Different Generics T in the same collection [duplicate]

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.

Answer 1

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());