What's the Difference Between Open and Closed Generic Types in .NET?

Detailed explanation of open generic types in .NET

In .NET, an "open generic type" refers to a type that contains type parameters that have not yet been specified. These type parameters act as placeholders for specific types, allowing a generic type to represent a range of related types.

Closed generic type

Contrary to open generic types, closed generic types specify type parameters for all type parameters. This means that a specific type has been assigned to the placeholder, and the generic type can now be used to instantiate objects of the specific type.

The difference between open generic types and unbound generic types

Open generic types are often confused with unbound generic types. An unbound generic type is a generic type that has unspecified type parameters and cannot be used or instantiated in expressions. Although both open generic types and unbound generic types involve type parameters, open generic types can be bound to a specific type, while unbound generic types cannot.

Examples of open and closed generic types

Consider the following example:

• List<T> is an open generic type, where T is a type parameter that can be bound to any specific type.
• List<int> is a closed generic type where int has been specified as a type parameter of T.

When to use open generic types

Open generic types are useful when you want to define a type that can handle any type of data. For example, List<T> can store a list of any type of object, allowing you to create common data structures and algorithms.

Restrictions on open generic types

In ASP.NET MVC, open generic types cannot be used as action methods because they may introduce potential performance and security issues. By requiring enclosing generic types, ASP.NET MVC ensures that type parameters are known at compile time, allowing for better optimization and improved security.

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