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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

.NET 1.1 is full of types that are replicated only by changing the types they are based on. Good examples are all delegates, which define the control's event handlers. You can find delegates such as

delegate void MouseEventHandler(object sender, MouseEventArgs args);

or

delegate void KeyEventHandler(object sender, KeyEventArgs args);

These delegates are only different in the type of the parameter args. Now instead of defining events as:

public event MouseEventHandler MouseUp;
public event KeyEventHandler KeyDown;

the following generic delegate instantiation can be used:

public event EventHandler<MouseEventArgs> MouseUp;
public event EventHandler<KeyEventArgs> MouseUp;

The following handling methods can be used to directly hook to the events defined above:

private void MyControl_MouseUp(object sender, MouseEventArgs args) {...}
private void MyControl_KeyDown(object sender, KeyEventArgs args) {...}

The collection hierarchy of the .NET 1.1 is based on object, thereby forcing the use of casting as it was previously explained. .NET 2.0 keeps these classes but also includes their generic versions, so it now has Stack<T>, List<T> (for ArrayList), Queue<T>, Dictionary<TKey, TValue>, and so on.

However, in .NET 1.1 there are types using specific collection classes that are not offered in their corresponding generic version. For instance, classes of the namespace System.Windows.Form have not being adapted to the new generic classes and they continue using the old delegates and specific classes as ControlsCollection or TabPagesCollections. These are only samples of how specific replicated types or "generic" types based on object will coexist with codes based on the new generic capabilities. The next section proposes a solution to do a seamless transition from non generic to generic and vice versa.

Between Generics and Non-Generics: A Seamless Transition
In contexts where types such as IList<T> have been used frequently, the following assignments could be very useful:

IList<T> controls = panel.Controls; 1
ControlsCollection controls = new List<Control>();

Unfortunately the aforementioned assignments are not allowed in C# 2.0. We can neither do the casting:

IList<Control> controls = (IList<Control>) panel.Controls;

nor the following:

ControlCollection controls = (ControlCollection) new List<Control>();

because there is not a type-subtype relationship between IList<Control> and ControlCollection types.

However, by using the classes of the namespaces Reflection and Reflection.Emit, we implemented the generic method:

class Converters
{
    ...
public static T TypeConvert<T>(object source);
}

With the method TypeConverter it is possible to emulate the casting by doing:

IList<Control> controls =
Converters.TypeConvert<IList<Control>> (panel.Controls);
ControlCollection controls =
Converters.TypeConvert<ControlCollection> (new List<Control>());

At run time, the method TypeConverter emits a proxy type that inherits from the actual type used to instantiate T. For each method in this latter type a proxy method is emitted in the proxy type. The proxy method calls the corresponding method through the source parameter.

For example, when executing the call (Controls is a property of type ControlsCollection):

Converters<IList<Control>>.TypeConvert(panel.Controls);

a type that is similar to the code excerpt shown in Listing 5 will be emitted.

Simulating Multiple Inheritance with Genericity
.NET lacks multiple inheritance. This section shows how to emulate multiple inheritance based on genericity and reflection.

Suppose a C class would like to inherit from both A and B classes. Listing 6 shows the classic simulation pattern of the multiple inheritance. Type C will inherit only from A and will delegate its B functionality in a private object of type B. The implicit operator allows the assignment B b = new C(); as if C were inherited also from B.

Now a similar pattern can be obtained by using genericity (Listing 7). The generic class inherits from A and uses its generic type to emulate the other base class. Then in order to obtain the effect of class C inheriting from A and B, C can simply be defined as follows:

public class C : A<B>{...}

Note that an assignment A a = new C(); is allowed because C inherits A<B> and A<B> inherits from A, and an assignment B b = new C(); is also allowed because of the implicit conversion operator that returns an object of the generic parameter (B in this example).

Overriding Methods
In a real multiple-inheritance scenario where a class C inherits from A and B, the following code should execute the F method of C (since F is a virtual method).

B b = new C();
b.F();

However, for a class C obtained with the pattern of Listing 7 the F method called is still the F of B.

To solve this problem, suppose that all virtual methods of B that should be polymorphic are collected in the interface

interface IBMethods
{
...
void F(...);
}

Now we could try to define the generic class A<T> as shown in Listing 8. Note that the constrained genericity forces the type used to instantiate the generic parameter to implement the interface IBMethods. Thus, B must implement IBMethods.

We use an inner class to obtain the polymorphic effect of the B methods. Then we have the code

class C: A<B>{...}
...
C c = new C(...);
B b = c;
b.F()

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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