Related
Looking over my Raku code, I've realized that I pretty much never use CATCH blocks to actually catch/handle error. Instead, I handle errors with try blocks and testing for undefined values; the only thing I use CATCH blocks for is to log errors differently. I don't seem to be alone in this habit – looking at the CATCH blocks in the Raku docs, pretty much none of them handle the error in any sense beyond printing a message. (The same is true of most of the CATCH blocks in Rakudo.).
Nevertheless, I'd like to better understand how to use CATCH blocks. Let me work through a few example functions, all of which are based on the following basic idea:
sub might-die($n) { $n %% 2 ?? 'lives' !! die 418 }
Now, as I've said, I'd normally use this function with something like
say try { might-die(3) } // 'default';
But I'd like to avoid that here and use CATCH blocks inside the function. My first instinct is to write
sub might-die1($n) {
$n %% 2 ?? 'lives' !! die 418
CATCH { default { 'default' }}
}
But this not only doesn't work, it also (very helpfully!) doesn't even compile. Apparently, the CATCH block is not removed from the control flow (as I would have thought). Thus, that block, rather than the ternary expression, is the last statement in the function. Ok, fair enough. How about this:
sub might-die2($n) {
ln1: CATCH { default { 'default' }}
ln2: $n %% 2 ?? 'lives' !! die 418
}
(those line numbers are Lables. Yes, it's valid Raku and, yes, they're useless here. But SO doesn't give line numbers, and I wanted some.)
This at least compiles, but it doesn't do what I mean.
say might-die2(3); # OUTPUT: «Nil»
To DWIM, I can change this to
sub might-die3($n) {
ln1: CATCH { default { return 'default' }}
ln2: $n %% 2 ?? 'lives' !! die 418
}
say might-die3(3); # OUTPUT: «'default'»
What these two reveal is that the result of the CATCH block is not, as I'd hopped, being inserted into control flow where the exception occurred. Instead, the exception is causing control flow to jump to the CATCH block for the enclosing scope. It's as though we'd written (in an alternate universe where Raku has a GOTO operator [EDIT: or maybe not that alternate of a universe, since we apparently have a NYI goto method. Learn something new every day…]
sub might-die4($n) {
ln0: GOTO ln2;
ln1: return 'default';
ln2: $n %% 2 ?? 'lives' !! GOTO ln1;
}
I realize that some critics of exceptions say that they can reduce to GOTO statements, but this seems to be carrying things a bit far.
I could (mostly) avoid emulating GOTO with the .resume method, but I can't do it the way I'd like to. Specifically, I can't write:
sub might-die5($n) {
ln1: CATCH { default { .resume('default') }}
ln2: $n %% 2 ?? 'lives' !! die 418
}
Because .resume doesn't take an argument. I can write
sub might-die6($n) {
ln1: CATCH { default { .resume }}
ln2: $n %% 2 ?? 'lives' !! do { die 418; 'default' }
}
say might-die6 3; # OUTPUT: «'default'»
This works, at least in this particular example. But I can't help feeling that it's more of a hack than an actual solution and that it wouldn't generalize well. Indeed, I can't help feeling that I'm missing some larger insight behind error handling in Raku that would make all of this fit together better. (Maybe because I've spent too much time programming in languages that handle errors without exceptions?) I would appreciate any insight into how to write the above code in idiomatic Raku. Is one of the approaches above basically correct? Is there a different approach I haven't considered? And is there a larger insight about error handling that I'm missing in all of this?
"Larger insight about error handling"
Is one of the approaches [in my question] basically correct?
Yes. In the general case, use features like try and if, not CATCH.
Is there a different approach I haven't considered?
Here's a brand new one: catch. I invented the first version of it a few weeks ago, and now your question has prompted me to reimagine it. I'm pretty happy with how it's now settled; I'd appreciate readers' feedback about it.
is there a larger insight about error handling that I'm missing in all of this?
I'll discuss some of my thoughts at the end of this answer.
But let's now go through your points in the order you wrote them.
KISS
I pretty much never use CATCH blocks to actually catch/handle error.
Me neither.
Instead, I handle errors with try blocks and testing for undefined values
That's more like it.
Logging errors with a catchall CATCH
the only thing I use CATCH blocks for is to log errors differently.
Right. A judiciously located catchall. This is a use case for which I'd say CATCH is a good fit.
The doc
looking at the CATCH blocks in the Raku docs, pretty much none of them handle the error in any sense beyond printing a message.
If the doc is misleading about:
The limits of the capabilities and applicability of CATCH / CONTROL blocks; and/or
The alternatives; and/or
What's idiomatic (which imo is not use of CATCH for code where try is more appropriate (and now my new catch function too?)).
then that would be unfortunate.
CATCH blocks in the Rakudo compiler source
(The same is true of most of the CATCH blocks in Rakudo.).
At a guess those will be judiciously placed catchalls. Placing one just before the callstack runs out, to specify default exception handling (as either a warning plus .resume, or a die or similar), seems reasonable to me. Is that what they all are?
Why are phasers statements?
sub might-die1($n) {
$n %% 2 ?? 'lives' !! die 418
CATCH { default { 'default' }}
}
this not only doesn't work, it also (very helpfully!) doesn't even compile.
.oO ( Well that's because you forgot a semi-colon at the end of the first statement )
(I would have thought ... the CATCH block [would have been] removed from the control flow)
Join the club. Others have expressed related sentiments in filed bugs, and SO Q's and A's. I used to think the current situation was wrong in the same way you express. I think I could now easily be persuaded by either side of the argument -- but jnthn's view would be decisive for me.
Quoting the doc:
A phaser block is just a trait of the closure containing it, and is automatically called at the appropriate moment.
That suggests that a phaser is not a statement, at least not in an ordinary sense and would, one might presume, be removed from ordinary control flow.
But returning to the doc:
Phasers [may] have a runtime value, and if evaluated [in a] surrounding expression, they simply save their result for use in the expression ... when the rest of the expression is evaluated.
That suggests that they can have a value in an ordinary control flow sense.
Perhaps the rationale for not removing phasers from holding their place in ordinary control flow, and instead evaluating to Nil if they don't otherwise return a value, is something like:
Phasers like INIT do return values. The compiler could insist that one assigns their result to a variable and then explicitly returns that variable. But that would be very un Raku-ish.
Raku philosophy is that, in general, the dev tells the compiler what to do or not do, not the other way around. A phaser is a statement. If you put a statement at the end, then you want it to be the value returned by its enclosing block. (Even if it's Nil.)
Still, overall, I'm with you in the following sense:
It seems natural to think that ordinary control flow does not include phasers that do not return a value. Why should it?
It seems IWBNI the compiler at least warned if it saw a non-value-returning phaser used as the last statement of a block that contains other value-returning statements.
Why don't CATCH blocks return/inject a value?
Ok, fair enough. How about this:
sub might-die2($n) {
ln1: CATCH { default { 'default' }}
ln2: $n %% 2 ?? 'lives' !! die 418
}
say might-die2(3); # OUTPUT: «Nil»
As discussed above, many phasers, including the exception handling ones, are statements that do not return values.
I think one could reasonably have expected that:
CATCH phasers would return a value. But they don't. I vaguely recall jnthn already explaining why here on SO; I'll leave hunting that down as an exercise for readers. Or, conversely:
The compiler would warn that a phaser that did not return a value was placed somewhere a returned value was probably intended.
It's as though we'd written ... a GOTO operator
Raku(do) isn't just doing an unstructured jump.
(Otherwise .resume wouldn't work.)
this seems to be carrying things a bit far
I agree, you are carrying things a bit too far. :P
.resume
Resumable exceptions certainly aren't something I've found myself reaching for in Raku. I don't think I've used them in "userspace" code at all yet.
(from jnthn's answer to When would I want to resume a Raku exception?.)
.resume doesn't take an argument
Right. It just resumes execution at the statement after the one that led to an exception being thrown. .resume does not alter the result of the failed statement.
Even if a CATCH block tries to intervene, it won't be able to do so in a simple, self-contained fashion, by setting the value of a variable whose assignment has thrown an exception, and then .resumeing. cf Should this Raku CATCH block be able to change variables in the lexical scope?.
(I tried several CATCH related approaches before concluding that just using try was the way to go for the body of the catch function I linked at the start. If you haven't already looked at the catch code, I recommend you do.)
Further tidbits about CATCH blocks
They're a bit fraught for a couple reasons. One is what seems to be deliberate limits of their intended capability and applicability. Another is bugs. Consider, for example:
My answer to SO CATCH and throw in custom exception
Rakudo issue: Missing return value from do when calling .resume and CATCH is the last statement in a block
Rakudo issue: return-ing out of a block and LEAVE phaser (“identity”‽)
Larger insight about error handling
is there a larger insight about error handling that I'm missing in all of this?
Perhaps. I think you already know most of it well, but:
KISS #1 You've handled errors without exceptions in other PLs. It worked. You've done it in Raku. It works. Use exceptions only when you need or want to use them. For most code, you won't.
KISS #2 Ignoring some native type use cases, almost all results can be expressed as valid or not valid, without leading to the semi-predicate problem, using simple combinations of the following Raku Truth value that provide ergonomic ways to discern between non-error values and errors:
Conditionals: if, while, try, //, et al
Predicates: .so, .defined, .DEFINITE, et al
Values/types: Nil, Failures, zero length composite data structures, :D vs :U type constraints, et al
Sticking with error exceptions, some points I think worth considering:
One of the use cases for Raku error exceptions is to cover the same ground as exceptions in, say, Haskell. These are scenarios in which handling them as values isn't the right solution (or, in Raku, might not be).
Other PLs support exceptions. One of Raku's superpowers is being able to interoperate with all other PLs. Ergo it supports exceptions if for no other reason than to enable correct interoperation.
Raku includes the notion of a Failure, a delayed exception. The idea is you can get the best of both worlds. Handled with due care, a Failure is just an error value. Handled carelessly, it blows up like a regular exception.
More generally, all of Raku's features are designed to work together to provide convenient but high quality error handling that supports all of the following coding scenarios:
Fast coding. Prototyping, exploratory code, one-offs, etc.
Control of robustness. Gradually narrowing or broadening error handling.
Diverse options. What errors should be signalled? When? By which code? What if consuming code wants to signal that producing code should be more strict? Or more relaxed? What if it's the other way around -- producing code wants to signal that consuming code should be more careful or can relax? What can be done if producing and consuming code have conflicting philosophies? What if producing code cannot be altered (eg it's a library, or written in another language)?
Interoperation between languages / codebases. The only way that can work well is if Raku provides both high levels of control and diverse options.
Convenient refactoring between these scenarios.
All of these factors, and more, underlie Raku's approach to error handling.
CATCH is a really old feature of the language.
It used to only exist inside of a try block.
(Which is not very Rakuish.)
It is also a very rarely used part of Raku.
Which means that not a lot of people have come up with “pain points” of the feature.
So then very rarely has anyone done any work to make it more Rakuish.
Both of those combined make it so that CATCH is a rather featureless part of the language.
If you look at the test file for the feature, you will note that most of it was written in 2009 when the test suite was still a part of the Pugs project.
(And most of the rest are tests for bugs that have been found over the years.)
There is a very good reason that few people have tried to add new behaviours to CATCH, there are plenty of other features that are much nicer to work with.
If you want to replace a result in the event of an exception
sub may-die () {
if Bool.pick {
return 'normal'
} else {
die
}
}
my $result;
{
CATCH { default { $result = 'replacement' }}
$result = may-die();
}
It is much easier to just use try without CATCH, along with defined‑or // to get something that works very similarly.
my $result = try { may-die } // 'replacement';
It is even easier if you are dealing with soft failures instead of hard exceptions, because you can just use defined‑or by itself.
sub may-fail () {
if Bool.pick {
return 'normal'
} else {
fail
}
}
my $result = may-fail() // 'replacement';
In fact the only way to use CATCH with a soft failure is to combine it with try
my $result;
try {
CATCH { default { $result = 'replacement' }}
$result = may-fail();
}
If your soft failure is the base of all failure objects Nil, you can either use // or is default
my $result = may-return-nil // 'replacement';
my $result is default<replacement> = may-return-nil;
But Nil won't just work with CATCH no matter how much you try.
Really the only time I would normally use CATCH is when I want to handle several different errors in different ways.
{
CATCH {
when X::Something { … }
when X::This { … }
when X::That { … }
default { … }
}
# some code that may throw X::This
…
# some code that may throw X::NotSpecified (default)
…
# some code that may throw X::Something
…
# some code that may throw X::This or X::That
…
# some code that may fail instead of throw
# (sunk so that it will throw immediately)
sink may-fail;
}
Or if I wanted to show how you could write this [terrible] Visual Basic line
On Error Resume Next
In Raku
CATCH { default { .resume } }
That of course doesn't really answer your question in the slightest.
You say that you expected CATCH to be removed from the control flow.
The whole point of CATCH is to insert itself into the exceptional control flow.
Actually that's not accurate. It doesn't so much insert itself into the control flow as ending the control flow while doing some processing before moving on to the caller/outside block. Presumably because the data of the current block is in an erroneous state and should no longer be trusted.
That still doesn't explain why your code fails to compile.
You expected CATCH to have its own special syntax rule when it comes to the semicolon ending a statement.
If it worked the way you expected it would fail one of the important [syntax] rules in Raku, “there should be as few special cases as possible”. Its syntax is not special in any way unlike what you seem to expect.
CATCH is just one of many phasers with one important extra bit of functionality, it stops exception propagation down the call stack.
What you seem to be asking for it to instead alter the result of an expression that may throw.
That doesn't seem like a good idea.
$a + may-die() + $b
You want to be able to replace the exception from may-die with a value.
$a + 42 + $b
Basically you are asking for the ability to add action‑at‑a‑distance as a feature.
There is also a problem, what if you actually wanted $a + may‑die to be replaced instead.
42 + $b
There is no way in your idea for you to specify that.
Even worse, there is a way that could accidently happen. What if may‑die started returning a failure instead of exception. Then it would only cause an exception when you tried to use it, for example by adding it to $a.
If some code throws an exception, the block is in an unrecoverable state and it needs to halt execution. This far, no farther.
If an expression throws an exception, the result of executing the statement it is in, is suspect.
Other statements may rely on that broken statement, so then the whole block is also suspect.
I do not think it would be that good of an idea if it instead allowed the code to continue but with a different result for the current expression. Especially if that value can be far removed from the expression somewhere else inside of the block. (action‑at‑a‑distance)
If you could come up with some code that would be vastly improved with .resume(value), then maybe it could be added.
(I personally think that leave(value) would be more useful in such a circumstance.)
I will grant that .resume(value) seems like it may be useful for control exceptions.
(Caught with CONTROL instead of CATCH.)
Sometimes you need to work with unmaintained, old, dirty, huge and sort of libraries that can be dangerous for our program.
Is there have the best practices for execution this code in a safe way?
Recently I found (probably on my knowledge and experience level) non-catching exception. The common practice that I used until today is to wrap an code into Fiber, capture exception inside and sending out through Channel. For now it is not work (I can't put Yield or Proc inside Fiber).
The dangerous lib can be looks like common class with method that encapsulates the Fiber with Fiber.yield for swap execution to other fibers right now. In real life this Fiber may contain inside work with IO, it does not matter.
class LibDangerous
def exec_remote
spawn do
raise IO::Error.new
end
Fiber.yield
end
end
Wrapper that should be handle exception consists of nested methods over begin ... rescue. I call methods from the top-level and from the last wrapper method I return lib method that always blow up the program even with block begin ... rescue.
class Wrapper
def capture
begin
yield self
rescue
puts "rescued from :capture"
end
end
def guard
begin
capture do |this|
yield this
end
rescue
puts "rescued from :guard"
end
end
def run
begin
yield LibDangerous.new
rescue ex
puts "rescued from :run"
end
end
end
This seems to be because you need to handle the exception at the same level where it happened, but I can't modify the code of someone else's library for a variety of reasons.
wrapper = Wrapper.new
result = wrapper.guard do |sandbox|
begin
sandbox.run do |library|
library.exec_remote
end
rescue
puts "rescued from top-level"
end
end
Boom! (this code on play.crystal-lang.org)
Unhandled exception in spawn: (IO::Error)
from /eval:4:7 in '->'
from /usr/lib/crystal/fiber.cr:255:3 in 'run'
from /usr/lib/crystal/fiber.cr:92:34 in '->'
from ???
It may happens due to swap executable contexts: my code and exception are in different contexts and can't interact? If you remove the Fiber then the Exception is caught as usual.
Is it possible to solve this without modifying the original library?
No, you cannot handle this without patching into the original faulty code. However Crystal's open class system makes this possible entirely from your side until the upstream behavior is fixed, you can just redefine the method in your code.
Please note that this is only a problem in terms of dealing with the fact that the operation failed. In case you can obtain that information otherwise, for example by waiting on the result with a timeout using select, or you simply don't care if the operation succeeded or not, the only real problem is that of a bit of log spam. A fiber, that's not the main fiber, crashing doesn't take down your program! (see https://play.crystal-lang.org/#/r/98da)
Why is this? Raising an exception means walking up the current stack until a handler is found. When you see "Unhanded exception", that's just the default handler Crystal puts at the root of each stack. Now what is a fiber? It's a separate stack! So raising inside a fiber does not unwind any other stacks, especially not your main fiber's.
I'm making a custom plugin to query a database for user info to aide customer support. My backend is slack.
Everytime I start the bot command I'm greeted with:
Computer says nooo. See logs for details:
catching classes that do not inherit from BaseException is not allowed
I'm not sure if this is warning me that I'm attempting to catch an exception that isn't a BaseClass in my code or if an unknown exception was raised and caught elsewhere outside of my plugin.
To debug I tried:
try:
do_the_thing()
except (TypeError, ValueError) as e:
return('Something went wrong.')
I also tried:
try:
do_the_thing()
except Exception as e:
return('Something went wrong.')
And I still get the errbot admonition. Note that the command still runs and does the right thing where there is no exception raised by do_the_thing().
It means that:
Somewhere in your code you have an except ... statement where the exception ... (or one of the exceptions in the sequence ...) is not a subclass of BaseException, and
An exception is being thrown that is caught by that except ... statement.
The TypeError can be raised only when an exception is actually thrown because the names you give to except ... must be evaluated for their current values at that time; just because TypeError referenced a particular class at one point in the program's execution doesn't mean it won't be changed later to reference another object (though that would be admittedly perverse).
The Python interpreter should be giving you a full traceback of the exception; the first thing you need to do is find this. It could be occurring in one of two situations. (This is for single-threaded programs; I'm assuming your program is not multithreaded.)
During the execution of your program, in which case the program will be terminated by the exception, or
During finalization of objects (in their __del__(self) functions) in which case the error will be printed to stderr.
In both cases there should be a stack trace, not just the error message; I've confirmed that at least on Python ≥3.4 a stack trace is printed out for case 2.
You then need to follow this stack trace to see where the problem lies. Remember that the names you give to except ... are variables (even things like TypeError) that can be reassigned, so that you could conceivably be dealing with a (perverse) situation like:
TypeError = False
try:
...
except TypeError:
...
But more likely it will be something obvious such as:
class MyException: # Doesn't inherit from Exception
...
try:
...
except MyException:
...
There is one special case you need to be aware of: if you are seeing messages to stderr (case "2. During finalization," above) printed out as your program exits that means that the exception was thrown during cleanup as the interpreter shuts down, where random variables throughout the program may have already been set to None as part of the cleanup process. But in this case your program should still exit successfully.
I have used try-catch/except-finally variants in many languages for years, today someone asked me what is the point of finally and I couldn't answer.
Basically why would you put a statement in finally instead of just putting it after the whole try-catch block? Or in other words is there a difference between the following blocks of code:
try{ //a}
catch {//b}
finally {//c}
try{//a}
catch{//b}
//c
EDIT:
PEOPLE, I know what finally does, I have been using it for ages, but my question is in the above example putting //c in finally seems redundant, doesn't it?
The purpose of a finally block is to ensure that code gets run in three circumstances which would not very cleanly be handled using "catch" blocks alone:
If code within the try block exits via fallthrough or return
If code within a catch block either rethrows the caught exception, or--accidentally or intentionally--ends up throwing a new one.
If the code within the try block encounters an exception for which the try has no catch.
One could copy the finally code before every return or throw, and wrap catch blocks within their own try/catch to allow for the possibility of an accidental exception occurring, but it's far easier to forgo all that and simply use a finally block.
BTW, one thing I wish language designers would include would be an exception argument to the finally block, to deal with the case where one needs to clean up after an exception but still wants it to percolate up the call stack (e.g. one could wrap the code for a constructor in such a construct, and Dispose the object under construction if the constructor was going to exit with an exception).
Finally block is executed even if an exception thrown in the try block. Therefore, for instance if you opened a stream before, you may want to close that stream either an exception is thrown or not. Finally block is useful for such an issue.
finally is a syntactic sugar to allow DRY principle in try-catch pattern. Exception is usually thrown if the library code has not enough information to handle some state and wants the client code to solve it. If you don't have library-client code separation, you can handle everything by if instead of try.
Let's see a standard situation without finally:
void myFunction() {
var r = allocateResources();
r.doSomething();
if(somethingBadHappens) {
freeResources(r);
throw new Exception(CODE42);
}
r.doSomethingMore();
freeResources(r);
}
In the snippet above, you repeat freeResources(): this can be multiple statements which you need to repeat. This smells and finally block is the solution for clean code:
void myFunction() {
var r = allocateResources();
try {
r.doSomething();
if(somethingBadHappens) throw new Exception(CODE42);
r.doSomethingMore();
}
finally {
freeResources(r);
}
happyFunction();
}
Let's realize three levels of abstraction:
A1 is the library code providing allocateResources() function
A2 is our code providing myFunction, consuming A1
A3 is some client code consuming myFunction in try-catch block:
function A3code() {
try {
myFunction();
doSomething();
}
catch(Exception e) {
// no hanging resources here
Console.WriteLine(e);
}
}
Now let's see what can happen:
if allocateResources() throws in A1, we don't know how to handle it in A2 (A2 code can be run in Console-free environment), so we delagate the situation to A3 without adding any further code. If Exception is thrown here, the finally block is not executed, since finally is bound to try which was not entered.
if somethingBadHappens in try block, the stack unwinds to A3 where the situation is handled BUT before it finally block is executed, so we don't need to repeat it if no exceptions happen.
before finally we can add catch block and try to resolve some potential exceptions from A1 which may appear in calling r.doSomething methods. Usually we want to handle exceptions as soon as possible to make the client code (A3) more comfortable for client coders.
happyFunction() is executed only if nothing throws in myFunction() (inside or outside of try block).
As #supercat points out, the finally block is executed also if try block exits via return. I suggest you avoid this bad habit and have only one return in every function (maybe some early exists at the very beginning of functions). The reasons for single-return functions are:
The code is more readable: you look at the end and see WHAT the function returns. In multiple returns you must find all return occurences, inspect all the ifs, think about when the ifs are satisfied and only then you know what the function returns.
The code can be optimized by compilers, see copy elision.
The reason for multiple returns is avoiding many nested ifs, but there are other techniques to solve it. Exceptions are exception in this rule.
Learn by example
let v = 0;
function f() {
try {
v = 1;
return 2;
} finally {
v = 3;
return 4;
}
v = 5;
return 6;
}
const r = f();
console.log(r, v);
following prints "3, 4"
Finally make sure your code is executed even if you get an exception.
The finally block is useful for cleaning up any resources allocated in the try block as well as running any code that must execute even if there is an exception
http://msdn.microsoft.com/en-us/library/zwc8s4fz(v=vs.80).aspx
I dont even know what caused it in my application. What is it? I created a new instance of a class (the class was in another file), but at the first time I call a method it throws a StackOverFlow exception.
The only thing that I think would logically throw a stackoverflow exception would be if someone downvoted Jon Skeet.
But seriously now, what is it? I got around it by creating another class in the same file as the first class and using that to call the methods for me.
As a general rule, a stack overflow exception is caused by a recursive algorithm where the depth of recursion has exceeded the (typically) fixed stack limit. This is usually a result of a bug in the algorithm, but it may also be caused by the data structure you are applying the algorithm to being too "deep".
Here's a trivial example of a buggy recursion (in no particular PL).
function int length(list l) {
if (empty(l)) {
return 0;
} else {
return 1 + length(l); // should be 'return 1 + length(tail(l));
}
}
Calling length for any non-empty list will give a stack overflow in a typical programming language. But even if you correct the bug, calling the method for a long list is likely to cause a stack overflow.
(The exception is when you use a language ... or more strictly a compiler ... that supports tail recursion optimization.)
A stackoverflow exception is when you exceed the allocated stack size, this generally occurs from recursively calling methods and never leaving, it can also be cause by various obscure method chaining. The issue is you probably have something to the extent of the following in the object.
void MyMethod()
{
MyMethod();
}
The calls will eat up and never free the stack space used because the calls never end execution and the entry point must remain.
P.S. SO was named for the particular exception in question (this is fundamental and not limited to .NET), it's just a clever name for a developer site.
StackOverFlows Exceptions are exactly what they sound like, the stack overflows. Usually this is because you have a circular dependency in your methods. For instance method A calls B and B calls A. Or it could be a recursive method without a base case.
Without seeing the code it is impossible to tell why this happened but a StackOverflowException is thrown when a thread overflows its call stack. This most often occurs when a method calls itself recursively without any conditional break thus creating infinite recursion. Since each recursion creates a new stack frame an infinite recursion would theoretically create an infinite number of stack frames, I am sure you can now see why the term "stack overflow" is apt.
The Stack is where the computer stores a list of the functions that are currently being called, and the variables and parameters used. So if function Main calls function A, and then function A calls function B, and they use variables c, d and e, the stack will contain all of that information. However, the stack is only so big. So if function B then calls function C, which calls function D... etc, ending up with hundreds of nested functions, eventually, the stack will "overflow" - there isn't enough space to store another function call.
As other people have noted, this usually happens with a recursive function (where function B calls function B, which then calls function B...) - eventually, the stack will overflow. You will need to find where that recursive function is being called, and why it isn't breaking out of the recursive loop when it's supposed to.
Of course, the problem may not be that it's a buggy recursive algorithm - it may just be that the number of function calls exceeds the size of the stack. So if your algorithm has the potential to call a recursive function a few hundred times, it may be that.
This is usually caused by a recursive call to a function where that recursive call never terminates. You may get this in several ways. One way could be a recursive algorithm without a base case, another common one is creating objects A and B that create one of each other in the their constructors, etc.
i recommend you step through the debugger and find out :)
I recently ported an old VB6 application to VB.NET which used a monstrous recursive function to sort an equally large amount of data. The algorithm was fine but execution consistently caused a stack overflow error. After much fiddling, I realized that VB was doing to much magic behind the code: easy type casting comes with a price. So the recursive function was relying way too much on late binding instead of using type variables and this resulted in HUGE amount of casting, parsing, etc. overhead (a single line of code could call from 2 to 10 functions...) which, obviously, made the stack overflow.
TL;DR: Use DirectCast() and static binding (typed variables) to prevent VB from flooding the stack at runtime in a recursive function.
I had this problem occur, I noticed that I mis-typed lstEncounter. As I learned in my C++ class, the problem is the recursive algorithm of basically calling itself with the same parameters. My example where I got the error:
Property Encounter(ByVal N As Integer)
Get
If N < lstEncounters.Count Then
Return Encounter(N)
Else
Return Nothing
End If
End Get
Set(value)
lstEncounters(N) = value
End Set
End Property
I was having a Stackoverflow error.
I was using a routine that added 1 to a counter then re called the same routine.
About every 2500 to 3000 loops I got the stackoverflow error.
I have added a DO...Loop that calls the routine, I use VB Express:
BEFORE:
Public Sub mainloop()
Dim cntr as integer
If cntr >= 5000 ( I just picked a number at random)
me.close ( I close the program)
... (This is where I would manipulate the cntr for diff
results)
cntr = cntr + 1 ( increment the cntr)
mainloop() (re call my loop)
End IF
End Sub
(as I said before, after about 2500-3000 I would get the Stackoverflow error)
AFTER: (Place this to execute first)
Dim LoopCntr as integer
Do While LoopCntr <= 40000 (this my number.. Use your own number)
If LoopCntr > 40000 Then
Exit Do
End If
mainloop() (The mainloop() has not been changed just the method of calling it)
LoopCntr = LoopCntr + 1
Loop
me.close (When LoopCntr reaches max it closes the program)
(After adding the Do..Loop my program ran 40000 time without "Stackoverflow")