How does .Net CLR work?
Within the domain of CLR are executables (consisting of code, data, and metadata), assemblies (consisting of a manifest and zero or more modules), and the Common Type System (CTS) convention set. When programmers write code in their favorite languages, that code is translated into IL prior to being compiled into a portable executable (PE).
The main difference between a Windows PE and a .NET PE is that the Windows PE is executed by the operating system, but .NET PEs are turned over to the .NET Framework’s CLR. Recognition of a PE as being .NET or Windows occurs because of the Common Object File Format (COFF) used by Windows operating systems. The COFF specifies two parts of any file: the file data itself and a bunch of header data describing the contents of the data portion. Note: To allow all Microsoft platforms to handle COFF modifications that enable .NET PEs, Microsoft has released new loaders for all of .NET’s supported systems (98, 2000, and Me).
Metadata is information about a PE. In COM, metadata is communicated through nonstandardized type libraries. In .NET, this data is contained in the header portion of a COFF-compliant PE and follows certain guidelines; it contains information such as the assembly’s name, version, language (spoken, not computer—a.k.a., “culture”), what external types are referenced, what internal types are exposed, methods, properties, classes, and much more. The CLR uses metadata for a number of specific purposes. Security is managed through a public key in the PE’s header. Information about classes, modules, and so forth allows the CLR to know in advance what structures are necessary. The class loader component of the CLR uses metadata to locate specific classes within assemblies, either locally or across networks. Just-in-time (JIT) compilers use the metadata to turn IL into executable code. Other programs take advantage of metadata as well. A common example is placing a Microsoft Word document on a Windows 2000 desktop. If the document file has completed comments, author, title, or other Properties metadata, the text is displayed as a tool tip when a user hovers the mouse over the document on the desktop. You can use the Ildasm.exe utility to view the metadata in a PE. Literally, this tool is an IL disassembler.