Open vs. Closed Generic Types in .NET: What's the Difference?

Open generic types vs. closed generic types in .NET: What’s the difference?

In .NET, a generic type refers to a class, interface, or delegate that defines type parameters, such as List<T> or Dictionary<TKey, TValue>. An open generic type is a generic type in which one or more type parameters are not specified. A closed generic type is a generic type whose type parameters are specified.

Example

Consider the following generic types:

• T: Open generic type
• List<T>: Open generic type
• Dictionary<string, T>: Open generic type
• List<int>: closed generic type
• Dictionary<string, int>: closed generic type

Unbound generic type

An unbound generic type is a special case of an open generic type in which all type parameters are unspecified. These types can only be used in certain contexts, such as reflection. Examples of unbound generic types include List and Dictionary.

Related concepts

The term "open generic type" is not commonly used. Instead, you may encounter the following terms:

• Type parameters: Placeholder for a specific type.
• Generic type definition: Unbound generic type.
• Constructed type: Bound generic type.

Note: It is important to understand that open generic types cannot be instantiated directly. Type parameters must first be specified to create a closed generic type.

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