Error communication and recovery approaches in .NE

Question

I am trying to do error communication and recovery in my C# code without using Exceptions. To give an example, suppose there is a Func A, which can be called by Func B or Func C or other functions. Func A has to be designed keeping reuse in mind. (This application has an evolving library where new features will keep getting added over a period of time)

If Func A is not able to do what it is supposed to do, it returns an int, where any non-zero value indicates failure. I also want to communicate the reason for failure. The caller function can use this information in multiple ways:

1. It can show the error message to the user,
2. It may display its own error message more relevant to its context
3. It may itself return an int value indicating failure to further ancestor caller functions.
4. It may try to recover from the error, using some intelligent algorithm.

Hypothetically, any function on which other functions depend, may need to communicate multiple things to its caller function to take appropriate action, including status code, error message, and other variables indicating the state of data. Returning everything as a delimited string may not allow the caller function to retrieve the information without parsing the string (which will lead to its own problems and is not recommended).

The only other way is to return an object containing member variables for all required information. This may lead to too many 'state' objects, as each function will need to have its state object.

I want to understand how this requirement can be designed in the most elegant way. Note that at the time of coding, Func A may not know whether the caller function will have the intelligence to recover from the error or not, so I do not want to throw exceptions. Also, I want to see whether such a design is possible (and elegant at the same time) without using exceptions.

If the only way is to communicate using data objects for each function, then is it the way professional libraries are written. Can there be a generic data object? Note new functions may be added in future, which may have different state variables, or supporting information about their errors.

Also note that since the function's return value is a 'state' object, the actual data what it is supposed to return may need to be passed as a ref or out parameter.

Is there a design pattern for this?

I have read the following articles before posting this question:

http://blogs.msdn.com/b/ricom/archive/2003/12/19/44697.aspx

Do try/catch blocks hurt performance when exceptions are not thrown?

Error Handling without Exceptions

I have read many other articles also, which suggest not to use exceptions for code flow control, and for errors which are recoverable. Also, throwing exceptions have their own cost. Moreover, if the caller function wants to recover from exception thrown by each of the called functions, it will have to surround each function call with a try catch block, as a generic try catch block will not allow to 'continue' from the next line of the error line.

EDIT:

A few specific questions: I need to write an application which will synchronize 2 different databases: one is a proprietory database, and the other is a SQL Server database. I want to encapsulate reusable functions in a separate layer.

The functionality is like this: The proprietory application can have many databases. Some information from each of these databases needs to be pushed to a single common SQL Server database. The proprietory application's databases can be read only when the application's GUI is open and it can be read only through XML.

The algorithm is like this:

1. Read List of Open databases in Proprietory Application
2. For each database, start Sync process.
3. Check whether the user currently logged in, in this database has the Sync Permission. (Note: each database may be opened using a different user id).
4. Read data from this database.
5. Transfer data to SQL Server
6. Proceed to next database.

While developing this application, I will be writing several reusable functions, like ReadUserPermission, ReadListOfDatabases, etc.

In this case, if ReadUserPermission finds that the permission does not exist, the caller should log this and proceed to next open database. If ReadListOfDatabases is not able to establish a connection with the Proprietory Application, the caller should automatically start the application, etc.

So which error conditions should be communicated should exceptions and which using return codes?

Note the reusable functions may be used in other projects, where the caller may have different error recovery requirements or capabilities, so that has to be kept in mind.

EDIT:

For all those advocating exceptions, I ask them: If Func A calls Func B,C,D,E,F,G and Func B throws an exception on some error condition, but Func A can recover from this error and will like to continue rest of execution i.e. call Func B,C,D,..., how does exception handling allow to do this 'elegantly'? The only solution will be to wrap calls to each of B,C,D,... within a try catch block, so that remaining statements get executed.

Please also read these 2 comments:

https://stackoverflow.com/a/1279137/1113579

https://stackoverflow.com/a/1272547/1113579

Note I am not averse to using exceptions, if error recovery and remaining code execution can be achieved elegantly and without impacting performance. Also, slight performance impact is not a concern, but I prefer the design should be scalable and elegant.

EDIT:

Ok, Based on "Zdeslav Vojkovic" comments', I am now thinking about using exceptions.

If I were to use exceptions, can you give some use case when not to use exception, but use return codes? Note: I am talking about return codes, not the data which function is supposed to return. Is there any use case of using return codes to indicate success / failure, or no use case? That will help me understand better.

One use case of exceptions what I have understood from "Zdeslav Vojkovic" is when the callee function wants to compulsorily notify caller function of some condition and interrupt the caller execution. In the absence of exception, the caller may or may not choose to examine the return codes. But in case of exceptions, the caller function must necessarily handle the exception, if it wants to continue execution.

EDIT:

I had another interesting idea. Any callee function which wants to support the idea of caller function recovering from its error can raise event, and check the event data after the event has been handled, and then decide to throw or not to throw exception. Error codes will not be used at all. Exceptions will be used for unrecovered errors. Basically when a callee function is unable to do what its contract says, it asks for "help" in the form of any available event handlers. Still if it is not able to perform the contract, it throws an exception. The advantage is that the added overhead of throwing exceptions is reduced, and exceptions are thrown only when the callee function or any of its caller functions are not able to recover from the error.

Suppose if the caller function does not want to handle the error, but rather the caller's caller function wants to handle the error, the custom event dispatcher will ensure that event handlers are called in the reverse order of event registration, i.e. the most recently registered event handler should be called prior to other registered event handlers, and if this event handler is able to resolve the error, the subsequent event handlers are not at all called. On the other hand, if the most recent event handler can not resolve the error, the event chain will propagate to the next handler.

Please give feedback on this approach.

Answer 1

"is it the way professional libraries are written?"

No, professional libraries are written by using exceptions for error handling - I am not sure if there is a pattern for using your suggested approach, but I consider it an anti-pattern (in .NET). After all, .NET itself is a professional framework and it uses exceptions. Besides, .NET developers are used to exceptions. Do you think that your library is really that special to force the users to learn completely different way of error handling?

What you just did is reinvent the COM error handling. If that is what you want to do then check this and ISupportErrorInfo interface for some ideas.

Why do you want to do this? I bet it is a performance 'optimization'.

Fear of performance issues with regard to exception handling is almost always a premature optimization. You will create an awkward API where each return value must be handled via ref/out parameters and which will hurt every user of your lib, just to solve the problem which likely doesn't exist at all.

"Func A may not know whether the caller function will have the intelligence to recover from the error or not, so I do not want to throw exceptions"

So you want to ensure that caller silently allows FuncA to mess up the system invariants and caller just goes on happily? It will just make it much harder to debug seemingly impossible bug which happens in another function later on due to this.

There are scenarios where it makes sense to avoid exceptions, but there should be a good justification for that. Exceptions are good idea.

EDIT: I see that you have added that you "have read many other articles also, which suggest not to use exceptions for code flow control". That is correct, exceptions in .NET are not for code flow but for error handling.

You ask:

If Func A calls Func B,C,D,E,F and it has to encapsulate each call with try catch because it can recover from error or it will still like to execute remaining function calls, then is not so many try catch statements awkward

not more than alternative. You are making a mistake that you can simple handle all errors returned from functions in a same way but you usually can't.

Consider if you need to handle every function separately - worst case scenario and code is usually not written like that:

Result x, y;
try {
   x = Function1();
}
catch(SomeException e) {
   // handle error   
}
try {
   y = Function2();
}
catch(SomeOtherException e) {
   // handle error   
}

against:

int error;
Result x, y;
error = Function1(out x);
if(error != SOME_KNOWN_ISSUE) {
   // handle error    
}
error = Function2(out y);
if(error != SOME_KNOWN_ISSUE) {
   // handle error
}

not a big difference. please don't tell me that you would not check the error code. However, if you decide to ignore all errors (a horrible idea) then exceptions are simpler:

try {
    var x = Function1();
    var y = Function2();    
    var z = Function3();
}
catch Exception() { you still can see the message here and possibly rethrow }

vs

Result1 r1;
Function1(out r1);
Result2 r2;
Function2(out r2);
Result3 r3;
Function3(out r3);
// and here you still don't know whether there was an error

Can you elaborate what do you mean by "I need predictability with regard to time constraints"?

in some system level software or realtime stuff, you can't afford stack unwinding related to exception handling, as you can't guarantee the duration, and that could violate your timing requirements. But this is never the case in .NET as garbage collection is far worse in this regard.

Also, when you say "In .NET I would always use the exceptions for error handling", can you explain how or what do you define as an error condition? Is a recoverable situation an error condition or not an error condition? –

@shambulater already gave a great example in comments. In FileStream, missing file is not recoverable and it will throw. In the client of FileStream it might be recoverable or not depending on context. Some clients will ignore it, some will exit the app, some will wrap it in another exception and let someone upstream to decide.

When will you not use exceptions?

In those cases where I would also not return an error code.

Answer 2

How about a common FunctionResult object that you use as an out param on all your methods that you don't want to throw exceptions in?

public class FuncResultInfo
    {
        public bool ExecutionSuccess { get; set; }
        public string ErrorCode { get; set; }
        public ErrorEnum Error { get; set; }
        public string CustomErrorMessage { get; set; }

        public FuncResultInfo()
        {
            this.ExecutionSuccess = true;
        }

        public enum ErrorEnum
        {
            ErrorFoo,
            ErrorBar,
        }
    }

    public static class Factory
    {
        public static int GetNewestItemId(out FuncResultInfo funcResInfo)
        {
            var i = 0;
            funcResInfo = new FuncResultInfo();

            if (true) // whatever you are doing to decide if the function fails
            {
                funcResInfo.Error = FuncResultInfo.ErrorEnum.ErrorFoo;
                funcResInfo.ErrorCode = "234";
                funcResInfo.CustomErrorMessage = "Er mah gawds, it done blewed up!";
            }
            else
            {
                i = 5; // whatever.
            }

            return i;
        }
    }

Make sure all of your functions that can fail without exceptions have that out param for FuncResultInfo

Answer 3

I use the FunctionResult approach extensively in ms-access and it works wonderfully. I consider it far better than error handling. For a start, each error message is application specific and is not the usually off target default error message. If the error propagates up a call list of functions, the error messages can be daisy chained together. This eventual error message looks like a call stack but is cleaner e.g. [Could not read photos from Drive F:, Could not read files, Drive not ready]. Wacko, I have just discovered that some Drives can be mounted but not ready. I could not have unit tested for that error as I didn't know that such an error could occur (means SD card reader is empty). Yet even without prior knowledge of this error, I could write an application that handled it gracefully.

My method is to call a method in a class that is written as a function that returns a boolean value. The return value is set to True in the last line of the function so if the function is exited before the last line, it is by default unsuccessful. I code, calling the function looks like if getphotos(folderID) then...do something .. Else report error. Inside the class module is a module level error variable (Str mEM) and it is read via a getter, so the class has an .em property which holds the error message. I also have a comment variable which is sometimes used like an error message, for example if the folder is empty, the code that looked for photos worked but did not return any photos. That would not be an error but it is something that I might want to communicate to the calling program. If there was an error, the user would get an error message and the calling procedure would exit. In contrast, if there was a cmt, such as 'no photos', then I might skill trying to read the photo metadata for example. How does Zdeslav Vojkovic handle subtlies like that with exceptions?

I am moving to C# hence finding this thread. I like the certainty of knowing why function calls failed (I interact with databases and filing systems all the time so I struggle to cover my projects with Unit Tests). I do agree with Zdeslav Vojkovic about using exceptions where their used is standard, but will not be be doing so in my own code. I am looking for a clean design pattern that allows me to validate parameters within the called function and to inform the caller if the parameters were not right.