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This issue has been on/off bugging me and I've written 4 wrappers now. I'm certain I'm doing it wrong and need to figure out where my understanding is branching off.

Let's say I'm using the Rectangle class to represent bars of metal. (this works better then animals)

So say the base class is called "Bar".

private class Bar
{
   internal Rectangle Area;
}

So. Now we make a rectangle, say 300 units by 10 units.

private Bar CreateBar()
{
    Bar1 = new Bar1();
    Bar1.Area = new Rectangle(new Point(0,0), new Size(300,10));
    return Bar1;
}

Fantastic, so we have a base bar.

Now let's say we want to make this bar have a material - say steel. So. . .

private class SteelBar : Bar
{
    string Material;
}

So if I did this. . .

private SteelBar CreateSteelBar()
{
    SteelBar SteelB = new SteelB();
    Bar B = CreateBar();
    SteelB = B;
    SteelB.Material = "Steel";
    return SteelB;
}

From what I get from this if I call CreateSteelBar, it creates a steelbar that calls CreateBar. So I end up with a steel bar with a 300 by 10 rectangle, and a nulled or empty string for material. Then I set the material to steel.

When I try something similar in my program, it keeps telling I cannot implicitly create a higher up class from a lower down class. I would have figured this is why inheritance exists considering all the inherits from animal examples I see, but am hoping someone can clear me up.

Also, I'm certain I could call SteelBar = CreateBar(); but I did it the long way here.

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  • 3
    It's not clear to me why you aren't using constructors. That would simplify your problem considerably. Commented Jun 13, 2012 at 17:10
  • You don't return value in CreateBar() Commented Jun 13, 2012 at 17:11
  • It would in this case - in my system I'm creating an extension for something that has anywhere from 1 to 500 items inside of it. Commented Jun 13, 2012 at 17:11
  • 1
    SteelBar = new SteelBar(); would be a syntax error, what is the name of the variable you are trying to declare here Commented Jun 13, 2012 at 17:12
  • 2
    A steelbar is a bar, that's all you know. You do not know that a bar is a steelbar. A bar could be a wetbar, for all you know. Similarly, a dog is an animal. But an animal may or may not be a dog. It could be a cat. Your method that returns an animal could return anything, trying to assign that to a dog isn't going to work. Trying to assign a bar to a steelbar is the same thing. The point is steelbar is implicitly convertible to bar, not the other way around. You essentially seem to have the wrong idea of how inheritance functions. Commented Jun 13, 2012 at 17:13

8 Answers 8

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16

Instead of having a method (CreateBar), you'd use a constructor:

public Bar()
{
    this.Area = new Rectangle(new Point(0,0), new Size(300,10));
}

Then, in SteelBar, you'd call the base class constructor:

public SteelBar()
{
    this.Material = "Steel";
}

The base class constructor will occur first, so your area will already be setup. You can be explicit about this, though, and show it in your code:

public SteelBar()
    : base()
{
    this.Material = "Steel";
}

However, this is typically only used if you want to have a specific constructor that takes arguments.

For details, see Constructors in the C# programming guide.

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6 Comments

This is in fact how you would achieve the desired result, as stated in the question
@KyleTrauberman That's untrue - the base class constructor is always called implicitly. You only need to specify it if you want to call a constructor other than the default, parameterless constructor. There is no way to construct a subclass without calling one of the base class constructors.
Well this is a massive joy kill. So what you're telling me is I can't create a base class, use it, and then extend it when required without recreating the base class? I'm just going to stop using extensions in this way and just start plopping references inside the class then.
@Charles You can't assign a base class to a subclass - no. The other way around is fine, of course. Typically, you just create it as the most derived class, and use it - since the derived class can always be used as the base class... (See: en.wikipedia.org/wiki/Liskov_substitution_principle)
@Charles Yeah - you can't "extend" a class that's already created. Composition or decoration works well in this case. See en.wikipedia.org/wiki/Decorator_pattern for one idea.
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Here's your problem:

Bar B = CreateBar();
SteelB = B;

B is a bar of any kind. You are trying to assign it to a SteelBar. That's wrong, it could be a bar of any kind (copper, wood, Tiki, ...). You can do this:

SteelBar sb = new SteelBar();
Bar b = sb;

Because a SteelBar is a bar of some kind, but the reverse is not true..

Also, I'm certain I could call SteelBar = CreateBar()

You're certain of something that is false. Try it; CreateBar returns an instance of a bar of any kind, not a SteelBar.

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1

As pycruft mentioned you cannot assign an object of a basetype to a subtype-variable. I don't understand why you add a SteelBar anyway, as you have a Material property too. This looks redundant to me.

To achieve what you're trying to do, you have to create a new factory method that builds a SteelBar from the very beginning. It may return it as a Bar anyway. This would be an implementation of the Factory Pattern.

However, some more information about what you're trying to achieve might help.

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1

it keeps telling I cannot implicitly create a higher up class from a lower down class

The best example I use for this is Fruit and apple. Inheritance is an "is-a" relationship. An apple is-a Fruit but a Fruit is not an apple as it can be an orange, banana ...

Back to your example, a SteelBar is a Bar but a Bar is not a SteelBar as it can be a WoodenBar and that explains why you can not cast the super class to the child class

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0

You can't do this : 

SteelBar = B;

You've created a new object of type Bar and you're trying to assign it to a reference of type SteelBar.

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0

Surely this would be easier to achieve with abstractions such as Interfaces ... e.g. IMeasureable and IPhysical and not Base Classes...

I would use a BaseClass only in the GUI Representation for easy of displaying them and I would force the base class to implement the required interfaces.

You could optionally also use a DisplayObject which is sealed and takes an IMeasurable or IPhysical...

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I'm pretty sure your problem is you're trying to upcast:

Bar B = CreateBar();
SteelBar = B;

This isn't right because even though SteelBar MUST be of type Bar (because it inherited from) the same cannot be done backwards.

The opposite would be legal:

SteelBar B = CreateBar();
Bar = (Bar)B;

Quite simply you are attempting to store a reference of SteelBar in Bar, whereas it could possibly be another bar type such as WoodenBar.

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0

Consider this example:

public class Super {
}

public class Sub : Super {
}

If you create object Super you cannot assign it to be a Sub because it is not a "Sub". OO programming allows you to "deal with" objects in terms of less derived classes but this simple fact still remains:

You have created a Super not a Sub.

The only way to create a Sub from a Super would be to create a new Sub and copy property values. A tool like AutoMapper works well to do this quickly otherwise you will end up with a bunch of lines of mundane brittle code.

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