CLR, CTS, CLS — what each one does — .NET Internals — CLR, CTS, CLS, IL, JIT, Memory Management & Garbage Collection

2. CLR, CTS, CLS — what each one does

Basic★ Must knowLesson 2 of 11

CLR responsibilities:

Component	Job
Class loader	Loads assemblies and types, builds MethodTables
JIT compiler	IL → native code
Garbage collector	Allocates and frees managed memory
Type checker / verifier	Type safety — no invalid casts or reading random memory
Exception manager	Structured exception handling across frames and languages
Thread support	Managed threads, thread pool, synchronisation
Security	Type safety, (Framework: Code Access Security — removed in Core)
Interop	P/Invoke, COM interop, marshalling
Debugging / profiling APIs	Debuggers, profilers, diagnostics

CTS — the type system all languages share:

text

System.Object
├── Value types (System.ValueType): int, double, bool, char, decimal, DateTime, struct, enum (System.Enum)
└── Reference types: class, string, array (System.Array), delegate (System.Delegate), interface, record

The CTS defines visibility (public, private, internal, protected), inheritance rules (single class inheritance, multiple interfaces), members (fields, methods, properties, events) and how values are boxed. Because C# int and VB.NET Integer are both System.Int32, the languages interoperate perfectly.

CLS — the common subset for cross-language libraries. Examples of non-CLS-compliant features: unsigned types in public APIs (uint — VB.NET historically lacked them), names differing only by case (Name and name — VB.NET is case-insensitive), pointers. Mark libraries with [assembly: CLSCompliant(true)] and the compiler warns about violations.