A generic list of generics

Question

I'm trying to store a list of generic objects in a generic list, but I'm having difficulty declaring it. My object looks like:

public class Field<T>
{
    public string Name { get; set; }
    public string Description { get; set; }
    public T Value { get; set; }

    /*
    ...
    */
}

I'd like to create a list of these. My problem is that each object in the list can have a separate type, so that the populated list could contain something like this:

{ Field<DateTime>, Field<int>, Field<double>, Field<DateTime> }

So how do I declare that?

List<Field<?>>

(I'd like to stay as typesafe as possible, so I don't want to use an ArrayList).

Answer 1

This is situation where it may benefit you to have an abstract base class (or interface) containing the non-generic bits:

public abstract class Field
{
    public string Name { get; set; }
    public string Description { get; set; }
}

public class Field<T> : Field
{    
    public T Value { get; set; }

    /*
    ...
    */
}

Then you can have a List<Field>. That expresses all the information you actually know about the list. You don't know the types of the fields' values, as they can vary from one field to another.

Answer 2

Perhaps implement an interface.

interface IField
{
}

class Field<T> : IField
{
}

...

List<IField> fields = new List<IField>() { new Field<int>(), new Field<double>() };

Answer 3

You can't declare a list of generic types without knowing the generic type at compile time.

You can declare a List<Field<int>> or a List<Field<double>>, but there is no other common base type for Field<int> and Field<double> than object. So the only List<T> that could hold different kinds of fields would be List<object>.

If you want a more specific type for the list, you would have to make the Field<T> class inherit a class or implement an interface. Then you can use that for the generic type in the list.