Program Flow via Custom Exception

Question

I have a process that is parsing an XML file.

This is occuring in the PAckage Class.

The Package class has a Delegate that sets the object to an invalid state and captures the detailed info on error that occured the Package Class

For simplicity I am showing the filitem being passed to the package..

i.e `

foreach( var package in Packages)
{
try
{

    package.ProcessXml(fileitem.nextfile);

}
catch (CustomeErrorException ex)
{
    Logger.LogError(ex)
}
}

Inside The Package my validations look something like this

var Album = xml.Descendants()
    .Select(albumShards => new Album {
      Label = (string)albumShards.Descendants(TempAlbum.LabelLoc).FirstOrDefault() == "" ?
FailPackage("Error on label Load",Componets.Package,SubComp.BuildAlbum ) :  (string)albumShards.Descendants(TempAlbum.LabelLoc).FirstOrDefault()

On this validation I check to see if "" is returned for label... if so Call Failpackage with error info and create exception

 protected override void FailPackage(string msg, LogItem logItem)
         {
             Valid = ProcessState.Bad;
             Logger.LogError(msg,logItem);
             throw CustomErrorException(msg, Logitem);

         }

that is captured via the containing try catch block

My concern is that I am using Exceptions for Program flow ... how else should I look at approaching this problem or is this a valid Pattern.

It should not be ...only on error – HoopSnake Dec 19 '11 at 01:11 — HoopSnake, Dec 19 '11 at 01:11

score 2 · Accepted Answer · answered Dec 19 '11 at 01:21

2

ProcessXml fails to do what its name implies in certain cases; these are error cases. Exceptions are for error handling, despite what the name implies.

One of the biggest misconceptions about exceptions is that they are for “exceptional conditions.” The reality is that they are for communicating error conditions.

Krzysztof Cwalina, Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries

In other words, you're in the right. :)

Read the chapter on exceptions in the above book for some excellent guidelines.

answered Dec 19 '11 at 01:21

TrueWill

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I think that sometimes the semantics of "exceptional conditions" versus "errors" is played up a bit too much. Those of us who use the term "exceptional conditions" usually are just cautioning against using Exceptions for every tiny, common error occurrence that can be easily and quickly handled without throwing an exception. For instance, prompting the user instead of relying on an exception to prevent user-supplied input from causing a divide-by-zero. – Andrew Barber Dec 19 '11 at 01:26
@AndrewBarber - fair point. This is also covered in the book. :) – TrueWill Dec 19 '11 at 01:27

score 0 · Answer 2 · answered Dec 24 '11 at 04:06

You can put the error along with the ProcessState:

foreach( var package in Packages)
{
    package.ProcessXml(fileitem.nextfile);
    if(!package.Valid)
        Logger.LogError(package.Error)
}



var albumShards = xml.Descendants().FirstOrDefault();
if((string)albumShards.Descendants(TempAlbum.LabelLoc).FirstOrDefault() == "")
    return FailPackage("Error on label Load",Componets.Package,SubComp.BuildAlbum );

album = (string)albumShards.Descendants(TempAlbum.LabelLoc);


 protected override void FailPackage(string msg, LogItem logItem)
 {
     Valid = ProcessState.Bad;
     Logger.LogError(msg,logItem);
     Error = msg;
 }

Also, you are already logging in your FailPackage method, you can just add the other logging and fail silently never throwing that exception. — Ivo, Dec 24 '11 at 04:07

Program Flow via Custom Exception

2 Answers2