Using a class nested in a generic class as type parameter in C#

Question

Having the following definition:

public class Generic<T>
{
    public class Nested { }
}

And given that ECMA ref §25.1 states:

Any class nested inside a generic class declaration or a generic struct declaration (§25.2) is itself a generic class declaration, since type parameters for the containing type shall be supplied to create a constructed type.

I understand that Nested requires the type parameter in order to be instantiated.

I can obtain the generic Type with typeof:

var type = typeof(Generic<>.Nested); 
// type.FullName: Namespace.Generic`1+Nested

Is there any way I can use it as a type parameter for a generic method such as the following?

var tmp = Enumerable.Empty<Generic<>.Nested>(); 
// Error: Unexpected use of an unbound generic

As stated before, from the ECMA specification I understand that all the type parameters must be satisfied before instancing but no objects are being created here. Furthermore, the Nested class does not make use of the type parameters in any way: I simply would like to define it nested for code organization purposes.

What would you do with an empty sequence of non-specified nested classes? I don't see any way around the limitation. The fact that Nested doesn't utilize T and that you're currently prototyping with an empty sequence do not appear to be relevant. For a fully constructible type for the sequence, you'll need to supply the type parameter. — Anthony Pegram
– Anthony Pegram, Commented Apr 14, 2016 at 8:30
@AnthonyPegram the example is poorly chosen, sorry. The actual motivation is defining message classes inside generic Akka.Net actors. Thus, I just want to define those nested classes where they belong, but they do not require type parameters in any way. — jnovo
– jnovo, Commented Apr 14, 2016 at 8:32
I assume you want to work with any instance (or list of) the nested class without regard to the containing outer class? I would suggest as an alternative to define an interface, have your nested class inherit from that interface, and then work via references (including lists) of that interface. Something like pastebin.com/xLQDKudb — Anthony Pegram
– Anthony Pegram, Commented Apr 14, 2016 at 8:37
Do you want to declare var tmp = Enumerable.Empty<Generic<>.Nested>(); outside of the parent class? Otherwise var tmp = Enumerable.Empty<Nested>(); should work. — Daniel van Heerden
– Daniel van Heerden, Commented Apr 14, 2016 at 8:39
Another approach is to create a builder function in your parent class that will build you an IEnumarable instance of your child class? — Daniel van Heerden
– Daniel van Heerden, Commented Apr 14, 2016 at 8:41

René Vogt · Accepted Answer · 2016-04-14 08:37:51Z

3

No, you can't do that. As you said

all the type parameters must be satisfied before instancing

and though no instance of Generic<>.Nested is actually generated

the compiler does not know the semantics of Empty (so doesn't know that no instance of Generic<>.Nested is created)
and the main problem: you do want to create an instance of IEnumerabe<Generic<>.Nested>, which would be a type with "unsatisfied type parameters", too

edited Apr 14, 2016 at 8:37

answered Apr 14, 2016 at 8:32

René Vogt

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Comments

Daniel van Heerden · Accepted Answer · 2016-04-14 09:19:50Z

0

I think your main goal will probably be achieved simply by putting the sub class next to the parent in the same namespace.

The namespace is really what should be organising the code in the way you are talking about in the comments under your post.

You should really only be defining a class as a sub class it only the parent class is going to be using it.

answered Apr 14, 2016 at 9:19

Daniel van Heerden

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