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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 CIL (Common Intermediate Language), and what role does it play between source code and execution? 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 the CLR (Common Language Runtime), and what are its core responsibilities?

Beginner

Answer

The CLR is the execution engine of .NET, responsible for managing memory (allocation and garbage collection), just-in-time (JIT) compiling IL code to native machine code, enforcing type safety, handling exceptions, managing threads, and providing security sandboxing. Any .NET language (C#, F#, VB.NET) compiles down to a common Intermediate Language (IL) that the CLR executes uniformly.
// C# source compiles to IL, which the CLR JIT-compiles to native code at runtime
public int Add(int a, int b) => a + b;
// Compiled IL (simplified): ldarg.1, ldarg.2, add, ret
Real-world example Because the CLR provides a common execution model, a library written in F# can be seamlessly consumed by a C# application, since both compile to the same IL that the CLR executes identically regardless of source language.

Common follow-ups: What's the difference between IL and native machine code?;How does the CLR differ from the JVM conceptually?

.NET vs .NET Framework;Memory Management & Garbage Collection
All 19 CLR & Runtime questions