Inheritance and Multiple Constructors

Question

I have a question regarding inheritance, so I will describe the scenario below:

I am reading a text file containing logs. (One log per line) Each log-line will have the following format: "Date Type Description"

However, depending on the "Type" of log, I will have to parse the "Description" differently and pull out different fields.

Here are some examples:

5/1/2011 Information Field1, Field2, Field3
5/2/2011 Error       Field1

-- So, what I tried to do was this:
-Get a line out of the log
-Parse it according to the pattern "Date Type Description"
-Look at the "Type" field, and create new objects/parse description as necessary

public class Log
{
   public DateTime Date;
   public String Type;
   public String Description;

   public Log(String line)
   {
      this.Date = GetDate();
      this.Type = GetType();
      this.Description = GetDescription();
   }
}

public class InformationLog : Log
{
   public String Field1;
   public String Field2;
   public String Field3;

   public InformationLog(Log log)
   {
      this.Field1 = GetField1(log.Description);
      this.Field1 = GetField2(log.Description);
      this.Field1 = GetField3(log.Description);
   }
}

public class Client
{
   public void Main()
   {
       String line = ReadFileAndGetLine();  // Get a line from the file
       Log log = new Log(line);
       if(log.Type == "Information")
          log = new InformationLog(log);    // Is this right?
   }
}

This works how I want it to, but it seems like this cannot be a good practice. The "log" variable is using itself as a parameter to its own constructor.

My question is: Is there a standard way of doing this? Or, is there anything wrong with this implemenation?

--
Edit:
Also, I should mention: My reasoning was that I would parse the line once to get out the date and type, and then parse it again to get the finer details.
I decided to use inheritance so I wouldn't have to parse out the Date and Type fields twice.

@Mr. Disappointment : Each type of log has different parameters, so I didn't want to put "Field1, Field2, Field3, etc." objects in "Log" if they weren't necessary. — Eric
– Eric, Commented Mar 1, 2011 at 14:24
Appreciated, but if they always contain Type and Description then this can still take place in the base class. — Grant Thomas
– Grant Thomas, Commented Mar 1, 2011 at 14:27
You can't do it nice because you each new log extension will provide self owned fields. In your example you haven't an ability to get FieldX value because "log" is base type. — Viacheslav Smityukh
– Viacheslav Smityukh, Commented Mar 1, 2011 at 14:30
Have you tried log4net? Its a great 3rd party tool for logging. — Ritch Melton
– Ritch Melton, Commented Mar 1, 2011 at 14:30

Stecya · Accepted Answer · 2011-03-01 14:25:08Z

4

Try to use Factory pattern

static class LogFactory
{
    public static Log Create(String line)
    {
        if(GetType(line) == "Information")
           return CreateInformationLog(line);
        return CreateLog(line);
    }

    private static Log CreateLog(String line)
    {
       return new Log(line);
    }

    private static Log CreateInformationLog(String line)
    {
       return new InformationLog(line);
    }
}

And then try to use

   String line = ReadFileAndGetLine();  // Get a line from the file
   Log log = LogFactory.Create(line);

answered Mar 1, 2011 at 14:25

Stecya

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2 Comments

Eric Over a year ago

Thanks. Is there a way to preserve the fields "Date" and "Type" without having to call "GetType(line)" twice? It seems like the constructor for InformationLog(line) would have to call GetType(line) again.

Stecya Over a year ago

you can then extend constructor to Log(String line,string type)

Grant Thomas · Accepted Answer · 2011-03-01 14:42:16Z

2

As per my comment, why not just do something a little like this:

    public enum LogEntryType
    {
        Error = -1,
        Information = 0,
    }

    public class LogEntry
    {
        public string Raw;
        public DateTime Date;
        public LogEntryType Type;
        public string Description;

        public LogEntry(String line)
        {
            Raw = line;
            Date = ParseDate();
            Type = ParseType();
            Description = ParseDescription();
        }

        public string ParseDescription()
        {
           var result = string.Empty;
           switch(Type)
           {
               case LogEntryType.Error:
                   //parse here
                   break;
               case LogEntryType.Information:
                   //parse here
                   break;
           }
           return result;
        }
    }

I notice you have fields in the derivative class, but the description could be parsed here; though, I can see why people may want to shift it to the place that actually knows how the description should be parsed, in which case you could use a factory pattern suggested in another answer, or implement a 'property bag' type scenario - but drifting away from strong typing is generally frowned upon these days, I reckon.

Another suggestion, though very similar to your initial attempt, tends to encapsulate management of the types, as opposed to having a detached class handle such stuff - a pattern a little (superficially) like Exception where you have a root entry and inner entries:

    public enum LogEntryType
    {
        Error = -1,
        Information = 0,
    }

    public class LogEntry
    {
        public string Raw;
        public DateTime Date;
        public LogEntryType Type;
        public string Description;

        public InnerLogEntry InnerEntry;

        public LogEntry(String line)
        {
            Raw = line;
            Date = ParseDate();
            Type = ParseType();
            //parse the 'raw' description...
            Description = ParseDescription();
            //determine the inner entry type...
            switch (Type)
            {
                case LogEntryType.Error:
                    InnerEntry = new ErrorLogEntry(this);
                    break;
                case LogEntryType.Information:
                    InnerEntry = new InformationLogEntry(this);
                    break;
            }                
        }
    }

    public abstract class InnerLogEntry
    {
        protected LogEntry Parent;

        public InnerLogEntry(LogEntry logEntry)
        {
            Parent = logEntry;
        }
    }

    public class InformationLogEntry : InnerLogEntry
    {
        public InformationLogEntry(LogEntry logEntry)
            : base(logEntry)
        {
            //parse custom data
        }
    }

    public class ErrorLogEntry : InnerLogEntry
    {
        public ErrorLogEntry(LogEntry logEntry)
            : base(logEntry)
        {
            //parse custom data
        }
    }

edited Mar 1, 2011 at 14:42

answered Mar 1, 2011 at 14:26

Grant Thomas

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3 Comments

Eric Over a year ago

Thanks, however, I can't see how one would go about storing the "Field1, Field2, etc." parameters? --"Description" may contain 3 strings, or it may have a DateTime and a Double, etc.

Eric Over a year ago

I like your second attempt much better. Actually looks like an Abstract Factory, to me. I think this is more of the direction I want to go in. Thanks.

Grant Thomas Over a year ago

No problem, I think it is more than suitable for certain cases - particularly here if it suits you. You could also have the InnerLogEntry derive from LogEntry so that every entry could have a basic set of data, and/or their own children, where necessary - this will take more tweaking, but just a thought.

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