Inherit in generic classes C#

Question

My brain is gonna to explode. :) So I would like to get help from you. Please, think about my question like about just programmer puzzle. (Actually. perhaps it is very easy question for you, but not for me.)

It is needed to create array of objects. For example List where T is class. (I will describe Class T below). Also it is needed create “container” that will contain this array and some methods for work with this array. For example Add(), Remove(int IndexToRemove). Class T must have field "Container", this way each elements of our array would be able to know where is it contained and has access its container's fields and methods. Notice, that in this case Class T should have type parameter. Indeed, it is not known beforehand which container's type is used. Let us denote this class container as A and class element (class T) as AUnit.

Code:

class Program
{
  static void Main(string[] args)
  {
      A a = new A();
      a.Add();
      a.Units[0].SomeField +=100;    
      Console.ReadKey();
  }
}


class A
{
    public List<AUnit> Units;

    public A()//ctor
    {
        Units = new List<AUnit>();
    }
    public void Add()
    {
        this.Units.Add(new AUnit(this));
    }
}
class AUnit
{
    public int SomeField;
    public A Container;
    public string Name { get; private set; }
    public AUnit(A container)
    {
        this.SomeField = 43;
        this.Container = container;
        this.Name = "Default";
    }
}

Public fields should be protected or private of course, but let think about this later. You can ask “why we create public A Container field in AUnit”? We create field public string Name{get;private set;} (actually property but nevermind). And also we would like to be able to change value of this field for example method [Class AUnit] public bool Rename(string newName)();. The main idea of this method is changing Name field only that case if no one element in array (public List Units; ) has the same name like newName. But to achieve this, Rename method has to have access to all names that is currently used. And that is why we need Container field.

Code of extended version AUnit

class AUnit
{
    public int SomeField;
    public A Container;
    public string Name { get; private set; }
    public AUnit(A container)
    {
        this.SomeField = 43;
        this.Container = container;
        this.Name = "Default";
    }
    public bool Rename(String newName)
    {
        Boolean res = true;
        foreach (AUnit unt in this.Container.Units)
        {
            if (unt.Name == newName)
            {
                res = false;
                break;
            }
        }
        if (res) this.Name = String.Copy(newName);
        return res;
    }
}

Ok. If you still read it let's continue. Now we need to create Class B and class BUnit which will be very similar like Class A and Class Aunit. And finally the main question of this puzzle is HOW WE CAN DO IT? Of course, I can CopyPaste and bit modify A and AUnit and create this code.

class B
{
    public List<BUnit> Units; //Only Type Changing

    public B()//ctor Name changing...
    {
        Units = new List<BUnit>();//Only Type Changing
    }
    public void Add()
    {
        this.Units.Add(new BUnit(this));//Only Type Changing
    }
}
class BUnit
{
    public int SomeField;
    public B Container;//Only Type Changing
    public string Name { get; private set; }
    public A a; //NEW FIELD IS ADDED (just one)

    public BUnit(B container) //Ctor Name and arguments type changing 
    {
        this.SomeField = 43;
        this.Container = container;
        this.Name = "Default";

        this.a=new A(); //New ROW (just one)
    }
    public bool Rename(String newName)
    {
        Boolean res = true;
        foreach (BUnit unt in this.Container.Units) //Only Type Changing
        {
            if (unt.Name == newName)
            {
                res = false;
                break;
            }
        }
        if (res) this.Name = String.Copy(newName);
        return res;
    }
}

And I can to use this classes this way.

static void Main(string[] args)
{
    B b = new B();
    b.Add();
    b.Units[0].a.Add();
    b.Units[0].a.Units[0].SomeField += 100;
    bool res= b.Units[0].a.Units[0].Rename("1");
    res = b.Units[0].a.Units[0].Rename("1");

    Console.ReadKey();
}

This construction is can be used to create “non-homogeneous trees”.

Help, I need somebody help, just no anybody…. [The Beatles]

I created B and BUnit using CopyPaste. But how it can be done using “macro-definitions” or “Generic”, inherit or anything else in elegant style? (C# language) I think that there is no reason to describe all my unsuccessful attempts and subquestions. Already topic is too long. : )

Thanks a lot if you still read it and understand what I would like to ask.

InBetween · Accepted Answer · 2016-10-03 10:53:43Z

You need to implement a base type, lets call it UnitBase, with all common functionality. I'd structure your code the following way:

Create an interface for your container, this way you can change implementation to more performant solutions without modifying the elements you will be adding to the container.
```
public interface IContainer
{
    Q Add<Q>() where Q : UnitBase, new();
    IEnumerable<UnitBase> Units { get; }
}
```

Following the idea stated in 1, why not make the search logic belong to the container? It makes much more sense, as it will mostly depend on how the container is implemented:

public interface IContainer
{
    Q Add<Q>() where Q : UnitBase, new();
    IEnumerable<UnitBase> Units { get; }
    bool Contains(string name);
}

A specific implementation of IContainer could be the following:

public class Container : IContainer
{
    public Container()
    {
        list = new List<UnitBase>();
    }

    private List<UnitBase> list;

    public Q Add<Q>() where Q: UnitBase, new()
    {
        var newItem = Activator.CreateInstance<Q>();
        newItem.SetContainer(this);
        list.Add(newItem);
        return newItem;
    }

    public IEnumerable<UnitBase> Units => list.Select(i => i);
    public bool Contains(string name) =>
        Units.Any(unit => unit.Name == name);
}

Create a base class for your AUnit and BUnit types condensing all common functionality:

public abstract class UnitBase
{
    protected UnitBase()
    {
    }

    public IContainer Container { get; private set; }
    public int SomeField;
    public string Name { get; private set; }
    public void SetContainer(IContainer container)
    {
        Container = container;
    }

    public bool Rename(String newName)
    {
         if (Container.Contains(newName))
             return false;

        this.Name = newName; //No need to use String.Copy
        return true;
    }
}

Implement your concrete types:

public class BUnit : UnitBase
{
    public int SpecificBProperty { get; private set; }
    public BUnit()
    {
    }
}

Shortcomings of this approach? Well, the container must be of type <UnitBase>, I've removed the generic type because it really wasn't doing much in this particular case as it would be invariant in the generic type.

Also, keep in mind that nothing in the type system avoids the following:

myContainer.Add<BUnit>();
myContainer.Add<AUnit>();

If having two different types in the same container is not an option then this whole set up kind of crumbles down. This issue was present in the previous solution too so its not something new, I simply forgot to point it out.

While I trying to understand and test your code, could you tell me to say: Can we avoid those problems with constructor that you describe if we would use parameterless construcor but add function SetContainer( some params) to Unit class and use it in public void Add(T item) { list.Add(item); item.SetContainer( some params) }. Also can we create item inside Add() function and use this function without params?
@JustOneQuestion Answering your first question: The problem is still the same. You want to have all common functionality in the base class UnitBase. This class knows nothing about what types might derive from it, so the Container will always be of type IContainer<UnitBase> which is invariant.
@JustOneQuestion Edited extensively the answer based on your second question and removed the generic type from IContainer<T> as it wasn't doing much in your particular scenario where T could only be UnitBase. Also notice that Add now returns the added element so you can get a reference to the newly created item. Also, the solution works if AUnit, BUnit etc. have a parameterless constructor in order to be able to create one inside Add.

Community · Accepted Answer · 2017-05-23 11:48:33Z

InBetween , I am very thankful to you for your advices. Actually I can't say that I understood your answer in full, but using your ideas I have done what I want.

Looks like my variant works well. However I would like to hear your (and everyone) opinions about code described below. The main goal of this structure is creating non-homogeneous trees. So could you estimate it from this side.

First of all. We need to create interfaces for both classes. We describe there all "cross-used" functions.

public interface IUnit<T>
{
    string Name { get;}
    void SetContainer(T t);
    bool Rename(String newName);   
}
public interface IContainer
{
   bool IsNameBusy(String newName);
   int Count { get; }
}

Next. Create Base for Unit Classes for future inheritance. We will use in this inheritors methods from Container Base so we need generic properties and IUnit interface.

 class UnitBase<T> : IUnit<T> where T : IContainer

Unfortunately I don't know yet how to solve the problem with Constructor parameters. That is why I use method

 SetContainer(T container).

Code:UnitBase

class UnitBase<T> : IUnit<T> where T : IContainer
{
    protected T Container;
    public string Name { get; private set; }
    public UnitBase()
    {
        this.Name = "Default";
    }
    public void SetContainer(T container)
    {
        this.Container = container;
    }
    public bool Rename(String newName)
    {
        bool res = Container.IsNameBusy(newName);
        if (!res) this.Name = String.Copy(newName);
        return !res;
    }
}

Next. Create ContainerBase

ContainerBase should:

1) has IContainer interface.

2)has information about what it will contain:

 ... where U : IUnit<C>, new()

3)and .... has information about what itself is. This information we need to pass as parameter to SetContainer() method.

Code ContainerBase:

class ContainerBase<U, C> : IContainer //U - Unit Class.  C-Container Class
    where U : IUnit<C>, new()
    where C : ContainerBase<U, C>
{
    protected List<U> Units;
    public U this[int index] { get { return Units[index]; } }
    public ContainerBase()//ctor
    {
        this.Units = new List<U>();
    }
    public void Add()
    {
        this.Units.Add(new U());
        this.Units.Last().SetContainer(((C)this));//may be a bit strange but actualy this will have the same type as <C>
    }
    public bool IsNameBusy(String newName)
    {
        bool res = false;
        foreach (var unt in this.Units)
        {
            if (unt.Name == newName)
            {
                res = true;
                break;
            }
        }
        return res;
    }
    public int Count { get { return this.Units.Count; } }
}

Cast ((TContainer)(this)) may be is a bit strange. But using ContainerBase we always should use NewInheritorContainer. So this cast is just do nothing…looks like...

Finally. This classes can be used like in this example.

class SheetContainer : ContainerBase<SheetUnit,SheetContainer> {public SheetContainer(){}}

class SheetUnit : UnitBase<SheetContainer>
{
    public CellContainer Cells;
    public PictureContainer Pictures;
    public SheetUnit()
    {
        this.Cells = new CellContainer();
        this.Pictures = new PictureContainer();
    }
}

class CellContainer : ContainerBase<CellUnit, CellContainer> { public CellContainer() { } }

class CellUnit : UnitBase<CellContainer>
{
    public string ValuePr;//Private Field
    private const string ValuePrDefault = "Default";  
    public string Value//Property for Value
    {
        //All below are Just For Example.
        get
        {
            return this.ValuePr;
        }
        set
        {
            if (String.IsNullOrEmpty(value))
            {
                this.ValuePr = ValuePrDefault;
            }
            else
            {
                this.ValuePr = String.Copy(value);
            }
        }
    }
    public CellUnit()
    {
        this.ValuePr = ValuePrDefault;
    }
}

class PictureContainer : ContainerBase<PictureUnit, PictureContainer> { public PictureContainer() { } }

class PictureUnit : UnitBase<PictureContainer>
{
    public int[,] Pixels{get;private set;}
    public PictureUnit()
    {
        this.Pixels=new int[,]{{10,20,30},{11,12,13}};
    }
    public int GetSizeX()
    {
        return this.Pixels.GetLength(1);
    }
    public int GetSizeY()
    {
        return this.Pixels.GetLength(0);
    }
    public bool LoadFromFile(string path)
    {
        return false;
    }
}
static void Main(string[] args)
    {
        SheetContainer Sheets = new SheetContainer();
        Sheets.Add();
        Sheets.Add();
        Sheets.Add();
        Sheets[0].Pictures.Add();
        Sheets[1].Cells.Add();
        Sheets[2].Pictures.Add();
        Sheets[2].Cells.Add();


        Sheets[2].Cells[0].Value = "FirstTest";
        bool res= Sheets[0].Rename("First");//res=true
        res=Sheets[2].Rename("First");//res =false

        int res2 = Sheets.Count;
        res2 = Sheets[2].Pictures[0].Pixels[1, 2];//13
        res2 = Sheets[2].Pictures.Count;//1
        res2 = Sheets[1].Pictures.Count;//0
        res2 = Sheets[0].Pictures[0].GetSizeX();//3
        Console.ReadKey();
    }

Looks like it works like I want. But I didn’t test it full. Let me say Thank you again, InBetween.

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