c# - Generics open and closed constructed types

Question

Recently I noticed that generic constructed types can be open and closed. But I do not understand what they actually mean. Can you give a simple example?

Answer 1

In practice the terminology doesn't really matter much - I can't remember the last time I had to worry about it except when trying 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 for nested types. Consult the C# 3.0 spec section 4.4 for gory details.)

As an example of an open constructed type, consider:

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

The 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 quite a useful little table.

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

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