[ACCEPTED]-Generics open and closed constructed types-generics

Accepted answer

Score: 85

In practice the terminology doesn't really 15 matter much - I can't remember the last 14 time I had to worry about it except when trying 13 to write about it.

An unbound type has no type arguments specified
A constructed type has at least one type argument specified
A type parameter is an open type
An array type where the element type is open is an open type
An open constructed type has at least one type argument which is an open type
A closed type is any type which isn't open

(There are further rules 12 for nested types. Consult the C# 3.0 spec 11 section 4.4 for gory details.)

As an example 10 of an open constructed type, consider:

public class NameDictionary<T> : Dictionary<string, T>

The 9 base class of typeof(NameDictionary<>) is:

Constructed because it specifies type arguments
Open because the second type argument (T) is an open type

The MSDN docs for Type.IsGenericType have 8 quite a useful little table.

Just to reiterate, this 7 is almost entirely unimportant in day to 6 day use.

I'm generally in favour of knowing 5 the correct terminology - particularly for 4 things like "pass by reference" etc 3 - but in this case it really, really doesn't 2 come up very often. I would like to actively 1 discourage you from worrying about it :)

Score: 9

From MSDN:

A generic type or method is closed 7 if instantiable types have been substituted 6 for all its type parameters, including all 5 the type parameters of all enclosing types. You 4 can only create an instance of a generic 3 type if it is closed.

So this works as List<int> is 2 closed:

var list = Activator.CreateInstance(typeof(List<int>));

But this throws an exception at run-time 1 because List<> is open:

var list = Activator.CreateInstance(typeof(List<>));
                                               ↑

Score: 3

I have mostly used open generics (basically 8 uninstantiated generics) in dependency injection 7 mappings. For example, something like

Bind<IRepository<>>()
   .To<BasicRepository<>>()

Then, when 6 my object constructor contains:

public SomethingController(IRepository<Something>) { ... }

My dependency 5 injection mechanism will instantiate a BasicRepository< Something 4 > automagically. (This works with Ninject 3 and StructureMap, and probably the Castle 2 Windsor library; I'm not sure about other 1 frameworks).

Source: stackoverflow.com

[ACCEPTED]-Generics open and closed constructed types-generics

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