Different Generics T in the same collection

Question

public abstract Column<T>
{
   private T Value {get;set;}    

   public abstract string Format();

}

public class DateColumn : Column<DateTime>
{
   public override string Format()
   {
      return Value.ToString("dd-MMM-yyyy");
   }
}

public class NumberColumn : Column<decimal>
{
   public override string Format()
   {
      return Value.ToString();
   }
}

The problem I have is adding these into a generic collection. I know its possible but how can I store multiple types in a collection etc

IList<Column<?>> columns = new List<Column<?>()

I would really appreciate any advice on achieving this goal. The goal being having different column types stored in the same List. Its worth mentioning I am using NHibernate and the discriminator to load the appropriate object.Ultimately the Value needs to have the type of the class.

Many thanks for your help in advance.

I am confused about the point of this. Generics helps with type safety. Regardlesss, if you really wanted to do this, couldn't you populate your list with Column — Killnine, May 15 '12 at 18:48
There are [a few](http://stackoverflow.com/q/3215402/590790) [duplicates](http://stackoverflow.com/q/3777057/590790) of this question already. In addition to the 'base-type' solution [posted here by JaredPar](http://stackoverflow.com/a/10606974/590790), I would like to propose [following the adapter pattern to create a non-generic wrapper to this purpose](http://stackoverflow.com/a/34417767/590790), so that the Liskov Substitution Principle is not broken. — Steven Jeuris, Dec 25 '15 at 15:29

score 23 · Accepted Answer · answered May 15 '12 at 18:48

In order to be stored in a List<T> together the columns must have a common base type. The closest common base class of DateColumn and NumberColumn is object. Neither derives from Column<T> but instead a specific and different instantiation of Column<T>.

One solution here is to introduce a non-generic Column type which Column<T> derives from and store that in the List

public abstract class Column { 
  public abstract object ValueUntyped { get; }
}

public abstract class Column<T> : Column {
  public T Value { get; set; }
  public override object ValueUntyped { get { return Value; } }
}

...

IList<Column> list = new List<Column>();
list.Add(new DateColumn());
list.Add(new NumberColumn());

You may also consider interface instead of base class. – Jakub Konecki May 15 '12 at 19:07 — Jakub Konecki, May 15 '12 at 19:07
Boxing happens here. T might be value type :( – Cristian E. Apr 28 '19 at 12:05 — Cristian E., Apr 28 '19 at 12:05

score 3 · Answer 2 · answered May 15 '12 at 18:47

3

Generics are all about specifying type. If you want to use dynamic types, use classic ArrayList instead.

answered May 15 '12 at 18:47

Pol

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In this case I don't believe the OP wants dynamic typing but instead wants to take advantage of base types and polymorphic behavior – JaredPar May 15 '12 at 18:55
Yes you're correct @JaredPar I need to Parse the values and trying to guess what an object is is extremely difficult. – Jonathan May 15 '12 at 18:57
2

You're right, I +1ed your detailed answer. – Pol May 15 '12 at 18:57

score 3 · Answer 3 · answered May 15 '12 at 18:48

3

It probably makes sense to derive from a non-generic Column class that wraps up as much of the non-generic common interface of a column... then to declare your list as List<Column>.

answered May 15 '12 at 18:48

spender

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...and what @JaredPar said... – spender May 15 '12 at 18:51

Different Generics T in the same collection

3 Answers3

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