Open vs. Closed Constructed Types in Generics: What's the Difference?

In-depth understanding of open and closed constructed types in generics

In generic programming, we sometimes encounter the terms "open" and "closed". They are used to describe the characteristics of a constructed type, clarifying its accessibility and limitations.

Understanding of constructed types

A constructed type is a generic type with one or more clearly defined type parameters. For example, the following is a constructed type inherited from the Dictionary base class:

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

The difference between open and closed construction types

The difference between open and closed constructed types is whether they contain open types in their type parameters:

• Open constructed type: A constructed type that contains at least one open type as a type parameter. An open type can be a type parameter or an array type with an open element type.
• Closed Construction Type: Any construction type that does not meet the criteria for an open construction type.

Example

Consider the NameDictionary type mentioned earlier. The type parameter T is an open type because it represents a generic type parameter. Therefore, NameDictionary is an open constructed type.

Practical meaning

The terms open and closed construction types have primarily a theoretical meaning. In everyday coding practice, their distinction rarely poses challenges or practical problems. However, a basic understanding of these concepts can help you gain a better grasp of generic types.

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