Common Language Runtime

As explained in .NET in Action, Second Edition

The .NET runtime is the execution environment that loads .NET assemblies and translates their intermediate code into processor-specific instructions.

The Common Language Runtime (CLR) is the execution environment shared by .NET languages. The modern .NET runtime is called CoreCLR; the original .NET Framework runtime was called CLR. C#, F#, and Visual Basic compilers produce Common Intermediate Language (CIL), which CoreCLR can execute. The runtime also provides memory management. section 1.1 section 1.5

A language compiler places CIL in a DLL. Because CIL does not target a particular processor, that DLL can run on another supported operating system or processor when a .NET runtime is available. CoreCLR normally uses just-in-time (JIT) compilation to turn CIL into machine code as the application needs it, rather than compiling the entire application at startup. section 1.5.1 section 1.5.2

For a known deployment target, such as a Docker container, ahead-of-time (AOT) compilation can reduce the initial wait for JIT compilation. AOT can also trim unused code, but trimming needs thorough testing because it may remove too much. The runtime’s other major concern, garbage collection, is introduced alongside CIL and JIT rather than detailed here. section 1.5.2 section 1.5

Figure 1.4 .NET languages compile to CIL in a DLL file that can be run on any machine with a .NET runtime.

Trace a .NET language from compilation into a CIL DLL, then through the runtime on the machine that executes it. section 1.5.1

How .NET in Action, Second Edition covers it

How deep it goes

Explains CIL portability and on-demand JIT compilation, while introducing garbage collection only at a high level.

Examples

Uses a compiler-to-CIL-to-runtime diagram and a Docker deployment target to illustrate portability and AOT.

What it assumes

Defines runtime and DLL terminology from basics; treats familiarity with Java bytecode as optional.

What you can do after

Evaluate/understand why a CIL DLL needs a .NET runtime and when JIT startup cost might favor AOT.

How it connects

Connects the CLR to CIL, DLLs, JIT compilation, garbage collection, and ahead-of-time compilation.

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Also explained in

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  ASP.NET Core in Action, 3e.NET Runtime is the execution environment that runs compiled .NET applications without providing the development and build tooling of the SDK.
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  .NET MAUI in ActionThe Common Language Runtime is the .NET execution environment that manages and runs compiled managed code.

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Related concepts

• .NETpart of · .NET is Microsoft's cross-platform development platform, providing runtimes, libraries, and tools for building and running applications.
• ASP.NET Core Windows Hosting Bundlepart of · The ASP.NET Core Windows Hosting Bundle is a Windows installation package containing the runtimes and IIS integration components required to host ASP.NET Core applications.
• self-contained deploymentused by · A self-contained deployment packages an application with its required runtime and framework components so it runs without a separately installed runtime.
• just-in-time compilationuses · Just-in-time compilation translates or optimizes program code during execution, often using runtime information to generate specialized executable instructions.
• machine languageproduces · Machine language is a processor-specific instruction language encoded in a form that computer hardware can execute directly.

This explainer was generated by Manning's knowledge graph from the text of .NET in Action, Second Edition. The section links point to the parts of the book it draws on.