docs.microsoft.com: "Because a generic type definition is only a template, you cannot create instances of a class, structure, or interface that is a generic type definition." so how come I can write this: Dictionary<int, string> d = new Dictionary<int, string>(); or it is not an instantiation? so what am I doing then?

CodePudding user response：

The docs are referring to the so called open generic types. I.e. you can't instantiate Dictionary<,>, you need to provide both generic type parameters, which results in so called closed (or constructed) generic type (i.e. Dictionary<int, string> in your case) which can be instantiated.

From Open and closed types docs:

All types can be classified as either open types or closed types. An open type is a type that involves type parameters. More specifically:

• A type parameter defines an open type.
• An array type is an open type if and only if its element type is an open type.
• A constructed type is an open type if and only if one or more of its type arguments is an open type. A constructed nested type is an open type if and only if one or more of its type arguments or the type arguments of its containing type(s) is an open type.

A closed type is a type that is not an open type.

CodePudding user response：

It is an instanciation. When you do Dictionary<int, string> d = new Dictionary<int, string>(), you instanicate Dictionary<TKey,TValue> with TKey : int and TValue : string.

What you cannot do is Dictionary d = new Dictionary() which should return the error you are describing.