Saturday, January 30, 2010

Metadata and Reflection

As I mentioned in the earlier section "Microsoft Intermediate Language and the JITters," the CLS-compliant compilers take your source code as input and produce MSIL code for the runtime to compile (via the JITters) and execute. In addition to mapping source code to MSIL instruction sequences, CLS-compliant compilers have another equally important task: embedding metadata into the resulting EXE.
Metadata is data that describes data. In this context, metadata is the collection of programmatic items that constitute the EXE, such as the types declared and the methods implemented. If this sounds vaguely familiar, it should. This metadata is similar to the type libraries (typelibs) generated with Component Object Model (COM) components. Not only is the metadata output from a .NET compiler substantially more expressive and complete than the COM typelibs we're accustomed to, but the metadata is also always embedded in the EXE. This way, there's no chance of losing the application's metadata or having a mismatched pair of files.
The reason for using metadata is simple. It allows the .NET runtime to know at run time what types will be allocated and what methods will be called. This enables the runtime to properly set up its environment to more efficiently run the application. The means by which this metadata is queried is called reflection. In fact, the .NET Framework class libraries provide an entire set of reflection methods that enable any application-not just the CLR-to query another application's metadata.
Tools such as Visual Studio.NET use these reflection methods to implement features such as IntelliSense. With IntelliSense, as you type in a method name, that method's arguments pop up in a list box on the screen. Visual Studio.NET takes that functionality even further, showing all the members of a type. I'll discuss the reflection APIs in Chapter 15, "Multithreaded Programming."-
Another incredibly useful .NET tool that takes advantage of reflection is the Microsoft .NET Framework IL Disassembler (ILDASM). This powerful utility parses the target application's metadata and then displays information about the application in a treelike hierarchy.

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