Question

I have a set of classes (MyClass1, MyClass2, MyClass3) that implement an interface (IMyClass).

I am interested in creating a method to fill and return a list (Collection.Generics.Lists<>) of objects that have common interface MyClassX (List<IMyClass>). But how?

Each class has a constructor MyClassX which does something different. So I can not create a BaseClass and use a common constructor for all.

By interface I can not do:

public List<IMyClass> Fill(string[] ItemsPassedByParam, IMyClass InstanceOfMyClassX)
    List MyList = new List<IMyClass>();
    foreach (item in ItemsPassedByParam)
    {
        MyList.Add (new IMyClass (item)); //can't do new from an Interface!!
    }
}

Also I tried using generics type, TMyClass in the next example, but neither has worked for me (though I have not very deep notions about generic types):

public List<TMyClass> Fill(string[] ItemsPassedByParam, Type TypeOfMyClassX)
    List MyList = new List <TMyClass> ();
    foreach (item in ItemsPassedByParam)
    {
        MyList.Add (new TMyClass (item)); //can't do new from a Generic Type!!
    }
}

I tried also to externalize the code inside the Constructor of MyClassX in a method, but I can’t call the method since de object is not initialized (there is no new () ).

In these cases which is the solution?

Thanks in advance!

Answer 1

It sounds like you want the Fill method to create a List<T> from the passed in string values where T is a derivation of IMyClass.

What you need to do is pass a factory function into the Fill method which allows for the creation of IMyClass derived instances.

public List<IMyClass> Fill(
  string[] ItemsPassedByParam, 
  Func<string,IMyClass> createFunc)

  List MyList = new List<IMyClass>();
  foreach (item in ItemsPassedByParam) {
    MyList.Add(createFunc(item));        
  }
}

Then at the place you call Fill you can choose which derivation of IMyClass to use

string[] items = ...;
List<IMyClass> list1 = Fill(items, item => new MyClass1(item));
List<IMyClass> list2 = Fill(items, item => new MyClass2(item));