Pitfalls of C# Generics and Their Solution Using Concepts

Abstract

As was shown in earlier studies, in comparison with Haskell type classes and C++ concepts such mainstream object-oriented languages as C# and Java provide much limited mechanisms of generic programming based on F-bounded polymorphism. Main pitfalls of C# generics are carefully considered in this paper. A special attention is given to drawbacks of recursive constraints (F-constraints), ambiguous semantics of interfaces, lack of language support for multi-type constraints and retroactive interface implementation, and subtle problems of the Concept design pattern, which is widely used not only in C#, but in Java and Scala as well. To solve the problems of C# generics, extending C# language with concepts is proposed: as a new language construct, concepts are to be used as constraints on type parameters exclusively, with object-oriented interfaces being used as types. In contrast to basic C++ concepts, C# concepts may include subtype and supertype constraints, allow constraints aliasing and automatic generation of default models. The major differ-ence of the concepts design proposed is language support for multiple models. The latter feature is supported neither in C++ concepts, nor in Haskell type classes. In conclusion, a mechanism of implementation of concepts via translation to basic C# is outlined. The most important property of the translation is a possibility to recover a source code in extended language from a compiled module.