Design and implementation of generics for the .NET common language runtime

The Microsoft .NET Common Language Runtime provides a shared type system, i ntermediate language and dynamic execution environment for the implementati on and inter-operation of multiple source languages. In this paper we exten d it with direct support for parametric polymorphism (also known as generic s), describing the design through examples written in an extended version o f the C# programming language, and explaining aspects of implementation by reference to a prototype extension to the runtime. Our design is very expressive, supporting parameterized types, polymorphic static, instance and virtual methods, "F-bounded" type parameters, instanti ation at pointer and value types, polymorphic recursion, and exact run-time types. The implementation takes advantage of the dynamic nature of the run time, performing justin-time type specialization, representation-based code sharing and novel techniques for efficient creation and use of Nn-time typ es. Early performance results are encouraging and suggest that programmers will not need to pay an overhead for using generics, achieving performance almo st matching hand-specialized code.