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Dealing With The C# 2.0 Genericity

Leverage generics for flexible code, the forthcoming .NET 2.0 Framework will introduce new important features

Genericity and Polymorphism
Arrays and stacks, which have been defined in the previous sections, are kinds of polymorphic data structures. Into a container object of type Stack<T> we can push objects whose type is not just T, but any subtype of T.

Does this mean that if B is a subtype of A, then SomeGenericType<B> is a subtype of SomeGenericType<A>? This is true for arrays: if a type Employee is a subtype of a type Person, then Employee[] is a subtype of Person[]. Thus, the following code is valid:

Person[] persons = new Person[10];
Employee[] employees = new Employee[5];
...
persons = employees;

The variable persons has static type Person[], so every item persons[j] has static type Person. However at run time, after executing the assignment persons = employees, the variable persons will dynamically have an object of type Employee[]. Now, if F is a virtual method, then the call persons[j].F() will invoke the F that was redefined in Employee.

CLR dynamically controls the fact that the variable persons holds an object of type Employee[]; then, although persons has been statically declared as Person[], any attempt to assign an object of type other than Employee, or a type derived from Employee, to an item of persons will produce an exception, i.e., the CLR knows that the entire array object holding in persons has the type Employee[].

The policy above can be applied for arrays because these are embedded in the framework and then the CLR has all the knowledge to check it. Unfortunately however, the same rule cannot be applied in C#2.0 to our custom generic types. If Employee is a subtype of Person, then Stack<Employee> is not a subtype of Stack<Person>. The following code produces a compiling error:

Stack<Person> sp = new Stack<Person>;
Stack<Employee> se = new Stack<Employee>;
...
sp = se;

This is a pessimistic, but safe, strong type checking. The CLR cannot guess the semantics of our custom generic definition. Suppose Student is also another subtype of Person. If the compiler would allow the assignment above, then if you do sp.Push(new Student());, you will be calling the Push method expecting an Employee, but passing a Student!

Constrained Genericity
Inheritance and genericity both support the task of defining types based on other types. With inheritance we can define a vertical hierarchy of types, and with genericity we can define a kind of "horizontal" family of types. This section illustrates how to combine inheritance with genericity by means of specifying whether a type is acceptable or not as the actual type to instantiate a certain generic class. Such capability, known as constrained genericity, was introduced in Eiffel language (see the second entry in the References section) and it is also included in C# 2.0.

In the Stack<T> example, it is possible to use any type as the actual type parameter for T because the implementation only does assignments and returns operations with objects of type T. Such operations can be done on objects of any type (value types or reference types). The compiler assumes those operations we can do with an object of type T to be general operations and the operations of the root System.Object type.

However, there are situations in which we can expect more specialized functionality from the type parameters. For example, suppose you have a Sort method in a generic class List<T> as shown in Listing 4. It is probable that in the implementation of Sort we need to do some comparison of objects of type T.

In this case the compiler needs to be sure that the actual type we use to instantiate the generic parameter T has the method CompareTo. Then Listing 4 will display a compilation error because the compiler cannot guarantee that a method CompareTo exists in any type T.

Nevertheless, in C# 2.0, an optional list of constraints can be supplied for each type parameter declared in a generic class. A constraint indicates a requirement that a type must fulfill in order to construct a generic type. Therefore, considering that the following interface exists:

interface IComparable
{
    int CompareTo(IComparable x);
}

we can write the generic class:

class List<T> where T:IComparable
{
    ...
}

Such generic definition tells the compiler that the actual type parameter used to instantiate List<T> must implement (or be a subtype of) IComparable. If the Person type implements IComparable, then the instantiation List<Person> myfamily; is correct.

Because there is no multiple inheritance in .NET, for a given type parameter you can specify any number of interfaces as constraints, but no more than one class.

Construction and Generic Parameters
There are situations in which objects of the same type might be created as the actual type used to instantiate the generic type. This could be done if it is expressed as a constraint that forces the parameter to have a constructor. The following generic definition compiles well:

class G<T> where T: new()
{
   ...
   public T SomeMethod()
{
    ...
    return new T();
}
}

The constraint where T: new() indicates that the actual type used to instantiate the generic class must have a default constructor. Then the statement return new T() and the following code excerpt:

class Person
{
   public Person(){...}
   ...
}
G<Person> gp;

will compile without errors, but

class Date
{
   public Date(int d, int m, int y){...}
   //No other constructors
   ...
}
G<Date> gd;

will display a compiling error because the type Date has no default constructor.

Unfortunately it is possible only to express construction constraints related to default constructors, i.e., constructors without parameters. It would be nice to express a constraint that requests a constructor signature. This would be a good reason to include constructor specifications in an interface. We hope this matter will be considered in the future.

The New Generic Types in the .NET 2.0 Library
.NET 2.0 is backward compatible, so it keeps all classes of the previous framework. Nevertheless, several types are now simplified and introducing their generic version.

More Stories By Miguel Katrib

Miguel Katrib is a PhD and a professor in the Computer Science Department at the University of Havana. He is also the head of the WEBOO group dedicated to Web and object-oriented technologies. Miguel is also a scientific advisor in .NET for the software enterprise CARE Technologies, Denia, Spain.

More Stories By Mario del Valle

Mario del Valle is working toward his MS at the Computer Science Department at the University of Havana, and is a software developer at the WEBOO group dedicated to Web and object-oriented technologies.

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Most Recent Comments
.NET News Desk 12/06/05 06:45:57 PM EST

The forthcoming .NET 2.0 Framework will introduce new important features. One of those features is genericity. Genericity is not really a new concept. It has been included in some previous languages as ADA, C++, Eiffel, and in the mathematical model of abstract data types (ADT). However, the C# 2.0 notation for genericity (see the first entry in the References section), the integration of genericity in the .NET type system, the efficient implementation of genericity in the CLR-JIT process, and the new generic features included in the reflection mechanism will strengthen .NET programmers' output.

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