OOP Inheriting from a .Net class

Question

3.50/5 (2 votes)

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O.k, it's late, I'm drawing a blank and am about to ask a stupid question.
OOP Basic Concepts.

I'm trying to inherit a .Net class called System.Printing.PrintSystemJobInfo
(From the .Net reference assemblies System.Printing.dll version 3.0.0.0)

Here is my non functioning code:

C#

namespace MyNameSpace
{
    class PrintSystemJobInfoCustom : System.Printing.PrintSystemJobInfo
    {
        protected PrintSystemJobInfoCustom(){}

        public DateTime TimeJobSubmittedCustom { get; set; }
    }
}

The error I'm getting with the constructor is :

The type 'System.Printing.PrintSystemJobInfo' has no constructors defined

But as you can see I did create a contructor, but obviously not correctly. When I remove this contructor, it gives me the same error, but at a class level.
I made the constructor protected because it gave me this error when it was public:

'System.Printing.PrintSystemJobInfo.PrintSystemJobInfo(System.Printing.PrintQueue)' is inaccessible due to its protection level

Is something in that class preventing inheritance, or am I overlooking something simple?

Posted 22-Aug-11 5:04am

MatthysDT

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3 solutions

Solution 1

You cannot inherit from a class with no public or protected constructors.
You get the first error because the compiler can't find a public default constructor of PrintSystemJobInfo.
And you get the second error because when you define no constructor the default constructor of the base is used which in this case is private.

May I ask why you want to inhirit from PrintSystemJobInfo?

Posted 22-Aug-11 5:10am

Simon Bang Terkildsen

Updated 22-Aug-11 5:11am

v2

Comments

TheyCallMeMrJames 22-Aug-11 11:14am

I'd like to know, too. What could you be doing by inheriting there?

MatthysDT 22-Aug-11 11:17am

Thank you.
I want to add my own TimeJobSubmitted field since the one I get is offset by 2 hours, I expect this is because it send me the GMT DateTime, I have no idea why. And I don't like hardcoding the GMT+2 for my timezone. And to add to this, at one stage, the DateTime actually came through as GMT+2 without me changing any code that should have affected it, so at this stage I do not trust the value of that field and would like to add my own.

If I can't inherit, I'll create a class that instantiates the PrintSystemJobInfo object, it's just more cumbersome to work with.

Manfred Rudolf Bihy 22-Aug-11 11:22am

Proposed as the solution! 5+

Solution 3

How about an extension method?

public DateTime GetTimeJobSubmittedCustom(this PrintSystemJobInfo target){
 return target.TimeJobSubmitted + get the time zone offset from settings;
}

You can get the current time zone from the locale settings somewhere.

Posted 22-Aug-11 5:58am

BobJanova

Comments

MatthysDT 22-Aug-11 12:03pm

Thank you.
Like I mentioned earlier, that I had an instance where that field was correct according to my timezone without my tampering, so at this stage I don't trust it enough to use it at all. Could be paranoia and there is probably a good explanation, but I'd rather populate and use a custom field.

[no name] 22-Aug-11 12:09pm

+5, Nice and simple. But I have an aversion to extension methods unless it is the only possible solution. They're not part of standard OOP.

BobJanova 22-Aug-11 14:09pm

In general I agree with you. But some people like them so I will still submit them as a solution where they are appropriate!

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Shameel · Accepted Answer · 2011-08-22T05:19:00

Solution 2

From MSDN, we see that the System.Printing.PrintSystemJobInfo does not have a public constructor (most probably it's private or internal), so you cannot inherit the class. You can get an instance of the class using the static Get() method.

However, if you must create an instance, you can try using the Decorator[^] pattern.

Posted 22-Aug-11 5:19am

Shameel

Comments

Manfred Rudolf Bihy 22-Aug-11 11:23am

Good idea! 5+

MatthysDT 22-Aug-11 11:24am

Thank you!
I'll check out and try implementing that pattern 1st thing tomorrow and get back to you ASAP!

BobJanova 22-Aug-11 11:57am

Decorator seems to require inheriting from the class you're 'decorating', too, so it won't work in this case.

[no name] 22-Aug-11 12:08pm

IMO, Decorator involves composition and not inheritance.