What's the Difference Between Open and Closed Constructed Generic Types?

In-depth understanding of open and closed constructed generic types

Generics in programming languages ​​allow defining types that can handle various data types at runtime. Generic types exist in two forms: open constructed types and closed constructed types. This article aims to clarify the differences between these two types.

Closed construction type:

A closed constructed type is a generic type in which all type parameters are specified as concrete types. In other words, a closed constructed type is a fully instantiated generic type. For example:

<code>Dictionary<string, int> myDictionary = new Dictionary<string, int>();</code>

In this example, Dictionary<string, int> is a closed constructed type. Dictionary is a generic type with two type parameters: TKey and TValue. However, in this case, TKey has been designated as string and TValue has been designated as int.

Open construction type:

An open constructed type is a generic type for which one or more type parameters have not been specified. In other words, an open constructed type is a partially instantiated generic type. For example:

<code>Dictionary<TKey, TValue> myDictionary2 = new Dictionary<TKey, TValue>();</code>

In this example, Dictionary<TKey, TValue> is an open constructed type. Dictionary is a generic type with two type parameters, but TKey and TValue are unspecified. Therefore, Dictionary<TKey, TValue> can accept any type of TKey and TValue.

Importance:

The difference between open and closed construction types may seem technical, but can be important in certain situations, such as the following:

• Code Universality: Closed constructed types are more specific and less general than open constructed types.
• Type safety: Closed constructed types ensure compile-time type safety, while open constructed types may cause runtime errors if the specified types are incompatible.
• Performance: Due to the lack of specificity of open construction types, their performance may not be as good as that of closed construction types.

In practice, however, the distinction between open and closed constructed types is usually not critical, and most programmers can work effectively without worrying about it explicitly.

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