C# child class returning a unknown type (futher: from parent)

Question

i have interface:

public interface Inx<T>
{
   T Set(Data data);
}

simple class with this metod

public class Base
{
   ??? Set(Data data) { ... }
}

and parent class like that:

public class Parent : Base, Inx<Parent>
{
   ...
}

i want to return Parent type from Set metod in child class It's possible ? I need it do to something like that:

list.Add(new Parent().Set(data));

Now i have to do that:

T t = new T();
t.Set(data);
list.Add(t);

And its little annoying, i have to use it many time

Answer 1

The only way you could do that would be to make the Base generic (Base<T>) and have Set return T - then have Parent : Base<Parent>. The problem is... how would Set know how to create the T? You could have the where T : new() clause...

A useful aside here is that you can move the interface implementation into Base<T>:

public class Base<T> : Inx<T> where T : new()
{
   public T Set(Data data) {
       T t = new T();
       ///
       return t;
   }
}
public class Parent : Base<Parent> { }

Answer 2

REPLY:

well, all methods from this base class can read all fields in parent class, if i make class

Parent : Child
{
   int aa;
   int bb;
}

and use

Child.Set() i this method can modify/use aa,bb value, and i can get parent type:

Type t = this.GetType();
foreach (FieldInfo f in t.GetFields(BindingFlags.NonPublic | BindingFlags.Instance))

but propably You have right, i can't know this type before compilation :\ so i should use then: class Parent : Base<Parent>, Inx<Parent> ... is no other solution ?

thx for help anyway :)

Answer 3

Sorry for spaming well hmm i can use something like that:

this.GetType().GetConstructor(new System.Type[] { typeof(Data) }).Invoke(new object[] { data })

so maybe good soluction is return a object from this method ;\ ?

well generic class with generic interface seem's be big memory waste ... beacose function of this class are same only return type is diffrent

Answer 4

Is this what you want?

interface Inx<T> { 
    T Set(Data data);
}
public class Base
{
    public virtual T Set<T>(Data data)
    {
        T t = default(T);
        return t;
    }
}
public class Parent : Base, Inx<Parent>
{
    public Parent Set(Data data)
    {
        return base.Set<Parent>(data);
    }
}
class Program
{
    static void Main(string[] args)
    {
        var data = new Data();
        var list = new List<Parent>();
        list.Add(new Parent().Set<Parent>(data));
        // or 
        list.Add(new Parent().Set(data));
    }
}

EDIT: It's better to move the interface implementation up to the Base class as Marc said:

interface Inx<T> { 
    T Set(Data data);
}
public class Base<T> : Inx<T>
{
    public virtual T Set(Data data)
    {
        T t = default(T);
        return t;
    }
}
public class Parent : Base<Parent>
{        
}
class Program
{
    static void Main(string[] args)
    {
        var data = new Data();
        var list = new List<Parent>();
        list.Add(new Parent().Set(data));            
    }
}

Answer 5

well if i use generic base Class i can't cast parent class to one type, i don't know why i tried use generic there ... i found solution now :)

thx for help

interface Inx
{
   object Set(Data data);
}
class Base
{
   object Set(Data data)
   {
      // ...
      return (object) this;
   }
}
class Parent: Base, Inx
{
   // i don't have too write here Set (im using from base one)
}
class ManageParent<T> where T: Inx, new()
{
   // ...
      list.Add((T) new T().Set(data));
   // ...
}