Reflective programming is becoming popular due to the increasing set of dynamic services provided by execution environments like JVM and CLR. With custom attributes Microsoft introduced an extensible model of reflection for CLR: they can be used as additional decorations on element declarations. The same notion has been introduced in Java 1.5. The annotation model, both in Java and in C#, limits annotations to classes and class members. In this paper we describe C# a, an extension of the C# programming language, that allows programmers to annotate statements and code blocks and retrieve these annotations at run-time. We show how this extension can be reduced to the existing model. A set of operations on annotated code blocks to retrieve annotations and manipulate bytecode is introduced. We also discuss how to use C# to annotate programs giving hints on how to parallelize a sequential method and how it can be implemented by means of the abstractions provided by the run-time of the language. Finally, we show how our model for custom attributes has been realized.

Reflection and dynamic loading are becoming essential elements of modern programs. Their usefulness is testified, for example, by the JDBC architecture that shows how to implement a driver-based architecture exploiting the Java dynamic loading.

Although reflection can be used to inspect the structure of types, to access fields and even to invoke methods dynamically, the concept of tagging has been anticipated as an interesting application. Consider for instance the Java serialization architecture: the programmer can declare the instances of a serializable class simply by implementing the Serializable interface, which in fact is an empty interface. Thus, two types that differ only for the implementation of the Serializable interface are indistinguishable from the execution standpoint. Besides, the serialization of the instances of non-serializable types will not be allowed by the serialization support. Java serialization taught us that the meta-data stored with the code can be used for other purposes than mere execution. Other programs may rely on the reflective abilities of inspecting the compiled types and act differently depending on what they have found.

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