What is good practice for abstracting generics?

Question

Consider the following scenario:

Public Class Condition(Of T)

    '...

End Class

Now, I create several instances of this class with various types. I now desire to populate a generic list with these instances. Something like:

Dim Conditions As New List(Of Condition....?)
Conditions.add(new Condition(Of String))
Conditions.add(new Condition(Of Double))
Conditions.add(new Condition(Of CustomClass))

What is the best way to do this? For now I am doing the following:

Public Class Condition(Of T)
    implements ICondition

    '...

End Class

And using it like so:

Dim Conditions As New List(Of ICondition)
Conditions.add(new Condition(Of String))
Conditions.add(new Condition(Of Double))
Conditions.add(new Condition(Of CustomClass))

However, ICondition is just totally empty. Which makes me think there is probably a better or more accepted way to go about things. I could also inherit from an empty base class, would that be better? What is the preferred method of doing this kind of thing?

I understand that so long as the list that contains the generics is of a broader type that I can insert conditions of any type. However I do not want to make it any broader than necessary - I only want to be able to insert objects that are instances of Public Class Condition(Of T), but whose types may vary.

Answer 1

The answer differs, depending upon the version of the .NET Framework that you're using. Look up articles about covariance and contravariance to understand the problem. Here's an example.

.NET 4 and 4.5 support this kind of usage.

Earlier versions of .NET would make you use something like the ICondition interface that you mention.