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What is the CLR (Common Language Runtime), and what are its core responsibilities? What is an assembly's manifest, and what identity information does it contain? What is .NET, and how does it work at a high level? What is the CLR, and why is it central to how .NET applications execute? What is the difference between managed and unmanaged code in .NET, and when would you need to work with the latter? What is JIT (Just-In-Time) compilation, and how does it differ from ahead-of-time (AOT) compilation? How does the CLR's type system distinguish between value types and reference types, and where is each typically allocated? What is the Common Type System (CTS), and how does it enable interoperability between different .NET languages? What is the relationship between the CLR and the .NET class library (BCL - Base Class Library)? What does the `unsafe` keyword and pointer usage in C# allow, and why would a developer opt into it?
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What is CIL (Common Intermediate Language), and what role does it play between source code and execution?

Beginner

Answer

CIL is the CPU-independent, low-level instruction set that every .NET compiler (csc for C#, the F# compiler, etc.) produces instead of native machine code -- this intermediate step is what enables both cross-platform execution (the CLR's JIT compiler translates CIL to whatever machine code the current OS/CPU needs) and language interoperability (a C# class and an F# class both compile down to the same CIL representation, so they can call each other directly without any bridging layer).
// You can inspect a compiled assembly's CIL using a tool like ildasm or dotnet-ildasm
// A simple method like this:
public int Add(int a, int b) => a + b;

// compiles to CIL roughly resembling:
// ldarg.1
// ldarg.2
// add
// ret
Real-world example A library written in F# exposing a public class is consumed directly from a C# project with zero special glue code, only possible because both languages' compilers target the identical CIL format that the CLR understands.

Common follow-ups: Why can't CIL be executed directly by a CPU without the JIT step?;How does CIL enable tools like decompilers to reconstruct readable source from a compiled DLL?

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All 19 CLR & Runtime questions