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

Detailed explanation of open generic types in .NET

In the context of ASP.NET MVC controller actions, one restriction is that "open generic types" cannot be used. This article delves into the concept of open and closed generic types in .NET.

What is an open generic type?

An open generic type in .NET is a type that contains type parameters, which are essentially placeholders for unspecified types. These types can be type parameters or generic types defined without specifying type parameters. For example, itself is an open generic type, as are List and Dictionary.

Closed generic type

Contrary to open generic types, closed generic types are types that do not contain type parameters. They are fully specified using concrete types as parameters. For example, List and Dictionary are closed generic types.

Open generic types and unbound generic types

While an open generic type contains type parameters, an unbound generic type is a generic type with unspecified type parameters. Unbound types cannot be used directly in expressions, nor can they be instantiated or called. They represent a generic definition before binding to a specific type.

Clarification of distinction

Consider the following code snippet:

<code class="language-c#">class Program {
   static void Main() { Test<int>(); }
   static void Test<T>() {
      Console.WriteLine(typeof(List<T>)); // 打印类型名称
   }
}</code>

When this code is executed, "System.Collections.Generic.List`1[System.Int32]" will be printed, which represents a bound open type because the type parameter is known at runtime: System.Int32.

Binding and type operations

Unbound generic types can be bound at runtime using the Type.MakeGenericType method. For example:

<code class="language-c#">Type unboundGenericList = typeof(List<>);
Type listOfInt = unboundGenericList.MakeGenericType(typeof(int));
if (listOfInt == typeof(List<int>))
     Console.WriteLine("构造了一个 List<int> 类型。");</code>

Conclusion

Understanding the difference between open, closed and unbound generic types is critical to using generics effectively in .NET. By leveraging these concepts, you can create flexible and reusable code that handles different data types efficiently.

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