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This chapter is from the book

This chapter is from the book

Generic Internals

Given the discussions in earlier chapters about the prevalence of objects within the CLI type system, it is no surprise that generics are also objects. In fact, the type parameter on a generic class becomes metadata that the runtime uses to build appropriate classes when needed. Generics, therefore, support inheritance, polymorphism, and encapsulation. With generics, you can define methods, properties, fields, classes, interfaces, and delegates.

To achieve this, generics require support from the underlying runtime. So, the addition of generics to the C# language is a feature of both the compiler and the platform. To avoid boxing, for example, the implementation of generics is different for value-based type parameters than for generics with reference type parameters.

Instantiating Generics Based on Value Types

When a generic type is first constructed with a value type as a type parameter, the runtime creates a specialized generic type with the supplied type parameter(s) placed appropriately in the CIL. Therefore, the runtime creates new specialized generic types for each new parameter value type.

For example, suppose some code declared a Stack constructed of integers, as shown in Listing 11.41.

Example 11.41. Stack<int> Definition

Stack<int> stack;

When using this type, Stack<int>, for the first time, the runtime generates a specialized version of the Stack class with int substituted for its type parameter. From then on, whenever the code uses a Stack<int>, the runtime reuses the generated specialized Stack<int> class. In Listing 11.42, you declare two instances of a Stack<int>, both using the code already generated by the runtime for a Stack<int>.

Example 11.42. Declaring Variables of Type Stack<T>

Stack<int> stackOne = new Stack<int>();
Stack<int> stackTwo = new Stack<int>();

If later in the code, you create another Stack with a different value type as its type parameter (such as a long or a user-defined struct), the runtime generates another version of the generic type. The benefit of specialized value type classes is better performance. Furthermore, the code is able to avoid conversions and boxing because each specialized generic class "natively" contains the value type.

Instantiating Generics Based on Reference Types

Generics work slightly differently for reference types. The first time a generic type is constructed with a reference type, the runtime creates a specialized generic type with object references substituted for type parameters in the CIL, not a specialized generic type based on the type parameter. Each subsequent time a constructed type is instantiated with a reference type parameter, the runtime reuses the previously generated version of the generic type, even if the reference type is different from the first reference type.

For example, suppose you have two reference types, a Customer class and an Order class, and you create an EntityDictionary of Customer types, like so:

 EntityDictionary<Guid, Customer> customers;

Prior to accessing this class, the runtime generates a specialized version of the EntityDictionary class that, instead of storing Customer as the specified data type, stores object references. Suppose the next line of code creates an EntityDictionary of another reference type, called Order:

EntityDictionary<Guid, Order> orders =
    new EntityDictionary<Guid, Order>();

Unlike value types, no new specialized version of the EntityDictionary class is created for the EntityDictionary that uses the Order type. Instead, an instance of the version of EntityDictionary that uses object references is instantiated and the orders variable is set to reference it.

To still gain the advantage of type safety, for each object reference substituted in place of the type parameter, an area of memory for an Order type is specifically allocated and the pointer is set to that memory reference. Suppose you then encountered a line of code to instantiate an EntityDictionary of a Customer type as follows:

 customers = new EntityDictionary<Guid, Customer>();

As with the previous use of the EntityDictionary class created with the Order type, another instance of the specialized EntityDictionary class (the one based on object references) is instantiated and the pointers contained therein are set to reference a Customer type specifically. This implementation of generics greatly reduces code bloat by reducing to one the number of specialized classes created by the compiler for generic classes of reference types.

Even though the runtime uses the same internal generic type definition when the type parameter on a generic reference type varies, this behavior is superseded if the type parameter is a value type. Dictionary<int, Customer>, Dictionary<Guid, Order>, and Dictionary<long, Order> will require new internal type definitions, for example.

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