I'm making a WP7 app, which is consuming an ASMX which I can't touch, nor adapt, since I'm not the creator, nor the provider, I'm just consuming it.
When I add the service reference to my WP7 solution (not mango - but it's the same behaviour in mango), I'll uncheck the "Reuse types in referenced assemblies" because I don't care about those.
Side Note: Even when I leave that checkbox checked it doesn't work either.
Then I add the following code to the constructor of a new WP7 page :
MobileWS.WebServiceSoapClient ws = new MobileWS.WebServiceSoapClient("WebServiceSoap", "http://www.somewhere.com/MobileService.asmx");
ws.getCountriesCompleted += new EventHandler<MobileWS.getCountriesCompletedEventArgs>(OnGetCountriesCompleted);
ws.getCountriesAsync("fr");
This goes out and fetches an array of Country objects ("fr" stands for "french", so it'll be "Etas Unis" instead of "United States")... at least that's the idea.
I even checked with Fiddler2 if it returned anything, and indeed, the ASMX responds with some XML that contains the countries.
Then my handler goes like this :
private void OnGetCountriesCompleted(object sender, MobileWS.getCountriesCompletedEventArgs e)
{
if (e.Cancelled == false && e.Error == null && e.Result != null)
{
List<MobileWS.Country> countries = e.Result.ToList<MobileWS.Country>();
CountriesListBox.ItemsSource = countries;
}
}
Unfortunatly the e.Result always returns an empty array of country objects (so non at all, but he knows there should be country objects in there, but there are 0 items in the array) !
Though, if I browse to : http://www.somewhere.com/MobileService.asmx I get the list when I invoke the getCountries function.
Even more strange, when I copy and past the exact same code in a WPF application it works like a charm, I get a filled array with 7 country objects in.
What's wrong ?
I'm refusing to parse the returned XML myself so far, but I'm feeling I will need to sooner or later because of this fail.
I'm pretty sure that the XML that is send back is correct, else the WPF application would have similar problems, no ?
So, looks like I'm doomed to parse it myself, then ?
I see a lot of examples on the web where they do that (parsing the XML result themselfs), so there must be a reason for that, and that reason is the one I describe above :).
Sounds like a serialization issue to me. And that's about all I can say, without knowing the service or how the used classes are written/generated.
Remember that Windows Phone have limited reflection abilities, and as such can only reflect on public/internal classes, and don't support dynamics.
What happens if you try to deserialize the result yourself? Granted, it's just a step away from parsing the xml manually, but it's something. If not, maybe you can find out what's going wrong.
This question is a result of the answers to this question that I just asked.
It was claimed that this code is "ugly" because it initializes a variable to a value that will never be read:
String tempName = null;
try{
tempName = buildFileName();
}
catch(Exception e){
...
System.exit(1);
}
FILE_NAME = tempName;
Is this indeed bad practice? Should one avoid initializing variables to dummy values that will never actually be used?
(EDIT -
And what about initializing a String variable to "" before a loop that will concatenate values to the String...? Or is this in a separate category?
e.g.
String whatever = "";
for(String str : someCollection){
whatever += str;
}
)
I think there's no point in initializing variables to values that won't be used unless required by the language.
For example, in C#, the default value for a declared string variable is null, so you're not even gaining anything by explicitly writing it out. It's mostly a style choice, but since strings are immutable, initializing it to something else would actually allocate an extra string in memory that you'd just throw away anyway. Other languages may impose other considerations.
Regarding the string loop, if you change it to a StringBuilder instead you don't even have to think about it.
Edit: removed bits better answered by others.
As a practice, I tend to avoid setting variables to arbitrary values, and instead initialize them to a default value.
i.e.
int x = 0;
string name = "";
bool done = false;
Person randomGuy = null; //or = new Person();
I like this method best because it brings a sense of uniformity to your code while not forcing the next guy that comes along to deal with stuff like: string name = "luke skywalker";
This is all more of a personal preference, so it will vary between programmers.
At the very lest, you should be following the standards that your project as set. You'll have an idea of how the legacy code handles these things, and it's probably best to conform to that so the overall coding style is the same throughout the system.
It depends on the compiler. C# compiler requires that the variable be initialized before using. But the CLR does not have this requirement. At run time the CLR verifies if the variable is initialized or no. If not it will throw a nullreference exception.
In my opinion, it might be more accurate to refer to it as a "code smell" - in the Martin Fowler sense of the word.
I don't think you can change your default initialisation in isolation - it would need to be made in conjunction with other refactoring methods. It is also assuming that you have refactored your code so that you don't need temp variables:
try{
FILE_NAME = buildFileName();
//Do other stuff with file name
}
catch(Exception e){
...
System.exit(1);
}
It also then makes the assumtion that this code segment is only code in the method in which it is contained - i.e. the method only does one thing
When I'm coding I would be concerned that I am using dummy values with temp variables but I would only change when I am finished coding that section and it solves the problem as intended - and only in conjunction with other refactoring steps.
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I've heard some voices saying that checking for a returned null value from methods is bad design. I would like to hear some reasons for this.
pseudocode:
variable x = object.method()
if (x is null) do something
The rationale behind not returning null is that you do not have to check for it and hence your code does not need to follow a different path based on the return value. You might want to check out the Null Object Pattern which provides more information on this.
For example, if I were to define a method in Java that returned a Collection I would typically prefer to return an empty collection (i.e. Collections.emptyList()) rather than null as it means my client code is cleaner; e.g.
Collection<? extends Item> c = getItems(); // Will never return null.
for (Item item : c) { // Will not enter the loop if c is empty.
// Process item.
}
... which is cleaner than:
Collection<? extends Item> c = getItems(); // Could potentially return null.
// Two possible code paths now so harder to test.
if (c != null) {
for (Item item : c) {
// Process item.
}
}
Here's the reason.
In Clean Code by Robert Martin he writes that returning null is bad design when you can instead return, say, empty array. Since expected result is an array, why not? It'll enable you to iterate over result without any extra conditions. If it's an integer, maybe 0 will suffice, if it's a hash, empty hash. etc.
The premise is to not force calling code to immediately handle issues. Calling code may not want to concern itself with them. That's also why in many cases exceptions is better than nil.
Good uses of returning null:
If null is a valid functional result, for example: FindFirstObjectThatNeedsProcessing() can return null if not found and the caller should check accordingly.
Bad uses: Trying to replace or hide exceptional situations such as:
catch(...) and return null
API dependency initialization failed
Out of disk space
Invalid input parameters (programming error, inputs must be sanitized by the caller)
etc
In those cases throwing an exception is more adequate since:
A null return value provides no meaningful error info
The immediate caller most likely cannot handle the error condition
There is no guarantee that the caller is checking for null results
However, Exceptions should not be used to handle normal program operation conditions such as:
Invalid username/password (or any user-provided inputs)
Breaking loops or as non-local gotos
Yes, returning NULL is a terrible design, in object-oriented world. In a nutshell, NULL usage leads to:
ad-hoc error handling (instead of exceptions)
ambiguous semantic
slow instead of fast failing
computer thinking instead of object thinking
mutable and incomplete objects
Check this blog post for a detailed explanation: http://www.yegor256.com/2014/05/13/why-null-is-bad.html. More in my book Elegant Objects, Section 4.1.
Who says this is bad design?
Checking for nulls is a common practice, even encouraged, otherwise you run the risk of NullReferenceExceptions everywhere. Its better to handle the error gracefully than throw exceptions when you don't need to.
Based on what you've said so far, I think there's not enough information.
Returning null from a CreateWidget()method seems bad.
Returning null from a FindFooInBar() method seems fine.
Its inventor says it is a billion dollar mistake!
It depends on the language you're using. If you're in a language like C# where the idiomatic way of indicating the lack of a value is to return null, then returning null is a good design if you don't have a value. Alternatively, in languages such as Haskell which idiomatically use the Maybe monad for this case, then returning null would be a bad design (if it were even possible).
If you read all the answers it becomes clear the answer to this question depends on the kind of method.
Firstly, when something exceptional happens (IOproblem etc), logically exceptions are thrown. When exactly something is exceptional is probably something for a different topic..
Whenever a method is expected to possibly have no results there are two categories:
If it is possible to return a neutral value, do so.
Empty enumrables, strings etc are good examples
If such a neutral value does not exist, null should be returned.
As mentioned, the method is assumed to possibly have no result, so it is not exceptional, hence should not throw an exception. A neutral value is not possible (for example: 0 is not especially a neutral result, depending on the program)
Untill we have an official way to denote that a function can or cannot return null, I try to have a naming convention to denote so.
Just like you have the TrySomething() convention for methods that are expected to fail, I often name my methods SafeSomething() when the method returns a neutral result instead of null.
I'm not fully ok with the name yet, but couldn't come up with anything better. So I'm running with that for now.
I have a convention in this area that served me well
For single item queries:
Create... returns a new instance, or throws
Get... returns an expected existing instance, or throws
GetOrCreate... returns an existing instance, or new instance if none exists, or throws
Find... returns an existing instance, if it exists, or null
For collection queries:
Get... always returns a collection, which is empty if no matching[1] items are found
[1] given some criteria, explicit or implicit, given in the function name or as parameters.
Exceptions are for exceptional circumstances.
If your function is intended to find an attribute associated with a given object, and that object does has no such attribute, it may be appropriate to return null. If the object does not exist, throwing an exception may be more appropriate. If the function is meant to return a list of attributes, and there are none to return, returning an empty list makes sense - you're returning all zero attributes.
It's not necessarily a bad design - as with so many design decisions, it depends.
If the result of the method is something that would not have a good result in normal use, returning null is fine:
object x = GetObjectFromCache(); // return null if it's not in the cache
If there really should always be a non-null result, then it might be better to throw an exception:
try {
Controller c = GetController(); // the controller object is central to
// the application. If we don't get one,
// we're fubar
// it's likely that it's OK to not have the try/catch since you won't
// be able to really handle the problem here
}
catch /* ... */ {
}
It's fine to return null if doing so is meaningful in some way:
public String getEmployeeName(int id){ ..}
In a case like this it's meaningful to return null if the id doesn't correspond to an existing entity, as it allows you to distinguish the case where no match was found from a legitimate error.
People may think this is bad because it can be abused as a "special" return value that indicates an error condition, which is not so good, a bit like returning error codes from a function but confusing because the user has to check the return for null, instead of catching the appropriate exceptions, e.g.
public Integer getId(...){
try{ ... ; return id; }
catch(Exception e){ return null;}
}
For certain scenarios, you want to notice a failure as soon as it happens.
Checking against NULL and not asserting (for programmer errors) or throwing (for user or caller errors) in the failure case can mean that later crashes are harder to track down, because the original odd case wasn't found.
Moreover, ignoring errors can lead to security exploits. Perhaps the null-ness came from the fact that a buffer was overwritten or the like. Now, you are not crashing, which means the exploiter has a chance to execute in your code.
What alternatives do you see to returning null?
I see two cases:
findAnItem( id ). What should this do if the item is not found
In this case we could: Return Null or throw a (checked) exception (or maybe create an item and return it)
listItemsMatching (criteria) what should this return if nothing is found?
In this case we could return Null, return an empty list or throw an Exception.
I believe that return null may be less good than the alternatives becasue it requires the client to remember to check for null, programmers forget and code
x = find();
x.getField(); // bang null pointer exception
In Java, throwing a checked exception, RecordNotFoundException, allows the compiler to remind the client to deal with case.
I find that searches returning empty lists can be quite convenient - just populate the display with all the contents of the list, oh it's empty, the code "just works".
Make them call another method after the fact to figure out if the previous call was null. ;-) Hey, it was good enough for JDBC
Well, it sure depends of the purpose of the method ... Sometimes, a better choice would be to throw an exception. It all depends from case to case.
Sometimes, returning NULL is the right thing to do, but specifically when you're dealing with sequences of different sorts (arrays, lists, strings, what-have-you) it is probably better to return a zero-length sequence, as it leads to shorter and hopefully more understandable code, while not taking much more writing on API implementer's part.
The base idea behind this thread is to program defensively. That is, code against the unexpected.
There is an array of different replies:
Adamski suggests looking at Null Object Pattern, with that reply being up voted for that suggestion.
Michael Valenty also suggests a naming convention to tell the developer what may be expected.
ZeroConcept suggests a proper use of Exception, if that is the reason for the NULL.
And others.
If we make the "rule" that we always want to do defensive programming then we can see that these suggestions are valid.
But we have 2 development scenarios.
Classes "authored" by a developer: The Author
Classes "consumed" by another(maybe) developer: the Developer
Regardless of whether a class returns NULL for methods with a return value or not,
the Developer will need to test if the object is valid.
If the developer cannot do this, then that Class/method is not deterministic.
That is, if the "method call" to get the object does not do what it "advertises" (eg getEmployee) it has broken the contract.
As an author of a class, I always want to be as kind and defensive ( and deterministic) when creating a method.
So given that either NULL or the NULL OBJECT (eg if(employee as NullEmployee.ISVALID)) needs to be checked
and that may need to happen with a collection of Employees, then the null object approach is the better approach.
But I also like Michael Valenty's suggestion of naming the method that MUST return null eg getEmployeeOrNull.
An Author who throws an exception is removing the choice for the developer to test the object's validity,
which is very bad on a collection of objects, and forces the developer into exception handling
when branching their consuming code.
As a developer consuming the class, I hope the author gives me the ability to avoid or program for the null situation
that their class/methods may be faced with.
So as a developer I would program defensively against NULL from a method.
If the author has given me a contract that always returns a object (NULL OBJECT always does)
and that object has a method/property by which to test the validity of the object,
then I would use that method/property to continue using the object, else the object is not valid
and I cannot use it.
Bottom line is that the Author of the Class/Methods must provide mechanisms
that a Developer can use in their defensive programming. That is, a clearer intention of the method.
The Developer should always use defensive programming to test the validity of the objects returned
from another class/method.
regards
GregJF
Other options to this, are:
returning some value that indicates success or not (or type of an error), but if you just need boolean value that will indicate success / fail, returning null for failure, and an object for success wouldn't be less correct, then returning true/false and getting the object through parameter.
Other approach would to to use exception to indicates failures, but here - there are actually many more voices, that say this is a BAD practice (as using exceptions may be convenient but has many disadvantages).
So I personally don't see anything bad in returning null as indication that something went wrong, and checking it later (to actually know if you have succeeded or not). Also, blindly thinking that your method will not return NULL, and then base your code on it, may lead to other, sometimes hard to find, errors (although in most cases it will just crash your system :), as you will reference to 0x00000000 sooner or later).
Unintended null functions can arise during the development of a complex programs, and like dead code, such occurrences indicate serious flaws in program structures.
A null function or method is often used as the default behavior of a revectorable function or overrideable method in an object framework.
Null_function #wikipedia
If the code is something like:
command = get_something_to_do()
if command: # if not Null
command.execute()
If you have a dummy object whose execute() method does nothing, and you return that instead of Null in the appropriate cases, you don't have to check for the Null case and can instead just do:
get_something_to_do().execute()
So, here the issue is not between checking for NULL vs. an exception, but is instead between the caller having to handle special non-cases differently (in whatever way) or not.
For my use case I needed to return a Map from method and then looking for a specific key. But if I return an empty Map, then it will lead to NullPointerException and then it wont be much different returning null instead of an empty Map.
But from Java8 onward we could use Optional. The above is the very reason Optional concept was introduced.
G'day,
Returning NULL when you are unable to create a new object is standard practise for many APIs.
Why the hell it's bad design I have no idea.
Edit: This is true of languages where you don't have exceptions such as C where it has been the convention for many years.
HTH
'Avahappy,
Should I be using this method of throwing errors:
if (isset($this->dbfields[$var])) {
return $this->dbfields[$var];
} else {
throw new FieldNotFoundException($var);
}
or this style:
try {
return $this->dbfields[$var];
} catch (Exception $e) {
throw new FieldNotFoundException($var);
}
...or something else altogether?
quick explanation of the code: $this->dbfields is an array. isset() checks if a variable is set, in this case, whether the array element exists.
The second example is bad. You're taking a lot of overhead to catch an exception when, as you demonstrate, it's just as easy to prevent the exception in the first place. Plus you also assume you know why that exception was thrown - if there was some other exception, like say an out of memory or something, you're reporting it as a "field not found" even if it wasn't.
Keep in mind that try/catch in languages like C++ and Java are very expensive because of all the state they have to save and restore. Python, on the other hand, has very cheap exceptions and they positively encourage you to use a try/except for simple validation. But even so, catching everything and pretending it's one type of exception is still bad.
//First let's do the checks.
if(!isset($this->dbfields[$var]))
throw new FieldNotFoundException($var);
//Now we're in the clear!
return $this->dbfields[$var];
Catching "Exception" is not, most of the time, considered a good practice, out of the two you displayed, I would use option 1.
Catching all exceptions may hide a different exception and mask it as a FileNotFoundException.
I prefer the first one, but if dbfields[$var] throws something reasonable when you access a non-existent element, then I'd prefer just returning it without checking.
I don't particularly like changing the exception type unless I have a good reason -- also if you do, make sure to try to preserve the original exception and stack trace.
Just re-read your explanation.
I guess your method there in #1 is going to catch any exceptions that might be thrown and simply return a bool. I definitely don't like the catching of the generic exception most of the time, so #2 wouldn't be my choice.
"...or something else altogether?"
Neither is very good, so something else would be appropriate.
Fix version 2 to catch the correct exception, not every possible exception. Post that as option 3. I'll upvote something that catches a specific exception instead of Exception.
This is far from language-agnostic.
Some languages won't throw errors for accessing non-existant fields, and the preferred pattern depends a lot on the implementations of the arrays, tables, objects, etc.