Error Handling without Exceptions

问题:

While searching SO for approaches to error handling related to business rule validation, all I encounter are examples of structured exception handling.

MSDN and many other reputable development resources are very clear that exceptions are not to be used to handle routine error cases. They are only to be used for exceptional circumstances and unexpected errors that may occur from improper use by the programmer (but not the user.) In many cases, user errors such as fields that are left blank are common, and things which our program should expect, and therefore are not exceptional and not candidates for use of exceptions.

QUOTE:

Remember that the use of the term exception in programming has to do with the thinking that an exception should represent an exceptional condition. Exceptional conditions, by their very nature, do not normally occur; so your code should not throw exceptions as part of its everyday operations.

Do not throw exceptions to signal commonly occurring events. Consider using alternate methods to communicate to a caller the occurrence of those events and leave the exception throwing for when something truly out of the ordinary happens.

For example, proper use:

private void DoSomething(string requiredParameter)
{
if (requiredParameter == null) throw new ArgumentExpcetion("requiredParameter cannot be null");
// Remainder of method body...
}

Improper use:

// Renames item to a name supplied by the user.  Name must begin with an "F".
public void RenameItem(string newName)
{
   // Items must have names that begin with "F"
   if (!newName.StartsWith("F")) throw new RenameException("New name must begin with /"F/"");
   // Remainder of method body...
}

In the above case, according to best practices, it would have been better to pass the error up to the UI without involving/requiring .NET's exception handling mechanisms.

Using the same example above, suppose one were to need to enforce a set of naming rules against items. What approach would be best?

1. Having the method return a enumerated result? RenameResult.Success, RenameResult.TooShort, RenameResult.TooLong, RenameResult.InvalidCharacters, etc.

2. Using an event in a controller class to report to the UI class? The UI calls the controller's RenameItem method, and then handles an AfterRename event that the controller raises and that has rename status as part of the event args?

3. The controlling class directly references and calls a method from the UI class that handles the error, e.g. ReportError(string text).

4. Something else... ?

Essentially, I want to know how to perform complex validation in classes that may not be the Form class itself, and pass the errors back to the Form class for display -- but I do not want to involve exception handling where it should not be used (even though it seems much easier!)

---

Based on responses to the question, I feel that I'll have to state the problem in terms that are more concrete:

UI = User Interface, BLL = Business Logic Layer (in this case, just a different class)

1. User enters value within UI.
2. UI reports value to BLL.
3. BLL performs routine validation of the value.
4. BLL discovers rule violation.
5. BLL returns rule violation to UI.
6. UI recieves return from BLL and reports error to user.

Since it is routine for a user to enter invalid values, exceptions should not be used. What is the right way to do this without exceptions?

回答1:

The example you give is of UI validating inputs.

Therefore, a good approach is to separate the validation from the action. WinForms has a built in validation system, but in principle, it works along these lines:

ValidationResult v = ValidateName(string newName);
if (v == ValidationResult.NameOk)
    SetName(newName);
else
    ReportErrorAndAskUserToRetry(...);

In addition, you can apply the validation in the SetName method to ensure that the validity has been checked:

public void SetName(string newName)
{
    if (ValidateName(newName) != ValidationResult.NameOk)
        throw new InvalidOperationException("name has not been correctly validated");

    name = newName;
}

(Note that this may not be the best approach for performance, but in the situation of applying a simple validation check to a UI input, it is unlikely that validating twice will be of any significance. Alternatively, the above check could be done purely as a debug-only assert check to catch any attempt by programmers to call the method without first validating the input. Once you know that all callers are abiding by their contract, there is often no need for a release runtime check at all)

To quote another answer:

Either a member fulfills its contract or it throws an exception. Period.

The thing that this misses out is: What is the contract? It is perfectly reasonable to state in the "contract" that a method returns a status value. e.g. File.Exists() returns a status code, not an exception, because that is its contract.

However, your example is different. In it, you actually do two separate actions: validation and storing. If SetName can either return a status code or set the name, it is trying to do two tasks in one, which means that the caller never knows which behaviour it will exhibit, and has to have special case handling for those cases. However, if you split SetName into separate Validate and Store steps, then the contract for StoreName can be that you pass in valid inputs (as passed by ValidateName), and it throws an exception if this contract is not met. Because each method then does one thing and one thing only, the contract is very clear, and it is obvious when an exception should be thrown.

回答2:

I assume that you are creating your own business rules validation engine, since you haven't mentioned the one you're using.

I would use exceptions, but I would not throw them. You will obviously need to be accumulating the state of the evaluation somewhere - to record the fact that a particular rule failed, I would store an Exception instance describing the failure. This is because:

1. Exceptions are serializable
2. Exceptions always have a Message property that is human-readable, and can have additional properties to record details of the exception in machine-readable form.
3. Some of the business rules failures may in fact have been signaled by exceptions - a FormatException, for instance. You could catch that exception and add it to the list.

In fact, this month's MSDN Magazine has an article that mentions the new AggregateException class in .NET 4.0, which is meant to be a collection of exceptions that occurred in a particular context.

---

Since you're using Windows Forms, you should use the built-in mechanisms for validation: the Validating event and the ErrorProvider component.

回答3:

I think you've gotten the wrong impression of the intended message. Here's a great quote I ran across yesterday from the current edition of Visual Studio magazine (Vol 19, No 8).

Either a member fulfills its contract or it throws an excetion. Period. No middle ground. No return codes, no sometimes it works, sometimes it doesn't.

Exceptions should be used with care as they are expensive to create and throw--but they are, however, the .NET framework's way of notifying a client (by that I mean any calling component) of an error.

回答4:

I agree part of Henk's suggestion.

Traditionally, "pass/fail" operations were implemented as functions with an integer or bool return type that would specifiy the result of the call. However, some are opposed to this, stating that "A function or method should either perform an action or return a value, but not both." In otherwords, a class memeber that returns a value should not also be a class memeber that changes the state of the object.

I've found the best solution to add a .HasErrors/.IsValid and an .Errors property within the class that generates the errors. The first two properties allow the client class to test wether or not errors exist, and if need be, can also read the .Errors property and report one or all of the errors contained. Each method then must be aware of these properties and manage the error state appropriately. These properties can then be rolled into an IErrorReporting interface that various business rule layer facade classes can incorporate.

回答5:

In my opinion, if in doubt, throw exceptions on business rule validation. I know this is somewhat counter-intuitive and I may get flamed for this, but I stand by it because what is routine and what is not depends on the situation, and is a business decision, not a programming one.

For example, if you have business domain classes that are used by a WPF app and a WCF service, invalid input of a field may be routine in a WPF app, but it would be disastrous when the domain objects are used in a WCF situation where you are handling service requests from another application.

I thought long and hard, and came up with this solution. Do the following to the domain classes:

• Add a property: ThrowsOnBusinessRule. Default should be true to throw exceptions. Set it to false if you don't want to throw it.
• Add a private Dictionary collection to store exceptions with key as domain property that has business rule violation. (Of course, you can expose this publicly if you want)
• Add a method: ThrowsBusinessRule(string propertyName, Exception e) to handle tho logic above
• If you want, you can implement IDataErrorInfo and use the Dictionary collection. Implementation of IDataErrorInfo is trivial given the setup above.

回答6:

Exceptions are just that: a way to handle exceptional scenarios, scenarios which should not ordinarily happen in your application. Both examples provided are reasonable examples of how to use exceptions correctly. In both instances they are identifying that an action has been invoked which should not be allowed to take place and is exceptional to the normal flow of the application.

The misinterpretation is that the second error, the renaming method, is the only mechanism to detect the renaming error. Exceptions should never be used as a mechanism for passing messages to a user interface. In this case, you would have some logic checking the name specified for the rename is valid somewhere in your UI validation. This validation would make it so that the exception would never be part of normal flow.

Exceptions are there to stop "bad things" happening, they act as last-line-of-defence to your API calls to ensure that errors are stopped and that only legal behaviour can take place. They are programmer-level errors and should only ever indicate one of the following has occured:

• Something catastrophic and systemic has occurred, such as running out of memory.
• A programmer has gone and programmed something wrong, be it a bad call to a method or a piece of illegal SQL.

They are not supposed to be the only line of defence against user error. Users require vastly more feedback and care than an exception will ever offer, and will routinely attempt to do things which are outside of the expected flow of your applications.