Partially applying generics for type constraints

Question

I currently try to construct a generic interface that every (generic) class deriving it will have a method accepting a delegate that accepts the type parameter and returns another class of the same type, with only another type parameter.

I tried the following:

public interface GenericInterface<out T, out SomeDerived> where SomeDerived<T> : GenericInterface<T, SomeDerived> { SomeDerived<NT> bind<NT>(bindee<T, NT, SomeDerived<NT>> bindFunc); } public delegate AnotherDerived<T2> bindee<in T1, out T2, out AnotherDerived>(T1 param) where AnotherDerived<T2> : GenericInterface<T2, AnotherDerived>; public class Derived<T> : GenericInterface<T, Derived> { Derived<NT> bind<NT>(bindee<T, NT, Derived<NT>> bindFunc); }

But it fails to compile and I get this error:

Invalid token '<' in class, struct, or interface member declaration

What is the correct design in such case?

EDIT:

I understand the syntatic reason for the compiler errors. You cannot apply a generic type argument a parameter in a where clause. I am asking what is the best way to mimic such behavior.

Can you show us that weird error - the complete and full error with all details, please. — marc_s
– marc_s, Commented Sep 14, 2013 at 12:20
well maybe weird is not the correct term... The compiler don't like having <> in the "where" specification — KanHar
– KanHar, Commented Sep 14, 2013 at 12:23
Invalid token '<' in class, struct, or interface member declaration; Invalid token '>' in class, struct, or interface member declaration; Invalid token '{' in class, struct, or interface member declaration; Syntax error, ':' expected — KanHar
– KanHar, Commented Sep 14, 2013 at 12:25
OK, now please also tell exactly WHERE in your code you're getting this error! — marc_s
– marc_s, Commented Sep 14, 2013 at 12:26
I agree with marc_s, questions must describe the specific problem — Sayse
– Sayse, Commented Sep 14, 2013 at 12:26

Meirion Hughes · Accepted Answer · 2013-09-14 13:08:31Z

I'll go out on a limb here and say what you're trying to do here with the Generic is impossible; I'll remove if someone thinks I'm wrong.

So lets start with this

interface IFoo<T> where T : IFoo<T>{} class Foo<T> : IFoo<T> where T : IFoo<T>{} class Bar<T> : Foo<T> where T : IFoo<T>{}

Lets try to instanciate this;

var foo = new Foo< Bar< ....errr what now? ad infinitum...

So to fix this, you need to redesign so you're classes looks more like this:

interface IBase {} interface IFoo<out T> where T : IBase { } class Foo<T> : IFoo<T> where T : IBase { }

which then allows:

IFoo<IBase> foo = new Foo<Base>();

[Addendum]

You can have function level generics that let you get around problems like these...

interface IFoo<out T> where T : IBase { IFoo<TBind> Bind<TBind>(Action<T, TBind> bindFunc) where TBind : IBase; }

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