Why Can't ASP.NET MVC Controller Actions Use Open Generic Types?

Unraveling the mystery of open generic types in .NET

In the ASP.NET MVC field, the key condition for a method to become a controller operation is that "open generic types" cannot be used. To get into the nitty-gritty of this concept and clear up any associated confusion, let’s explore open and closed generic types.

Open generic type

In .NET, an open generic type is defined as a type that contains type parameters or constitutes a generic type formulated with unspecified type parameters. Essentially, it represents a type, leaving room for later customization or instantiation with a specific type. Examples of open generic types include T, List, Dictionary.

Closed generic type

Counterintuitively, a closed generic type is just a non-open type. It represents a type that has been fully instantiated with specific type parameters, effectively "closing" the type definition. Examples of closed generic types include List and Dictionary.

Common misunderstandings

The term "open generic type" is not commonly used compared to related concepts, which often leads to confusion. Some interchangeable or similar terms include:

• Unbound generic type: A generic type that does not specify a type parameter, making it unavailable in expressions outside typeof().
• Generic type definition: A blueprint for a generic type that does not specify type parameters.

Elucidate open types and unbound types

Although open types and unbound types have similarities, they are different. Open types (e.g. List) have bound type parameters at runtime, while unbound types (e.g. List) remain unspecified and require further binding.

In summary, open generic types are a powerful tool in .NET that provide flexibility and extensibility to your code. Understanding the difference between open and closed generic types, and how they relate to unbound types and generic type definitions, is critical to mastering .NET programming.

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