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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

What happens in the background? Note that when doing an assignment B b = c;, the implicit conversion operator returns the object t that is of type InnerT, which inherits from the generic T (B in this example). Now the call b.F() will really call the method F of the inner class (since that call at.F()was originally at the c, then the method F that is called is the F of C, which is what we want).

Unfortunately however, the code class InnerT: T {...} displays a compiling error because in C# 2.0 we cannot inherit from a generic type parameter.

The true subject of the attempt to make InnerT inherit from T was that the InnerT object returned by the implicit conversion operator would be of type T. A similar result could be obtained by using our TypeConvert. The code of the operator T in Listing 8 could do so with the following:

public static implicit operator T(A<T> at){
    return Converters.TypeConvert<T>(at.t);
}

Note that TypeConvert receives an object at of type A<T>. Actually the real parameter passed to at is of type A<T> because the assignment B b = c; c is of type C, but you must remember that C inherits from A<B>. Then our conversion method generates a proxy type to ascertain that the at.t (that is of type InnerT) will be of type B. Also, this proxy object will polymorphically perform as a C.

Reflecting and Emitting Generic Code
The use of generic patterns as shown in the previous section reveals some of the benefits of the inclusion of genericity. Nevertheless, a manual reproduction of this pattern could be tedious and error prone, especially for nonskilled programmers.

It is possible to automate this "generation of the generic class" by once again using the .NET reflection magic. In.NET 2.0 the TypeBuilder class has been enhanced with new types and methods to deal with genericity. For example with the method:

GenericParameterBuilder[] DefineGenericParameters(string[] names);

users can define the generic parameters for a specific generic type being constructed by a TypeBuilder. The GenericParameterBuilder type is a descendant of TypeBuilder. In addition, this new type offers the SetBaseTypeConstraints method to specify constraints for each generic type parameter.

Based on these new reflection features the following tool was developed that generates an assembly that contains a type defined by the generic pattern for multiple inheritance, as previously explained above.

class MultipleInheritance
{
   ...
   public static Type EmitGenericPattern(String assemblyName, Type mainBaseType, Type
   constrainType)
   {...}
}

To obtain a multiple inheritance of C from A and B we must call

MultipleInheritance.EmitGenericPattern("myGenerics",typeof(A), typeof(IBMethods));

to obtain the type A<T> explained above. Then we must add a reference of the generated assembly myGenerics.dll in our project and write class C: A<B> in our code. Objects of type C will act as "inheriting" from A and B and could be assigned to variables of those types, thereby preserving the polymorphic behaviour and the type conformance from heirs to parents.

A common scenario of requesting for multiple inheritance is not only to inherit from a main class, but also to inherit from other additional classes so as to reuse their implemented functionality. Frequently in such scenarios the main interest is not in redefining the methods of those additional classes, but just in using them. In such a case calling the tool in the way presented below is enough.

MultipleInheritance.EmitGenericPattern("myGenericDLL", typeof(A), null);

Extending this pattern to more than two base classes is a straightforward task for the reader's discretion.

Summary
Genericity in the .NET Framework 2.0 and its inclusion in .NET languages, particularly in C# 2.0, will be very welcome and will enhance developers' productivity. This article has introduced and illustrated some of the basic notions of genericity and has also proposed how one could integrate non-generic code with generic code by using the proposed tools based on the generic capabilities, including integration with the reflection mechanism. Of course, not all specifications about genericity were explained, and readers who are interested in more details should refer to the first entry in the References section below.

References

  • C# version 2.0 Specifications, Microsoft Corporation, July 2003.
  • Meyer, B: Object-Oriented Software Construction 2nd Edition. Prentice Hall, 1997.

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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