.NET Platform Architecture

C# programs run on .NET, a virtual execution system called the common language runtime (CLR) and a unified set
of class libraries. The CLR is the commercial implementation by Microsoft of the common language infrastructure
(CLI), an international standard. The CLI is the basis for creating execution and development environments in whichlanguages and libraries work together seamlessly.
Source code written in C# is compiled into an intermediate language (IL) that conforms to the CLI specification. The
IL code and resources, such as bitmaps and strings, are stored in an assembly, typically with an extension of .dll. An
assembly contains a manifest that provides information about the assembly's types, version, and culture.
When the C# program is executed, the assembly is loaded into the CLR. The CLR performs Just-In-Time (JIT)
compilation to convert the IL code to native machine instructions. The CLR provides other services related to
automatic garbage collection, exception handling, and resource management. Code that's executed by the CLR is
sometimes referred to as "managed code", in contrast to "unmanaged code", which is compiled into native machine
language that targets a specific system.
Language interoperability is a key feature of .NET. IL code produced by the C# compiler conforms to the Common
Type Specification (CTS). IL code generated from C# can interact with code that was generated from the .NET
versions of F#, Visual Basic, C++, or any of more than 20 other CTS-compliant languages. A single assembly may
contain multiple modules written in different .NET languages, and the types can reference each other as if they
were written in the same language.
In addition to the run time services, .NET also includes extensive libraries. These libraries support many different
workloads. They are organized into namespaces that provide a wide variety of useful functionality for everything
from file input and output to string manipulation to XML parsing, to web application frameworks to Windows
Forms controls. The typical C# application uses the .NET class library extensively to handle common "plumbing"
chores.
For more information about .NET, see Overview of .NET.