're bouncing up and down the stack like a yo-yo, but it's a much more general idea than "oh crap, lets bail" kind of approach that most catch/throw exception systems embody.
I don't think there is anything wrong with using Exceptions for flow-control. Exceptions are somewhat similar to continuations and in statically typed languages, Exceptions are more powerful than continuations, so, if you need continuations but your language doesn't have them, you can use Exceptions to implement them.
Well, actually, if you need continuations and your language doesn't have them, you chose the wrong language and you should rather be using a different one. But sometimes you don't have a choice: client-side web programming is the prime example – there's just no way to get around JavaScript.
An example: Microsoft Volta is a project to allow writing web applications in straight-forward .NET, and let the framework take care of figuring out which bits need to run where. One consequence of this is that Volta needs to be able to compile CIL to JavaScript, so that you can run code on the client. However, there is a problem: .NET has multithreading, JavaScript doesn't. So, Volta implements continuations in JavaScript using JavaScript Exceptions, then implements .NET Threads using those continuations. That way, Volta applications that use threads can be compiled to run in an unmodified browser – no Silverlight needed.
But you won't always know what happens in the Method/s that you call. You won't know exactly where the exception was thrown. Without examining the exception object in greater detail....
I feel that there is nothing wrong with your example. On the contrary, it would be a sin to ignore the exception thrown by the called function.
In the JVM, throwing an exception is not that expensive, only creating the exception with new xyzException(...), because the latter involves a stack walk. So if you have some exceptions created in advance, you may throw them many times without costs. Of course, this way you can't pass data along with the exception, but I think that is a bad thing to do anyway.
There are a few general mechanisms via which a language could allow for a method to exit without returning a value and unwind to the next "catch" block:
Have the method examine the stack frame to determine the call site, and use the metadata for the call site to find either information about a try block within the calling method, or the location where the calling method stored the address of its caller; in the latter situation, examine metadata for the caller's caller to determine in the same fashion as the immediate caller, repeating until one finds a try block or the stack is empty. This approach adds very little overhead to the no-exception case (it does preclude some optimizations) but is expensive when an exception occurs.
Have the method return a "hidden" flag which distinguishes a normal return from an exception, and have the caller check that flag and branch to an "exception" routine if it's set. This routine adds 1-2 instructions to the no-exception case, but relatively little overhead when an exception occurs.
Have the caller place exception-handling information or code at a fixed address relative to the stacked return address. For example, with the ARM, instead of using the instruction "BL subroutine", one could use the sequence:
adr lr,next_instr
b subroutine
b handle_exception
next_instr:
To exit normally, the subroutine would simply do bx lr or pop {pc}; in case of an abnormal exit, the subroutine would either subtract 4 from LR before performing the return or use sub lr,#4,pc (depending upon the ARM variation, execution mode, etc.) This approach will malfunction very badly if the caller is not designed to accommodate it.
A language or framework which uses checked exceptions might benefit from having those handled with a mechanism like #2 or #3 above, while unchecked exceptions are handled using #1. Although the implementation of checked exceptions in Java is rather nuisancesome, they would not be a bad concept if there were a means by which a call site could say, essentially, "This method is declared as throwing XX, but I don't expect it ever to do so; if it does, rethrow as an "unchecked" exception. In a framework where checked exceptions were handled in such fashion, they could be an effective means of flow control for things like parsing methods which in some contexts may have a high likelihood of failure, but where failure should return fundamentally different information than success. I'm unaware of any frameworks that use such a pattern, however. Instead, the more common pattern is to use the first approach above (minimal cost for the no-exception case, but high cost when exceptions are thrown) for all exceptions.
One aesthetic reason:
A try always comes with a catch, whereas an if doesn't have to come with an else.
if (PerformCheckSucceeded())
DoSomething();
With try/catch, it becomes much more verbose.
try
{
PerformCheckSucceeded();
DoSomething();
}
catch
{
}
That's 6 lines of code too many.
This is the first time I face a problem like this. Not being this my profession but only my hobby, I have no previous references.
In my program I have added one by one several functions to control a machine. After I added the last function (temperature measurement), I have started experiencing problems on other functions (approx. 8 of them running all together. The problem I am experiencing is on a chart (RPM of a motor) that is not related to this function but is affected by it. You see the difference between these two charts with and without the temperature measurement running. The real speed of the motor is the same in both charts but in the second one I loose pieces on the fly because the application slows down.
Without the temperature function.
With the temperature function
Particularly this function is disturbing the above control and I think is because the work load is becoming heavy for the application and or because I need sampling so there is some time waiting to get them:
private void AddT(decimal valueTemp)
{
sumTemp += valueTemp;
countTemp += 1;
if (countTemp >= 20) //take 20 samples and make average
{
OnAvarerageChangedTemp(sumTemp / countTemp);
sumTemp = 0;
countTemp = 0;
}
}
private void OnAvarerageChangedTemp(decimal avTemp)
{
float val3 = (float)avTemp;
decimal alarm = avTemp;
textBox2.Text = avTemp.ToString("F");
if (alarm > 230)
{
System.Media.SoundPlayer player = new System.Media.SoundPlayer();
player.Stream = Properties.Resources.alarma;
player.Play();
timer4.Start();
}
else
{
timer4.Stop();
panel2.BackColor = SystemColors.Control;
}
}
I am wondering if running this function on a different thread would solve the problem and how I can do that? Or if there is a different way to solve the problem.Sample code will be appreciated.
Update, added method call.
This is how I call the method AddT
if (b != "")
{
decimal convTemp; //corrente resistenza
decimal.TryParse(b, out convTemp);
AddT(convTemp);}
This is how I receive the data from the serial and pass it to the class that strips out unwonted chars and return values to the different variables.
This is the class that strips out the unwonted chars and return the values.
And this is how I manage the serial incoming data. Please do not laugh at me after seeing my coding. I do a different job and I am learning on my own.
It's very hard to tell if there's anything wrong and what it might be - it looks like subtle problem.
However, it might be easier to get a handle on these things if you refactor your code. There are many things in the code you've shown that make it harder than necessary to reason about what's happening.
You're using float and decimal - float isn't that accurate but small and fast; decimal (tries) to be precise but especially is predictable since it rounds errors the way a human might in base-10 - but it is quite slow, and is usually intended for calculations where precise reproducibility is necessary (e.g. financial stuff). You should probably use double everywhere.
You've got useless else {} code in the Stripper class.
Your Stripper is an instatiable class, when it should simply be a static class with a static method - Stripper is stateless.
You're catching exceptions just to rethrow them.
You're using TryParse, and not checking for success. Normally you'd only use TryParse if you (a) expect parsing to fail sometimes, and (b) can deal with that parse failure. If you don't expect failure or can't deal with it, you're better off with a crash you learn about soon than a subtly incorrect values.
In stripper, you're duplicating variables such as _currentMot, currentMot, and param4 but they're identical - use only the parameter, and give it a logical name.
You're using out parameters. It's almost always a better idea to define a simple struct and return that instead - this also allows you to ensure you can't easily mix up variable names, and it's much easier to encapsulate and reuse functionality since you don't need to duplicate a long call and argument definition.
Your string parsing logic is too fragile. You should probably avoid Replace entirely, and instead explicitly make a Substring without the characters you've checked for, and you have some oddly named things like test1 and test2 which refer to a lastChar that's not the last character - this might be OK, but better names can help keep things straight in your head too.
You have incorrect code comments (decimal convTemp; //corrente resistenza). I usually avoid all purely technical code comments; it's better to use descriptive variable names which are another form of self-documenting code but one in which the compiler can at least check if you use them consistently.
Rather that return 4 possibly empty values, your Stripper should probably accept a parameter "sink" object on which it can call AddT AddD and AddA directly.
I don't think any of the above will fix your issue, but I do believe they're help keep your code a little cleaner and (in the long run) make it easier to find the issues.
your problem is in the parsing of the values you have
decimal.TryParse(a, out convRes);
AddA(convRes);
and don't check for failed values you only accept the value if it returns true
if(decimal.TryParse(a, out convRes))
{
AddA(convRes);
}
you may have more errors but this one is making you process 0 values every time the TryParse fails.
We have some unit tests that fail when run in Release mode vs debug mode. If I attach a debugger in release mode the tests pass. There is way too much code to publish here so I am really just looking for best practices in debugging Release mode issues. I have checked for:
DEBUG and RELEASE preprocessor directives but I did not find any.
Conditional Methods
SOLUTION: In this case it is because I was comparing floating point variables for equality. I could not change the floats to decimal without a major refactoring so I added an extension method:
public static class FloatExtension
{
public static bool AlmostEquals(this float f1, float f2, float precision)
{
return (Math.Abs(f1 - f2) <= precision);
}
public static bool AlmostEquals(this float f1, float f2)
{
return AlmostEquals(f1, f2, .00001f);
}
public static bool AlmostEquals(this float? f1, float? f2)
{
if (f1.HasValue && f2.HasValue)
{
return AlmostEquals(f1.Value, f2.Value);
}
else if (f1 == null && f2 == null)
{
return true;
}
return false;
}
}
One thing that might cause the behaviour that you are seeing is an error that causes a race condition. Attaching a debugger can change the timing of the code such that the race condition is no longer triggered.
To fix it, use synchronization appropriately whenever you have multiple threads accessing data.
I am comparing some float values in the IsEqual method.
That sounds like a very bad idea. You should not compare floats for equality because floating point calcualtions are not 100% precise and you can get representation and rounding errors. Compare to see whether they are sufficiently close together. For calculations involving money, you probably want to use the decimal type instead.
Since it seems to be floating point related there are so many things that can go wrong. See:
C# - Inconsistent math operation result on 32-bit and 64-bit
and
Double precision problems on .NET
There are so many things that can be trashed with floating points. And comparing floats for equality is a general no-no. You chould check the difference smaller than a reasonably epsilon.
Questions you should ask yourself -
Is my code threaded? Timing differences will affect output
Is someone calling Debug.Assert() with an expression that side effects?
What objects implement IDisposable() and do some do so in such a way that changes state?
Are you P/Invoking into unmanaged code?
Number 3 is a very likely bad-boy in this case. Garbage collection may be very different in debug and release and you may find that when an object is garbage collected is affecting the outcome of a later unit test.
And FYI, if you're using NUnit and TestDriven.NET - the two run tests in different orders.
This is often the case as the debug build is not optimized by default, and even if you enable it, the behavior when debugging is very different. You can disable the "Optimize code" from the project settings for all assemblies on the Properties->Build tab.
There are certainly other changes that can cause differences, like you mention Conditional Methods are one. These I've found to rarely be the cause of issues, for me it's almost always the optimizer.
Classic gotcha's of the optimizer include methods that get 'inlined' so that they fail to appear on a call stack. This causes problems when using System.Diagnostics.StackFrame classes to determine the current execution point. Similarly this will affect the result of MethodBase.GetCurrentMethod or other functions/behavior that rely on the executing method.
Then there are of course many things I've seen the optimizer do that I simply cannot explain at all. One such example was documented and discussed in a post 'HashDerivedBytes - replacing Rfc2898DeriveBytes, but why?' but I've never solved the mystery. I only know that the optimizer just flat broke Rfc2898DeriveBytes when used to generate a series of derived bytes. Oddly enough this only broke when the bytes generated were not evenly divisible by the size of the hash algorithm used (20) and only produced incorrect results after the first 20 bytes.
The fact is that optimizations adversely affecting code is not a new thing for compilers. Most of the old-school C++ developers will tell you that straight away and then, as I did, go into some long drawn out story about how they worked around it ;)
As Mark suggests, this is usually a result of a timing-related issue, often a race condition or synchronization problem.
One common way to handle this sort of problem is to use "print" statements in the affected areas to show you what's going on. If the print statements (Console.WriteLine, Response.Write, logging, or whatever) make the problem go away, store the values in global variables and print the globals once the problem has shown up.
The most recent time this has happened to me was in code that was reading from a serial port. The debugging activity caused just enough of a change in timing to affect how bytes from the serial port were buffered, which changed how the buffer was getting parsed. Since the print statements changed the timing, I had to store the data up to output later.
Just to add my two cents to this, I recently found that I had a date comparison in an sql procedure that the testing called. The dates were all auto-generated prior in the test procedure and values were inserted into the DB, and so occasionally they were exactly the same (when using RunTests) causing a null to be returned on a table join. Not what I was expecting. Obviously, in debug mode, since I'm slowly progressing through it, there will be a difference in the auto-generated times which meant that I never bumped into the error. I resolved this by inserting
Threading.Thread.Sleep(520)
wherever there would definitely be a delay between actions. Problem fixed.
I do not currently have this issue, but you never know, and thought experiments are always fun.
Ignoring the obvious problems that you would have to have with your architecture to even be attempting this, let's assume that you had some horribly-written code of someone else's design, and you needed to do a bunch of wide and varied operations in the same code block, e.g.:
WidgetMaker.SetAlignment(57);
contactForm["Title"] = txtTitle.Text;
Casserole.Season(true, false);
((RecordKeeper)Session["CasseroleTracker"]).Seasoned = true;
Multiplied by a hundred. Some of these might work, others might go badly wrong. What you need is the C# equivalent of "on error resume next", otherwise you're going to end up copying and pasting try-catches around the many lines of code.
How would you attempt to tackle this problem?
public delegate void VoidDelegate();
public static class Utils
{
public static void Try(VoidDelegate v) {
try {
v();
}
catch {}
}
}
Utils.Try( () => WidgetMaker.SetAlignment(57) );
Utils.Try( () => contactForm["Title"] = txtTitle.Text );
Utils.Try( () => Casserole.Season(true, false) );
Utils.Try( () => ((RecordKeeper)Session["CasseroleTracker"]).Seasoned = true );
Refactor into individual, well-named methods:
AdjustFormWidgets();
SetContactTitle(txtTitle.Text);
SeasonCasserole();
Each of those is protected appropriately.
I would say do nothing.
Yup thats right, do NOTHING.
You have clearly identified two things to me:
You know the architecture is borked.
There is a ton of this crap.
I say:
Do nothing.
Add a global error handler to send you an email every time it goes boom.
Wait until something falls over (or fails a test)
Correct that (Refactoring as necessary within the scope of the page).
Repeat every time a problem occurs.
You will have this cleared up in no time if it is that bad. Yeah I know it sounds sucky and you may be pulling your hair out with bugfixes to begin with, but it will allow you to fix the needy/buggy code before the (large) amount of code that may actually be working no matter how crappy it looks.
Once you start winning the war, you will have a better handle on the code (due to all your refactoring) you will have a better idea for a winning design for it..
Trying to wrap all of it in bubble wrap is probably going to take just a long to do and you will still not be any closer to fixing the problems.
It's pretty obvious that you'd write the code in VB.NET, which actually does have On Error Resume Next, and export it in a DLL to C#. Anything else is just being a glutton
for punishment.
Fail Fast
To elaborate, I guess I am questioning the question. If an exception is thrown, why would you want your code to simply continue as if nothing has happened? Either you expect exceptions in certain situations, in which case you write a try-catch block around that code and handle them, or there is an unexpected error, in which case you should prefer your application to abort, or retry, or fail. Not carry on like a wounded zombie moaning 'brains'.
This is one of the things that having a preprocessor is useful for. You could define a macro that swallows exceptions, then with a quick script add that macro to all lines.
So, if this were C++, you could do something like this:
#define ATTEMPT(x) try { x; } catch (...) { }
// ...
ATTEMPT(WidgetMaker.SetAlignment(57));
ATTEMPT(contactForm["Title"] = txtTitle.Text);
ATTEMPT(Casserole.Season(true, false));
ATTEMPT(((RecordKeeper)Session["CasseroleTracker"]).Seasoned = true);
Unfortunately, not many languages seem to include a preprocessor like C/C++ did.
You could create your own preprocessor and add it as a pre-build step. If you felt like completely automating it you could probably write a preprocessor that would take the actual code file and add the try/catch stuff in on its own (so you don't have to add those ATTEMPT() blocks to the code manually). Making sure it only modified the lines it's supposed to could be difficult though (have to skip variable declarations, loop constructs, etc to that you don't break the build).
However, I think these are horrible ideas and should never be done, but the question was asked. :)
Really, you shouldn't ever do this. You need to find what's causing the error and fix it. Swallowing/ignoring errors is a bad thing to do, so I think the correct answer here is "Fix the bug, don't ignore it!". :)
On Error Resume Next is a really bad idea in the C# world. Nor would adding the equivalent to On Error Resume Next actually help you. All it would do is leave you in a bad state which could cause more subtle errors, data loss and possibly data corruption.
But to give the questioner his due, you could add a global handler and check the TargetSite to see which method borked. Then you could at least know what line it borked on. The next part would be to try and figure out how to set the "next statement" the same way the debugger does it. Hopefully your stack won't have unwound at this point or you can re-create it, but it's certainly worth a shot. However, given this approach the code would have to run in Debug mode every time so that you would have your debug symbols included.
As someone mentioned, VB allows this. How about doing it the same way in C#? Enter trusty reflector:
This:
Sub Main()
On Error Resume Next
Dim i As Integer = 0
Dim y As Integer = CInt(5 / i)
End Sub
Translates into this:
public static void Main()
{
// This item is obfuscated and can not be translated.
int VB$ResumeTarget;
try
{
int VB$CurrentStatement;
Label_0001:
ProjectData.ClearProjectError();
int VB$ActiveHandler = -2;
Label_0009:
VB$CurrentStatement = 2;
int i = 0;
Label_000E:
VB$CurrentStatement = 3;
int y = (int) Math.Round((double) (5.0 / ((double) i)));
goto Label_008F;
Label_0029:
VB$ResumeTarget = 0;
switch ((VB$ResumeTarget + 1))
{
case 1:
goto Label_0001;
case 2:
goto Label_0009;
case 3:
goto Label_000E;
case 4:
goto Label_008F;
default:
goto Label_0084;
}
Label_0049:
VB$ResumeTarget = VB$CurrentStatement;
switch (((VB$ActiveHandler > -2) ? VB$ActiveHandler : 1))
{
case 0:
goto Label_0084;
case 1:
goto Label_0029;
}
}
catch (object obj1) when (?)
{
ProjectData.SetProjectError((Exception) obj1);
goto Label_0049;
}
Label_0084:
throw ProjectData.CreateProjectError(-2146828237);
Label_008F:
if (VB$ResumeTarget != 0)
{
ProjectData.ClearProjectError();
}
}
Rewrite the code. Try to find sets of statements which logically depend on each other, so that if one fails then the next ones make no sense, and hive them off into their own functions and put try-catches round them, if you want to ignore the result of that and continue.
This may help you in identifing the pieces that have the most problems.
# JB King
Thanks for reminding me. The Logging application block has a Instrumentation Event that can be used to trace events, you can find more info on the MS Enterprise library docs.
Using (New InstEvent)
<series of statements>
End Using
All of the steps in this using will be traced to a log file, and you can parse that out to see where the log breaks (ex is thrown) and id the high offenders.
Refactoring is really your best bet, but if you have a lot, this may help you pinpoint the worst offenders.
You could use goto, but it's still messy.
I've actually wanted a sort of single statement try-catch for a while. It would be helpful in certain cases, like adding logging code or something that you don't want to interrupt the main program flow if it fails.
I suspect something could be done with some of the features associated with linq, but don't really have time to look into it at the moment. If you could just find a way to wrap a statement as an anonymous function, then use another one to call that within a try-catch block it would work... but not sure if that's possible just yet.
If you can get the compiler to give you an expression tree for this code, then you could modify that expression tree by replacing each statement with a new try-catch block that wraps the original statement. This isn't as far-fetched as it sounds; for LINQ, C# acquired the ability to capture lambda expressions as expression trees that can be manipulated in user code at runtime.
This approach is not possible today with .NET 3.5 -- if for no other reason than the lack of a "try" statement in System.Linq.Expressions. However, it may very well be viable in a future version of C# once the merge of the DLR and LINQ expression trees is complete.
Why not use the reflection in c#? You could create a class that reflects on the code and use line #s as the hint for what to put in each individual try/catch block. This has a few advantages:
Its slightly less ugly as it doesn't really you require mangle your source code and you can use it only during debug modes.
You learn something interesting about c# while implementing it.
I however would recommend against any of this, unless of course you are taking over maintance of someelses work and you need to get a handle on the exceptions so you can fix them. Might be fun to write though.
Fun question; very terrible.
It'd be nice if you could use a macro. But this is blasted C#, so you might solve it with some preprocessor work or some external tool to wrap your lines in individual try-catch blocks. Not sure if you meant you didn't want to manually wrap them or that you wanted to avoid try-catch entirely.
Messing around with this, I tried labeling every line and jumping back from a single catch, without much luck. However, Christopher uncovered the correct way to do this. There's some interesting additional discussion of this at Dot Net Thoughts and at Mike Stall's .NET Blog.
EDIT: Of course. The try-catch / switch-goto solution listed won't actually compile since the try labels are out-of-scope in catch. Anyone know what's missing to make something like this compile?
You could automate this with a compiler preprocess step or maybe hack up Mike Stall's Inline IL tool to inject some error-ignorance.
(Orion Adrian's answer about examining the Exception and trying to set the next instruction is interesting too.)
All in all, it seems like an interesting and instructive exercise. Of course, you'd have to decide at what point the effort to simulate ON ERROR RESUME NEXT outweighs the effort to fix the code. :-)
Catch the errors in the UnhandledException Event of the application. That way, unhandled execptions can even be logged as to the sender and whatever other information the developer would reasonable.
Unfortunately you are probably out of luck. On Error Resume Next is a legacy option that is generally heavily discouraged, and does not have an equivalent to my knowledge in C#.
I would recommend leaving the code in VB (It sounds like that was the source, given your specific request for OnError ResumeNext) and interfacing with or from a C# dll or exe that implements whatever new code you need. Then preform refactoring to cause the code to be safe, and convert this safe code to C# as you do this.
You could look at integrating the Enterprise Library's Exception Handling component for one idea of how to handle unhandled exceptions.
If this is for ASP.Net applications, there is a function in the Global.asax called, "Application_Error" that gets called in most cases with catastrophic failure being the other case usually.
Ignoring all the reasons you'd want to avoid doing this.......
If it were simply a need to keep # of lines down, you could try something like:
int totalMethodCount = xxx;
for(int counter = 0; counter < totalMethodCount; counter++) {
try {
if (counter == 0) WidgetMaker.SetAlignment(57);
if (counter == 1) contactForm["Title"] = txtTitle.Text;
if (counter == 2) Casserole.Season(true, false);
if (counter == 3) ((RecordKeeper)Session["CasseroleTracker"]).Seasoned = true;
} catch (Exception ex) {
// log here
}
}
However, you'd have to keep an eye on variable scope if you try to reuse any of the results of the calls.
Hilite each line, one at a time, 'Surround with' try/catch. That avoids the copying pasting you mentioned
The IT department of a subsidiary of ours had a consulting company write them an ASP.NET application. Now it's having intermittent problems with mixing up who the current user is and has been known to show Joe some of Bob's data by mistake.
The consultants were brought back to troubleshoot and we were invited to listen in on their explanation. Two things stuck out.
First, the consultant lead provided this pseudo-code:
void MyFunction()
{
Session["UserID"] = SomeProprietarySessionManagementLookup();
Response.Redirect("SomeOtherPage.aspx");
}
He went on to say that the assignment of the session variable is asynchronous, which seemed untrue. Granted the call into the lookup function could do something asynchronously, but this seems unwise.
Given that alleged asynchronousness, his theory was that the session variable was not being assigned before the redirect's inevitable ThreadAbort exception was raised. This faulure then prevented SomeOtherPage from displaying the correct user's data.
Second, he gave an example of a coding best practice he recommends. Rather than writing:
int MyFunction(int x, int x)
{
try
{
return x / y;
}
catch(Exception ex)
{
// log it
throw;
}
}
the technique he recommended was:
int MyFunction(int x, int y, out bool isSuccessful)
{
isSuccessful = false;
if (y == 0)
return 0;
isSuccessful = true;
return x / y;
}
This will certainly work and could be better from a performance perspective in some situations.
However, from these and other discussion points it just seemed to us that this team was not well-versed technically.
Opinions?
Rule of thumb: If you need to ask if a consultant knows what he's doing, he probably doesn't ;)
And I tend to agree here. Obviously you haven't provided much, but they don't seem terribly competent.
I would agree. These guys seem quite incompetent.
(BTW, I'd check to see if in "SomeProprietarySessionManagementLookup," they're using static data. Saw this -- with behavior exactly as you describe on a project I inherited several months ago. It was a total head-slap moment when we finally saw it ... And wished we could get face to face with the guys who wrote it ... )
If the consultant has written an application that's supposed to be able to keep track of users and only show the correct data to the correct users and it doesn't do that, then clearly something's wrong. A good consultant would find the problem and fix it. A bad consultant would tell you that it was asynchronicity.
On the asynchronous part, the only way that could be true is if the assignment going on there is actually an indexer setter on Session that is hiding an asynchronous call with no callback indicating success/failure. This would seem to be a HORRIBLE design choice, and it looks like a core class in your framework, so I find it highly unlikely.
Usually asynchronous calls have a way to specify a callback so you can determine what the result is, or if the operation was successful. The documentation for Session should be pretty clear though on if it is actually hiding an asynchronous call, but yeah... doesn't look like the consultant knows what he is talking about...
The method call that is being assigned to the Session indexer cannot be asynch, because to get a value asynchronously, you HAVE to use a callback... no way around that, so if there is no explicit callback, it's definitely not asynch (well, internally there could be an asynchronous call, but the caller of the method would perceive it as synchronous, so it is irrelevant if the method internally for example invokes a web service asynchronously).
For the second point, I think this would be much better, and keep the same functionality essentially:
int MyFunction(int x, int y)
{
if (y == 0)
{
// log it
throw new DivideByZeroException("Divide by zero attempted!");
}
return x / y;
}
For the first point, that does indeed seem bizarre.
On the second one, it's reasonable to try to avoid division by 0 - it's entirely avoidable and that avoidance is simple. However, using an out parameter to indicate success is only reasonable in certain cases, such as int.TryParse and DateTime.TryParseExact - where the caller can't easily determine whether or not their arguments are reasonable. Even then, the return value is usually the success/failure and the out parameter is the result of the method.
Asp.net sessions, if you're using the built-in providers, won't accidentally give you someone else's session. SomeProprietarySessionManagementLookup() is the likely culprit and is returning bad values or just not working.
Session["UserID"] = SomeProprietarySessionManagementLookup();
First of all assigning the return value from an asynchronously SomeProprietarySessionManagementLookup() just wont work. The consultants code probably looks like:
public void SomeProprietarySessionManagementLookup()
{
// do some async lookup
Action<object> d = delegate(object val)
{
LookupSession(); // long running thing that looks up the user.
Session["UserID"] = 1234; // Setting session manually
};
d.BeginInvoke(null,null,null);
}
The consultant isn't totally full of BS, but they have written some buggy code. Response.Redirect() does throw a ThreadAbort, and if the proprietary method is asynchronous, asp.net doesn't know to wait for the asynchronous method to write back to the session before asp.net itself saves the session. This is probably why it sometimes works and sometimes doesn't.
Their code might work if the asp.net session is in-process, but a state server or db server wouldn't. It's timing dependent.
I tested the following. We use state server in development. This code works because the session is written to before the main thread finishes.
Action<object> d = delegate(object val)
{
System.Threading.Thread.Sleep(1000); // waits a little
Session["rubbish"] = DateTime.Now;
};
d.BeginInvoke(null, null, null);
System.Threading.Thread.Sleep(5000); // waits a lot
object stuff = Session["rubbish"];
if( stuff == null ) stuff = "not there";
divStuff.InnerHtml = Convert.ToString(stuff);
This next snippet of code doesn't work because the session was already saved back to state server by the time the asynchronous method gets around to setting a session value.
Action<object> d = delegate(object val)
{
System.Threading.Thread.Sleep(5000); // waits a lot
Session["rubbish"] = DateTime.Now;
};
d.BeginInvoke(null, null, null);
// wait removed - ends immediately.
object stuff = Session["rubbish"];
if( stuff == null ) stuff = "not there";
divStuff.InnerHtml = Convert.ToString(stuff);
The first step is for the consultant to make their code synchronous because their performance trick didn't work at all. If that fixes it, have the consultant properly implement using the Asynchronous Programming Design Pattern
I agree with him in part -- it's definitely better to check y for zero rather than catching the (expensive) exception. The out bool isSuccessful seems really dated to me, but whatever.
re: the asynchronous sessionid buffoonery -- may or may not be true, but it sounds like the consultant is blowing smoke for cover.
Cody's rule of thumb is dead right. If you have to ask, he probably doesn't.
It seems like point two its patently incorrect. .NET's standards explain that if a method fails it should throw an exception, which seems closer to the original; not the consulstant's suggestion. Assuming the exception is accurately & specifically describing the failure.
The consultants created the code in the first place right? And it doesn't work. I think you have quite a bit of dirt on them already.
The asynchronous answer sounds like BS, but there may be something in it. Presumably they have offered a suitable solution as well as pseudo-code describing the problem they themselves created. I would be more tempted to judge them on their solution rather than their expression of the problem. If their understanding is flawed their new solution won't work either. Then you'll know they are idiots. (In fact look round to see if you have a similar proof in any other areas of their code already)
The other one is a code style issue. There are a lot of different ways to cope with that. I personally don't like that style, but there will be circumstances under which it is suitable.
They're wrong on the async.
The assignment happens and then the page redirects. The function can start something asynchronously and return (and could even conceivably alter the Session in its own way), but whatever it does return has to be assigned in the code you gave before the redirect.
They're wrong on that defensive coding style in any low-level code and even in a higher-level function unless it's a specific business case that the 0 or NULL or empty string or whatever should be handled that way - in which case, it's always successful (that successful flag is a nasty code smell) and not an exception. Exceptions are for exceptions. You don't want to mask behaviors like this by coddling the callers of the functions. Catch things early and throw exceptions. I think Maguire covered this in Writing Solid Code or McConnell in Code Complete. Either way, it smells.
This guy does not know what he is doing. The obvious culprit is right here:
Session["UserID"] = SomeProprietarySessionManagementLookup();
I have to agree with John Rudy. My gut tells me the problem is in SomeProprietarySessionManagementLookup().
.. and your consultants do not sound to sure of themselves.
Storing in Session in not async. So that isn't true unless that function is async. But even so, since it isn't calling a BeginCall and have something to call on completion, the next line of code wouldn't execute until the Session line is complete.
For the second statement, while that could be used, it isn't exactly a best practice and you have a few things to note with it. You save the cost of throwing an exception, but wouldn't you want to know that you are trying to divide by zero instead of just moving past it?
I don't think that is a solid suggestion at all.
Quite strange. On the second item it may or may not be faster. It certainly isn't the same functionality though.
Typical "consultant" bollocks:
The problem is with whatever SomeProprietarySessionManagementLookup is doing
Exceptions are only expensive if they're thrown. Don't be afraid of try..catch, but throws should only occur in exceptional circumstances. If variable y shouldn't be zero then an ArgumentOutOfRangeException would be appropriate.
I'm guessing your consultant is suggesting use a status variable instead of exception for error handling is a better practice? I don't agree. How often does people forgot or too lazy to do error checking for return values? Also, pass/fail variable is not informative. There are more things can go wrong other than divide by zero like integer x/y is too big or x is NaN. When things go wrong, status variable cannot tell you what went wrong, but exception can. Exception is for exceptional case, and divide by zero or NaN are definitely exceptional cases.
The session thing is possible. It's a bug, beyond doubt, but it could be that the write arrives at whatever custom session state provider you're using after the next read. The session state provider API accommodates locking to prevent this sort of thing, but if the implementor has just ignored all that, your consultant could be telling the truth.
The second issue is also kinda valid. It's not quite idiomatic - it's a slightly reversed version of things like int.TryParse, which are there to avoid performance issues caused by throwing lots of exceptions. But unless you're calling that code an awful lot, it's unlikely it'll make a noticeable difference (compared to say, one less database query per page etc). It's certainly not something you should do by default.
If SomeProprietarySessionManagementLookup(); is doing an asynchronous assignment it would more likely look like this:
SomeProprietarySessionManagementLookup(Session["UserID"]);
The very fact that the code is assigning the result to Session["UserID"] would suggest that it is not supposed to be asynchronous and the result should be obtained before Response.Redirect is called. If SomeProprietarySessionManagementLookup is returning before its result is calculated they have a design flaw anyway.
The throw an exception or use an out parameter is a matter of opinion and circumstance and in actual practice won't amount to a hill of beans which ever way you do it. For the performance hit of exceptions to become an issue you would need to be calling the function a huge number of times which would probably be a problem in itself.
If the consultants deployed their ASP.NET application on your server(s), then they may have deployed it in uncompiled form, which means there would be a bunch of *.cs files floating around that you could look at.
If all you can find is compiled .NET assemblies (DLLs and EXEs) of theirs, then you should still be able to decompile them into somewhat readable source code. I'll bet if you look through the code you'll find them using static variables in their proprietary lookup code. You'd then have something very concrete to show your bosses.
This entire answer stream is full of typical programmer attitudes. It reminds me of Joel's 'Things you should never do' article (rewrite from scratch.) We don't really know anything about the system, other than there's a bug, and some guy posted some code online. There are so many unknowns that it is ridiculous to say "This guy does not know what he is doing."
Rather than pile on the Consultant, you could just as easily pile on the person who procured their services. No consultant is perfect, nor is a hiring manager ... but at the end of the day the real direction you should be taking is very clear: instead of trying to find fault you should expend energy into working collaboratively to find solutions. No matter how skilled someone is at their roles and responsibilities they will certainly have deficiencies. If you determine there is a pattern of incompentencies then you may choose to transition to another resource going forward, but assigning blame has never solved a single problem in history.
On the second point, I would not use exceptions here. Exceptions are reserved for exceptional cases.
However, division of anything by zero certainly does not equal zero (in math, at least), so this would be case specific.