Can I have a base class where each derived class has its own copy of a static property?

Question

I have something like the following situation below:

class Base
{
     public static int x;
     public int myMethod()
     {
          x += 5;
          return x;
     }

}

class DerivedA : Base
{
}

class DerivedB : Base
{
}

I am trying to set this up so that each derived class has its own static instance of x, if I do something like this:

 DerivedA.x = 5;
 DerivedB.x = 10;

then when I run:

 DerivedA.myMethod(); //The result will be 10
 DerivedB.myMethod(); //The reusult will be 15

Can i do something like this? How can I setup the derived classes to achieve this? Thanks guys.

EDIT: Basically, I have a bunch of derived classes that each have a property unique to that class. It does not vary for each instance, and thus I believe should be a static variable. Also, that property is set by a method, that is the same for each of these classes. I am trying to avoid having to copy and paste the logic for that property and method in each of these derived classes. I thought it best to somehow move that logic to the base class from which each of these classes are derived from. But, I need each derived class to have its own copy of that property. I do not necessarily have to do it this way, and I will be happy to hear some better practices suggestions if you guys have any.. Thanks!

score 12 · Answer 1 · answered Feb 20 '11 at 21:35

12

Well, yes, you can, but it revolves around a bit of a trick with generics.

It is much better if you fix your design so that you don't need that static field, or at least not per descendant, but here goes:

class Base<TDescendant>
    where TDescendant : Base
{
     public static int x;
     public int myMethod()
     {
          x += 5;
          return x;
     }

}

class DerivedA : Base<DerivedA>
{
}

class DerivedB : Base<DerivedB>
{
}

This relies on the fact that a generic type with static fields will get a separate copy of those static fields for each type you invoke it with.

However, if you intend to descend from DerivedA or DerivedB, it gets tricky, so I wouldn't recommend going down this lane.

answered Feb 20 '11 at 21:35

Lasse V. Karlsen

380,855
102
628
825

I chose to implement this pattern for what I believe to be a legit pattern. I welcome feedback. My static property is one which contains a static list of token replacement mappings. Each email type has its own class, which inherits from a base class. The classes are used in the processing of emails - the email template has tokens in it which are replaced with actual values at run time. The tokens don't change, so I use a static property: private static Dictionary _TokenReplacementMappings = new Dictionary() {...}; – Vince Horst Apr 03 '14 at 17:44
I Prefer this method too. – Tejasvi Hegde Mar 13 '15 at 05:44
But for multi level inheritence, one need to mix and match with with techniques like this and the above one. – Tejasvi Hegde Mar 13 '15 at 05:55
Thanks for the insight. I had completely forgotten that generic types create a new copy of static fields. Like you said though, this can quickly get tricky if care is not taken. – Alex Essilfie Aug 20 '15 at 03:32

Femaref · Accepted Answer · 2011-02-20T22:17:07.490

5

You will need to redefine and hide the field and method in all derived types.

Example:

class DerivedA : Base
{
  public new static int x;
  public new int myMethod()
  {
    x += 5;
    return x;
  }
}

Note: don't do it this way. Fix your design.

Edit:

Actually, I have a similar construct. I solve it with an abstract (if you need a default value, use virtual) property which then gets used from the base class:

public abstract class Base
{
   public abstract string Name { get; }

   public void Refresh()
   {
     //do something with Name
   }
}

public class DerivedA
{
  public override string Name { get { return "Overview"; } }
}

You should be able to adjust that for your use case. You can of course make the property protected if only deriving classes should be able to see it.

edited Feb 20 '11 at 22:17

answered Feb 20 '11 at 21:31

Femaref

60,705
7
138
176

So, what is the best way to avoid repeating code in each of my derived classes? The method would be the same for each of my derived classes. – ntsue Feb 20 '11 at 21:37
1

Please edit your question to display what you want to achieve. We will be able to help you more that way. – Femaref Feb 20 '11 at 21:39
So, I took the code you gave and modified it by adding a private static variable to DerivedA... and I use the overridden property to get and set that static field... Is that ok? Or is that not a good way to do it? It seems to do what I want.. thanks!! – ntsue Feb 21 '11 at 14:41
Well, the point is to not use any static field at all - you should use just the property on its own. The point is that you don't have to add a static field everytime you derive but simply override the property and handle your case there. – Femaref Feb 21 '11 at 14:44
If I don't use a static field, and I just use the property.. wouldn't every instance of DerivedA have it's own copy of "Name"? I wanted the entire class to share a copy of "Name". Forgive me if I am not understanding correctly... Thanks! – ntsue Feb 21 '11 at 15:16
Yes it would, but what is the problem with that? Fact is, you can't require a class to implement a static field. However, you can require a class to implement a public or protected property. A small price to pay to have this desire to have the information required by code rather than by documentation. – Femaref Feb 21 '11 at 15:21

score 1 · Answer 3 · answered May 14 '16 at 19:29

Static dictionary with Type as keys should do. I mean avoid logic repetition and different values for each derived type but shared among instances.

public class Base
{
    private static Dictionatry<Type,int> _values;

    public int MyMethod()
    {
        _values[this.GetType()]+=5;
        return _values[this.GetType()];
    }
}

score 0 · Answer 4 · answered Feb 20 '11 at 21:58

I usually implement subclass specific stuff as an abstract get property

public class Base
{
    // You must pick one option below

    // if you have a default value in the base class
    public virtual int x { get { return 7; /* magic default value */} }

    // if you don't have a default value
    // if you choose this alternative you must also make the Base class abstract
    public abstract int x { get; }
}

public class DerivedA : Base
{
    public override int x { get { return 5; } }
}

public class DerivedB : Base
{
    public override int x { get { return 10; } }
}

The key (that is missing here) is that the property is `static`. — neizan, May 23 '17 at 20:11

Can I have a base class where each derived class has its own copy of a static property?

4 Answers4

Linked