If I need to throw an exception from within my application which of the built-in .NET exception classes can I use? Are they all fair game? When should I derive my own?
See Creating and Throwing Exceptions.
On throwing built-in exceptions, it says:
Do not throw System.Exception, System.SystemException, System.NullReferenceException, or System.IndexOutOfRangeException intentionally from your own source code.
and
Do Not Throw General Exceptions
If you throw a general exception type, such as Exception or SystemException in a library or framework, it forces consumers to catch all exceptions, including unknown exceptions that they do not know how to handle.
Instead, either throw a more derived type that already exists in the framework, or create your own type that derives from Exception."
This blog entry also has some useful guidelines.
Also, FxCop code analysis defines a list of "do not raise exceptions" as described here. It recommends:
The following exception types are too general to provide sufficient information to the user:
System.Exception
System.ApplicationException
System.SystemException
The following exception types are reserved and should be thrown only by the common language runtime:
System.ExecutionEngineException
System.IndexOutOfRangeException
System.NullReferenceException
System.OutOfMemoryException
So in theory you can raise any other framework exception type, providing you clearly understand the intent of the exception as described by Microsoft (see MSDN documentation).
Note, these are "guidelines" and as some others have said, there is debate around System.IndexOutOfRangeException (ie many developers throw this exception).
On the subject of System.Exception and System.ApplicationException: The latter was meant to be used as the base class of all custom exceptions. However, this hasn't been enforced consistently from the beginning. Consequently, there's a controversy whether this class should be used at all rather than using System.Exception as the base class for all exceptions.
Whichever way you decide, never throw an instance of these two classes directly. It's actually a pity that they aren't abstact. For what it's worth, always try using the most specific exception possible. If there is none to meet your requirement, feel free to create your own. In this case, however, make sure that your exception has a benefit over existing exceptions. In particular, it should convey its meaning perfectly and provide all the information necessary to handle the situation in a meaningful manner.
Avoid to create stub exceptions that don't do anything meaningful. In the same vein, avoid creating huge exception class hierarchies, they're rarely useful (although I can imagine a situation or two where I would use them … a parser being one of them).
I use the ArgumentException (and its “friends”) regularly.
NotSupportedException and NotImplementedException are also common.
My advice would be to focus on two things:
Scenarios
User expectations
In otherwords, I would sit down and identify:
Under what scenarios do you want to throw exceptions.
In those scenarios, what would the users of your API expect
The answer to #1 is, of course, application specific. The answer to #2 is "what ever similar code they are already familiar with does".
The behavior that comes out of this is:
Under the scenarios that arise in your programs that also arrive inside the
framework, such as arguments being null, out of range, being invalid, methods not
being implemented, or just not supported, then you should use the same exceptions the
framework uses. The people using your APIs are going to expect that they behave that
way (because that's how everything else behaves), and so will be better able to use
your api from the "get go".
For new scenarios that don't exist in the framework, you should go ahead and invent
your own exception classes. I would say that you should prefer Exception as your base
class unless their is some other base exception that provides services you need.
Generally speaking I don't think something like "ApplicationException" will help you
much. When you start defining your own exceptions there are a few things you should
keep in mind though:
a. The primary purpose of an exception is for human communication. They convey
information about something that happened that shouldn't have. They should provide
enough information to identify the cause of a problem and to figure out how to
resolve it.
b. Internal consistency is extremely important. Making your app behave as universally
as possible under similar circumstances will make you API's users more productive.
As far as there being hard and fast rules about what you should and should not do... I wouldn't worry about that stuff. Instead I would just focus on identifying scenarios, finding the existing exception that fits those scenarios, and then carefully desining your own if an existing one doesn't exist.
You can create and throw pretty much any of them, but you generally shouldn't. As an example, the various argument validation exceptions (ArgumentException, ArgumentNullException, ArgumentOutOfRangeException, etc) are suitable for use in application code, but AccessViolationException isn't. ApplicationException is provided as a suitable base class for any custom exception classes you may require.
See this MSDN article for a list of best practices - it refers to handling exceptions, but also contains good advice on creating them...
Related
One thing I find very frustrating with C# is when I find some issue and I want to throw a meaningful exception. I find it very difficult to find those exceptions in intellisense. There is no Exception namespace, so I cant list all exceptions via intellisense without digging around looking for the exception.
I am not looking to create my own exceptions, I am only trying to see if there are any other options than googling an exception to find its namespace so I can use it.
For the most part, this is bad practise. There are a small handful of exceptions that you should reuse (InvalidOperation, NullReference, ArgumentException, a few others). But you should not, for example, throw SqlException yourself - because you don't know what the framework might do with it.
Creating your own exception hierarchy adds meaning to your application at times of error. Reusing exceptions that have already-understood meanings leads to confusion - loss of meaning.
You can browse the entire Exception Class Tree in object Browser. Look for System.Exception and then click derived types. not 100% sure if all of them are there but the most of them are there for sure.
System.Exception -> Derived types (also in the root of the System.Excecption tree)
You can find some exceptions in the MSDN, here.
In general, these are the exceptions you'll ever tend to throw, and in many cases, you'll derive your own exceptions from these exceptions. However, a single method throwing too many different exceptions is generally frowned upon.
Also, recall the <exception> xml documentation tag. Using this tag goes a long way towards enlightening users which exceptions your method throws, and when. It's probably more important, in terms of clarity, than throwing exceptions of specific classes.
In Java, you explicitly define what exceptions are thrown using the "throws" keyword. That way, anyone calling your method knows what to catch.
Is there something in C#? If not, how do I know what exceptions to catch, or how do I let others know what exceptions to catch?
Also, if I am defining an interface, is there a way to say "methodX() should throw this exception on error"?
There is nothing equivalent in C#: The Trouble with Checked Exceptions
Other than documentation, there is no way to declare an interface to say "methodX() should throw this exception on error".
C#/.net does not have checked Exceptions, they proved to be less useful in large scale systems than first thought. In a lot of projects the time to maintain the check exception specs was a lot greater than the debugging time saved by having them.
Checked Exceptions seem like a good ideal until you have methods that can take delegates or calls into object you pass in. Take a simple case, the Sort() method on a list can’t know what exceptions it will throw, as it does not know what exceptions the Compar() method on the objects being sorted will throw.
So the spec for the exceptions a method may throw must be able to include information on how exceptions are populated from pass in objects and delegates. No one knows how to do this!
However there are tools that you check if you are catching all exceptions – see Exception Hunter by Red Gate. I personally don’t see much value in these tool, however if you like checked exceptions you may find them useful. ==> Looks like Exception Hunter wasn't too useful, so Redgate discontinued Exception Hunter a long while ago:
This feature is not available in C#. You can make proper XML documentation (3 slashes ///) and state what exceptions are being thrown.
This will be picked up by the IntelliSense mechanism and will be visible for the users of the class/method before they use it.
C# does not support this. (Not that I know anyway).
What you can do is use Xml Comments so that while calling you methods this data will be shown by intellisense.
As far as I'm aware there is no throws declaration in C# you can document your method indicating that it throws an exception but no forced error handling.
C# doesn't support checked exceptions. The language designers consider checked exceptions in the way java uses them a bad idea.
Some workarounds
Let me cite this medium article: It's almost 2020 and yet... Checked exceptions are still a thing
Among the many reasons why it's a bad idea, putting the checked exceptions in the contract (interfaces):
makes it impossible to change the implementation of an interface with a different one which throws different exceptions
exposes implementation details
a change of the checked exceptions of an API interface, makes it necessary to change the whole chain of interfaces in the call stack
For example, imagine that you are implementing a repository based on SQL Server, so you expose all kind of SQL Server specific exceptions. Then you want to move it to MySQL or Cosmos BD. Of course:
the implementation can't be changed to a new one that need to throw different exceptions. Also related to this, if you have different implementations of the storage, you can't just change them by configuration, but you need to have different compilations of the code for each storage backend
this is the explanation for 1: as the interface showed the implementation details (SQL Server exceptions) know you can't just change it
if you need to make the change, prepare to change the interface at all levels, since the API that uses the database up to the lates consumer in the call stack chain.
The articles cited above includes pointers to many explanations to discourage the use of checked exceptions, included this by the creator of C#: The trouble with checked exceptions
I am continuing with my exam revision.
I have come across the usage of the Base Exception class and I have seen it on exam papers also.
My question is when do you derive from the Base Exception class?
I am of the impression if you want a custom class to throw an exception with more meaningful information, then you can create a custom exception class that contains the exact data that is representative of how your custom class is used and what scenario it is designed to be used for?
Why can't my custom exception class derive from 'ApplicationException' or 'SecurityException' or the base 'Exception' class?
I am of the impression that I should derive from the base Exception class and not the previous two.
My question second is, when would you derive from the other two??? Are there any clear-cut
distinctions as to when you would derive from either one of these three? Assuming there are no others I have I have missed out?
SMALL UPDATE:
This question from transcender pretty much hits the nail on the head.
*Which class should you use to generate an application-specific exception?
Answer: The ApplicationException class*
This is all discussed in the Design Guidelines document.
In our most recent project we used a base exception class. We used it to get the following functionality:
All exceptions needed a number, so defining the property for the number was done in the base class
All exception messages needed to be formatted the same way, with the number, reason and type. This get formmated message was done in the base class.
Our base exception class derives from ApplicationException. This may have been a mistake, there is a lot of discussion about problems with too much depth of inheritance. However, we have not had any problems with this.
One tip for the exam: Read the question very carefully. Good luck.
In general, you want to derive from the Exception class which most closely resembles the type of exception you want to throw. If the trouble is that some Argument or Parameter has been passed which causes a problem, use ArgumentException. If you need some customization with that, inherit from ArgumentException.
In my experience, the only two reasons to use the base Exception are: 1) when you need some custom exception that completely does not fit one of the current exception models or 2) When a method could theoretically throw a number of exceptions, but you've already caught the ones you find most likely to be thrown.
Typically, I don't inherit from exceptions at all. Simply setting the Message property tends to be enough.
Ideally, exceptions should be grouped in a hierarchy such that if code will want to handle several exceptions the same way, they will all be derived from a common base class. If the base throw-able type had been an interface rather than a class, such an ideal might have been somewhat achievable. As it is, however, the single-inheritance limitation for classes severely limits the usefulness of the hierarchy.
The only time an exception hierarchy is apt to be a useful concept is when an implementation of an interface, or a new version of a class, which is documented as throwing certain exceptions wants to allow code to distinguish among more distinct conditions than are reported by those exceptions. In such a scenario, having a method throw exceptions which do not derive from the documented ones would be a breaking change, so one must throw an exception that inherits from the documented one which best describes the previously-unanticipated condition. That's rather ugly, but the exception-handling mechanism doesn't really provide any better alternative. It's rather unfortunate that things like IEnumerator<T>.MoveNext() aren't documented as throwing any exception which would simply mean "Sorry--the system isn't on fire or anything, and I don't know that anybody's changed the collection, but I can neither advance to the next item nor truthfully say the enumeration is complete", but they don't.
Other than the case where one needs to throw an exception that's compatible with existing code, it may be helpful to have the exceptions used by an application or library derive from a common base. Instead of using ApplicationException it should be something like YourApplicationNameException or YourLibraryNameException--something that nothing else is apt to derive from. Something like ApplicationException is bad because code which does a catch ApplicationException will get not only the exceptions which it derived from that type, but also any exceptions which any other libraries derived from it.
There are already lots of questions on SO about exceptions, but I can't find one that answers my question. Feel free to point me in the direction of another question if I've missed it.
My question is quite simple: how do other (C#) developers go about choosing the most appropriate type of exception to throw? Earlier I wrote the following code:
if (Enum.IsDefined(enumType, value))
{
return (T)Enum.Parse(enumType, value);
}
else
{
throw new ArgumentException(string.Format("Parameter for value \"{0}\" is not defined in {1}", value, enumType));
}
I have since realised that throwing an InvalidEnumArgumentException would probably have been more appropriate had I known of its existence at the time.
Is there an authoritative resource available that helps developers chose exception types, or is it simply a matter of experience?
Edit
I've given the points to Noldorin for providing a range of ideas in a well thought-out answer. The points could have gone to any one of you really - thanks for all the suggestions.
Krzysztof Cwalina has a good post on this see chapter "1.1.1 Choosing the Right Type of Exception to Throw"
PS Consider subscribing to his blog. Good reading!
To answer your question: InvalidEnumArgumentException
because throw the most specific (the most derived) exception that makes sense.
AND callers that catch ArgumentException, catch InvalidEnumArgumentException too...
I would say it's just down to experience. There's still new exceptions I discover every so often, and I've been working with many aspects of .NET for a while now! What would you want this source to tell you, anyway? Choosing the appropriate exception type would seem highly context-specific, so I'm doubtful over the level of advice it could offer. Listing the more common ones would be most it could provide. The names and Intellisense descriptions of the exception types typically explain with good clarity their usage scenarios.
My recommendation is simply to familiarize yourself with all of the fundamental ones (specifically, those in System, System.IO, and any other namespaces you often use), and learn the others along the way. I find that I generally get away using just a small number. If you accidentally use a more generic exception type when there already exists a more specific one in the BCL, then it's no great crime, and can be changed later easily enough. To be honest, for any error that's particularly specific, you will often need to create your own class inheriting from Exception anyway.
Hope that helps.
Edit: If you want a brief guide to the very common ones, see the Common Exception Classes page on MSDN.
Common Exception Types and their Explanations
I think this will probably help you find out what the most appropriate exceptions for you to use are. You can also look into the MSDN documentation for more information on the Exception class and all of its types if you need.
MSDN Exception Class (System)
MSDN SystemException Class
(System) - More thorough list of
exception types
If you check out the MSDN article on System.Exception, right at the bottom of the page is a whole list of BCL exception types that inherit Exception. It's not quite a definitive list on what should be used for what - but it does give you a great place to start checking out exception types. Going through each will tell you what they should be used for:
http://msdn.microsoft.com/en-us/library/system.exception.aspx
There is also a fairly comprehensive article regarding the .NET Exception Hierarchy found at:
http://msdn.microsoft.com/en-us/library/z4c5tckx(VS.71).aspx
In fact the whole section in the MSDN library about handling and throwing exceptions is pretty good:
http://msdn.microsoft.com/en-us/library/5b2yeyab(VS.71).aspx
I have most of the common exceptions written on postcards on my desk and use them when people try and interop with me in a way that is exceptional.
This is really good practice for working out which exception fits best...
The one I find myself using mostly is OperationNotSupportedException
I think the real question is to ask yourself "What type of exception do I want to handle?" If you're not going to have special handling for an InvalidEnumArgumentException or an ArgumentException, then they offer no advantage over a plain old Exception.
I typically either throw Exception or wrap the exception in a custom exception.
Edited to add:
At one point, I recall that the guidance from Microsoft was that your application should never throw framework exceptions, rather it should only throw extensions of ApplicationException.
I usually define my own structure of exceptions, first creating some base exception:
class MyProjectNameException:Exception
{
...constructors, etc
}
and then derived classes for more precise exceptions. In this way you know exactly when to expect which exception, and you know if the exception was thrown by you or by the framework.
If you find an exception that is a good fit, you can use it.
If in doubt, just use ApplicationException. It's the exception that is intended for code that is not part of a standard library.
If the exception is not intended to be used in a standardised way (like catching the base class IOException to catch any I/O related error), the message that you put in the exception is more useful than the type of the exception.
I have an application which tries to load some expected registry settings within its constructor.
What is the most appropriate .NET Exception from the BCL to throw if these (essential, non-defaultable) registry settings cannot be loaded?
For example:
RegistryKey registryKey = Registry.LocalMachine.OpenSubkey("HKLM\Foo\Bar\Baz");
// registryKey might be null!
if (registryKey == null)
{
// What exception to throw?
throw new ???Exception("Could not load settings from HKLM\foo\bar\baz.");
}
Why not create your custom exception?
public class KeyNotFoundException : RegistryException
{
public KeyNotFoundException(string message)
: base(message) { }
}
public class RegistryException : Exception
{
public RegistryException(string message)
: base(message) { }
}
....
if (registryKey == null)
{
throw new KeyNotFoundException("Could not load settings from HKLM\foo\bar\baz.");
}
Also, instead of inheriting from Exception you could inherit from ApplicationException. This depends on the kind of failure you want your application to have in this situation.
actually, I wouldn't throw an exception here. I would have a default value, and then create the key using that default value.
If you MUST have a user-defined value, I'd use the ArgumentException (as that's fundamentally what you're missing, an argument for your constructor--where you store it is irrelevant to the type of exception you're trying to generate).
I'd go with ArgumentException or ArgumentOutOfRangeException..
throw new ArgumentException("Could not find registry key: " + theKey);
Quoting MSDN:
The exception that is thrown when one
of the arguments provided to a method
is not valid.
...
IMO writing a proper exception message is more important.
It depends on why it failed. If it's a permissions issue, the I'd go with System.UnauthorizedAccess exception:
The exception that is thrown when the operating system denies access because of an I/O error or a specific type of security error.
I don't know if it matches the "specific type", but it is a security error, and access wasn't authorized.
On the other hand, if the item just doesn't exist then I'd thrown a FileNotFound exception. Of course, a registry key isn't a file, but FileNotFound is pretty well understood.
Since this entry is as you put it an essential value, what is the impacts to your application if this value cannot be obtained? Do you need to hault operations or do you simply need to notify the application.
Also, there are a number of reasons that the value could be null
User doesn't have permission to read the key
The key doesn't exist
Does this impact the action you take when working with the application?
I think that these types of scenarios play into what exception to throw. Personally I would never throw just Exception, as it really is a "no-no" from a standard design practice.
If it is due to a user not having permissions, AND then not having this permission might cause future problems I would vote for an UnauthroizedAccess exception.
If the issue is not a critical issue, but you really need to know that the key isn't there I would strongly recommend the "KeyNotFoundException" implementation mentioned elsehwere.
When throwing an exception you want to make sure that the exception being thrown is descriptive and provides all needed information, thus why I think it depends on the root cause as well as the overall impacts to the application.
To quote MSDN's "Design Guidelines for Developing Class Libraries"
ApplicationException
If you are designing an application
that needs to create its own
exceptions, you are advised to derive
custom exceptions from the Exception
class. It was originally thought that
custom exceptions should derive from
the ApplicationException class;
however in practice this has not been
found to add significant value. For
more information, see Best Practices
for Handling Exceptions.
I think that the best approach is to take a step back. If there is not a clear cut exception that describes what is happening, it takes only minutes to define one. Try to avoid repurposing exceptions because it "is close enough".
My recommendation is that you should create a base exception class which inherits from either Exception or ApplicationException. This will allow for you to easily identify, from your stack trace, whether the exception is a custom exception that you defined or whether it originated somewhere else. All of your custom exceptions should inherit from the base exception that you create.
Note: I am not going to recommend the use of either Exception or ApplicationException. There is enough debate in the community vs. Microsoft's documentation over which should be used and for what purpose. On a personal level, I choose Exception as my base exception.
If there is not a clearly predefined exception that matches your intent, going forward, derive a custom exception from your base exception. It definitely helps in tracing down the origin of a problem, makes them easier to handle (imagine that an existing framework exception was thrown in the block of code, but by the framework or another method), and just plain makes sense.
Keep in mind, you can have multiple exception hierarchies to group like exceptions together. For example, I can have MyBaseException which inherits either ApplicationException or Exception. I then can have a more generalized MyRegsitryException which inherits from MyBaseException. Then I can have specific exceptions, such as MyRegistryKeyNotFoundException or MyRegistryKeyPermissionException.
This allows you to catch a grouped exception on a higher level and reduce the number of catches that you might have that contain redundant handling mechanism. Combine this with isolating the scope of the exceptions to specific namespaces that would use them, and you have the start of a very clean exception handling scheme.
I would probably throw an ApplicationException since this is specifically related to your application. Alternatively, you could throw a ConfigurationErrorsException, though this is usually associated with an error parsing an application configuration file, not reading the configuration from the registry.
The other potential exceptions that come to mind are ArgumentException or ArgumentNullException, but these have a connotation of being related to parameters that are passed into the method and are not, therefore, appropriate to my mind. It could easily lead someone using/modifying your code astray when trying to determine why it is not working.
In any case, a good, descriptive error message is probably the most effective way of communicating what the problem is.
EDIT: Note that throwing an exception on a null value doesn't mask any exceptions that would occur when attempting to read the registry value. I believe that any SecurityException that gets thrown when you attempt to read the value without sufficient privileges will still occur as long as you don't wrap it in a try/catch block.
I think just Exception itself could do the job. If your message is descriptive enough, then it's all good. If you really want to be precise about it, then I'd agree with petr k. Just roll your own.