Re: [ADVANCED-DOTNET] Partially constructed objects in C#

[Stefan Holdermans]

Richard,

>Actually the field initializer of log will run before the base 
>class ctor [...]

Well, you're right about that offcourse. I was making the example to simple.
I should have let the BamBam constructor take the log string as a parameter,
in order to show the danger, shouldn't I?

  public class BamBam : Fred {
    private string log;

    public BamBam(string log) { this.log = log; }

    public override void LogMessage(string msg) {
    // DANGER: Fred's constructor calls this method before
    // BamBam's constructor assigned to the field log
      if (msg != null) {
        this.log += "\n";
        if this.log += msg;
      }
    }
  }

Regards,

Stefan


>-----Original Message-----
>From: [EMAIL PROTECTED] 
>[mailto:[EMAIL PROTECTED] On Behalf 
>Of Richard Blewett
>Sent: Friday, June 20, 2003 11:31 AM
>To: 'Moderated discussion of advanced .NET topics.'
>Subject: RE: [ADVANCED-DOTNET] Partially constructed objects in C#
>
>
>Actually the field initializer of log will run before the base 
>class ctor - I guess precisely because of this situation (well 
>this is true in C# but not in VB.NET). 
>
>I can see the point of not disallowing the behaviour as its 
>quite permissible for me to seal a class so I might have 
>inheritance in my implementation that results in a sealed 
>class at the end - and in this class I specialize the construction.
>
>Personally I wouldn't do it as I think it is a serious 
>maintenance issue in any application architecture - we end up 
>with behaviour that is not
>*obvious* and obvious visible behaviour is always easier to maintain.
>
>Regards
>
>Richard Blewett
>DevelopMentor
>
>-----Original Message-----
>From: Moderated discussion of advanced .NET topics. 
>[mailto:[EMAIL PROTECTED] On Behalf Of 
>Stefan Holdermans
>Sent: 19 June 2003 21:42
>To: [EMAIL PROTECTED]
>Subject: Re: [ADVANCED-DOTNET] Partially constructed objects in C#
>
>Robert,
>
>>This is a bad example, but let's say we had the following
>>code-- why wouldn't this be a valid use of a virtual method in 
>>a constructor?
>
>Because as you publish your Fred class in a library, users of 
>your library can provide a class BamBam as follows:
>
>  public class BamBam : Fred {
>    private string log = "log:";
>
>    public override void LogMessage(string msg) {
>    // DANGER: Fred's constructor call this method before
>    // BamBam's field log is initialized
>      if (msg != null) {
>        this.log += "\n";
>        if this.log += msg;
>      }
>    }
>  }
>
>The comment says it all.
>
>(Maybe I'm wrong, but isn't this where this discussion is 
>about? Can someone please confirm? Either Rob or myself is 
>missing the point completely.)
>
>Regards,
>
>Stefan
>
>
>>-----Original Message-----
>>From: [EMAIL PROTECTED]
>>[mailto:[EMAIL PROTECTED] On Behalf 
>>Of Streno, Robert
>>Sent: Thursday, June 19, 2003 9:30 PM
>>To: 'Moderated discussion of advanced .NET topics.'
>>Subject: RE: [ADVANCED-DOTNET] Partially constructed objects in C#
>>
>>
>>I'm not sure I see the harm in providing a virtual function to
>>be called from my constructor if it provides for "optional" 
>>constructor functionality.
>>
>>This is a bad example, but let's say we had the following
>>code-- why wouldn't this be a valid use of a virtual method in 
>>a constructor?
>>
>>public class Fred
>>{
>>        public Fred()
>>        {
>>                LogMessage("Constructor");
>>      }
>>
>>      protected virtual void LogMessage(string strMessage)
>>      {
>>                System.Console.WriteLine(strMessage);
>>        }
>>}
>>
>>public class Pebbles : Fred
>>{
>>        //Override LogMessage function to write to debug
>>instead of Console
>>        public override void LogMessage(string strMessage)
>>        {
>>                System.Environment.Debug.WriteLine(strMessage);
>>        }
>>}
>>
>>
>>-----Original Message-----
>>From: Stefan Holdermans [mailto:[EMAIL PROTECTED]
>>Sent: Thursday, June 19, 2003 3:11 PM
>>To: [EMAIL PROTECTED]
>>Subject: Re: [ADVANCED-DOTNET] Partially constructed objects in C#
>>
>>
>>John,
>>
>>>You missed his point.
>>
>>Then, exactly, what is his point?
>>
>>>So break the init into critical and non-critical, with the
>>non-critical
>>>being virtal.
>>
>>That won't do the trick though. Calling the virtual method
>>from the constructor is not reliable. That's why FxCorp issues 
>>a warning. The method call being critical or non-critical to 
>>construction does not matter here.
>>
>>I think, as a rule, it's best not to call virtual methods from
>>constructors. Hence, the FxCorp rule. If you disagree, please 
>>let me know why... Maybe it is me missing the point.
>>
>>Furthermore, if you want initialization to be customizable,
>>you should introduce some idiom for it. Perhaps a Initialize 
>>method this is *not* called from the constructor. (This leaves 
>>it up to the client to call it---and I can tell why you're not 
>>happy with that, but the life of a programmer is not always a 
>>walk in park, you know... Just think of it as the 
>>construction/initialization complement of Dispose. Maybe you 
>>should define an IDispoable-like interface for it: 
>>IInitializable or something...)
>>
>>Cheers,
>>
>>Stefan
>>
>
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