Have you ever stopped to think about all the things that have to take place when you execute a .NET program? As the quote from Neal Ford says "Understand one level below your usual abstraction", this talk will look at why this is important and how can it help you if we apply it to the .NET framework. We will delve into the internals of the recently open-sourced .NET Core Runtime, looking at what happens, when it happens and why. Using freely available diagnostic tools such as PerfView, libraries including ClrMD and even the source code itself! Along the way we'll examine the Execution Engine, Type Loader, Just-in-Time (JIT) Compiler and the CLR Hosting API, to see how all these components play a part in making a 'Hello World' app possible.

1. From ‘dotnet run’
to “Hello World”
@matthewwarren
mattwarren.org

2. From ‘dotnet run’
to “Hello World”
http://raygun.com/apm

3. I ❤ ‘.NET Internals’

4. Understand one level below
your usual abstraction
Neal Ford

5. Demo – “Hello World”

6. Outline
• Main Components
• Hosting the runtime
• CLR Initialisation
• Type Loader
• JIT Compilation
• Everything Else

7. Main Components

8. CoreCLR Repository
• 2.6 million of source code, under src
• 8.2 million lines of test code
• 24,485 Files (7,466 source & 17,019 test)
– 6,626 C# (956 source & 5,670 test)
– 2,074 C and C++
– 3,701 IL
– 93 Assembler, 43 Python & 6 Perl!!

17. Virtual Machine (VM)
• Type System, Generics
• Built-in Types
• Loading types/classes
• Threading
• Exception Handling and Stack Walking
• Event Tracing and Profiling
• P/Invoke

18. Hosting

19. Hosting API
1. Identify the managed entry point
2. Find and load CoreCLR.dll
3. Get an ICLRRuntimeHost2 Instance
4. Set start-up flags and start the runtime
5. Preparing AppDomain settings
6. Create the AppDomain
7. Run managed code
8. Clean up
https://docs.microsoft.com/en-us/dotnet/core/tutorials/netcore-hosting

20. Demo – Hosting API

21. Initialisation

22. Initialising the CLR
• Phase 1 - Set-up infrastructure for all components
• Phase 2 - Initialise the core, low-level components
• Phase 3 - Start-up the low-level components, i.e. error
handling, profiling API, debugging
• Phase 4 - Start the main components, i.e. GC & AppDomains
• Phase 5 - Final setup and send out a notification
http://mattwarren.org/2017/02/07/The-68-things-the-CLR-does-before-
executing-a-single-line-of-your-code/

23. Demo - PerfView

24. ImageCourtesyof
JuliaEvans
@b0rk–jvns.ca

27. https://docs.microsoft.com/en-us/dotnet/standard/garbage-collection/fundamentals

28. Type Loader

31. http://mattwarren.org/2017/06/15/How-the-.NET-Rutime-loads-a-Type/

32. JIT Compilation

33. http://mattwarren.org/2017/12/15/How-does-.NET-JIT-a-
method-and-Tiered-Compilation/
Before JIT-ing

34. http://mattwarren.org/2017/12/15/How-does-.NET-JIT-a-
method-and-Tiered-Compilation/
Before JIT-ing
After JIT-ing

35. http://mattwarren.org/2017/12/15/How-does-.NET-JIT-a-
method-and-Tiered-Compilation/
Before JIT-ing
After JIT-ing with ‘Tiered Compilation’

36. http://mattwarren.org/2017/12/15/How-does-.NET-JIT-a-
method-and-Tiered-Compilation/
After JIT-ing with ‘Tiered Compilation’ - Optimised

37. Everything Else

38. Marshalling and P/Invoke

39. Resources
• http://mattwarren.org/tags/#Internals
– Resources for Learning about .NET Internals
– Exploring the Internals of the .NET Runtime
– Tools for Exploring .NET Internals
– Presentations and Talks covering '.NET Internals'
• Book of the Runtime (BotR)
– https://github.com/dotnet/coreclr/tree/master/Document
ation/botr

40. I ❤ ‘.NET Internals’
AMA