The Metadata System1

2 Introduction Metadata is data that describes data. Traditionally, metadata has been found in language- specific files (e.g. C/C++ header files), but providing interoperability between such files across multiple languages is difficult (or almost impossible) Compilers usually remove most of the metadata information when source files are compiled, so executable files often have little or no metadata about their types available at runtime In some systems, components’ metadata is not stored with the components, but as separate files (e.g. IDL in CORBA), may cause versioning and inconsistency problems Traditional metadata facilities are primitive, allow developers to specify the syntax of an interface but not its semantics Thus, compilers and execution engine require metadata.

The Metadata System3.NET Metadata System The metadata system is the part of CLR that describes the types in the CLR. It is the essential facility that enables type sharing between compilers (languages). Allows metadata to be persisted along with types at compile time. Can be interrogated by other CLR compilers, and compilers can use the metadata to make types available in their own languages. Can be used by the execution system at runtime (to manage types) Enables other languages to inspect, use and extend types

The Metadata System4

5.NET Metadata System CLR extends the use of metadata to provide not only information about types but also information about assemblies, the unit of deployment in the.NET framework. Both assemblies and types are self-describing. Execution engines uses this information to ensure correct runtime execution of code. Some metadata facilities are needed to access the metadata

The Metadata System6 Reflection The act of inspecting a type’s metadata at runtime is called reflection. System.Object has a method GetType() specifically designed to facilitate this inspection. GetType() returns an object of type Type. Listing 3.1 ILDASM: a tool for examining metadata

The Metadata System7 ILDASM Tool

The Metadata System8 Reflection Classes Object MemberInfo EventInfoFieldInfoMethodBasePropertyInfoType ConstructorInfoMethodInfo

The Metadata System9 Metadata Extensiblity To provide more semantic information By Attributes, essentially a class, an instance of an attribute class is attached to a type or its members and persisted in the metadata for that type Custom Attributes: simple and easy means for developers to annotate a type or its members Standard Attributes: Framework Class Library predefined standard attributes

The Metadata System10 Using a standard attribute (Listing 3.5) using System; namespace Sample { public class TestClass { [Obsolete("Use NewMthod instead of OldMethod")] public static void OldMethod() { Console.WriteLine("Hi World"); } public static void NewMethod() { Console.WriteLine("Hello World"); }

The Metadata System11 Dynamic Discovery of Types Becoming increasingly important in modern software architectures Traditionally programming languages and systems used to have such information to be known at compile time, more type-safe (type checking) and efficient But leads to platform-specific code that is very difficult to migrate and execute on different systems Even, types can be known only at runtime.NET framework provides dynamic type discovery through its Reflection facilities.

The Metadata System12 Assemblies and Manifests (1) Assembly is basically a collection of files containing types and methods that can be used (similar to DLL libraries) Can be viewed as a collection of files that form a functionally unit, contain types, resources available to other assemblies. An assembly houses the components, but also provides facilities such as version information and type resolution. An assembly comprises one or more modules, a module is a file that contains CLR metadata. One of the modules has a manifest. The manifest contains information about the assembly, such as its name, version information, and names of all other modules and files Using an assembly (Listing 3.8)

The Metadata System13 Assemblies and Manifests (2) Assembly A File AModule B Module A Manifest Note: File A is not a module, would not contain any CLR metadata, just hold other resources such as bitmaps.

The Metadata System14 Assemblies and Manifests (3)

The Metadata System15 Assemblies and Manifests (3).assembly extern mscorlib {.publickeytoken = (B7 7A 5C E0 89 ) //.z\V.4...ver 1:0:3300:0 }.assembly AssemblyInformation { // --- The following custom attribute is added automatically, do not uncomment //.custom instance void [mscorlib]System.Diagnostics.DebuggableAttribute::.ctor(bool, // bool) = ( ).hash algorithm 0x ver 0:0:0:0 }.module AssemblyInformation.exe // MVID: {ADDEC506-DB9F-4E90-80D5-5712F39A3081}.imagebase 0x subsystem 0x file alignment 512.corflags 0x // Image base: 0x

The Metadata System16 Meta-Programming The ability to dynamically create new types at runtime is known as meta- programming. Supported by CLR’s Reflection.Emit Creating dynamic types is a fundamental facility in the remoting services, automatic generation of proxy objects at runtime.