Understanding the Common Language Runtime

In addition to the CTS and CLS specifications, the final TLA (three-letter abbreviation) to contend with at the moment is the CLR. Programmatically speaking, the term runtime can be understood as a collection of external services that are required to execute a given compiled unit of code. For example, when developers make use of the MFC to create a new application, they are aware that their program requires the MFC runtime library (i.e., mfc42.dll). Other popular languages also have a corresponding runtime. VB6 programmers are also tied to a runtime module or two (e.g., msvbvm60.dll). Java developers are tied to the Java Virtual Machine (JVM), and so forth. The .NET platformoffers yet another runtime system. The key difference between the .NET runtime and the various other runtimes I just mentioned is the fact that the .NET runtime provides a single well-defined runtime layer that is shared by all languages and platforms that are .NETaware. The crux of the CLR is physically represented by a library named mscoree.dll (a.k.a. the Common Object Runtime Execution Engine). When an assembly is referenced for use, mscoree.dll is loaded automatically, which in turn loads the required assembly into memory. The runtime engine is responsible for a number of tasks. First and foremost, it is the entity in charge of resolving the location of an assembly and finding the requested type within the binary by reading the contained metadata. The CLR then lays out the type in memory, compiles the associated CIL into platformspecific instructions, performs any necessary security checks, and then executes the code in question. In addition to loading your custom assemblies and creating your custom types, the CLR will also interact with the types contained within the .NET base class libraries when required. Although
the entire base class library has been broken into a number of discrete assemblies, the key assembly is mscorlib.dll. mscorlib.dll contains a large number of core types that encapsulate a wide variety of common programming tasks as well as the core data types used by all .NET languages. When you build .NET solutions, you automatically have access to this particular assembly.