Modding Tutorials/Decompiling source code

The base game provides a bunch of code snippets in ../Source/, relative to your Rimworld installation. Since this isn't a lot, one might want to take a look at the game's full source code. RimWorld's EULA allows you to decompile the game for personal use. It's recommended to read it.
The following programs are used and recommended by modders:
=Decompiling source code=
===ILSpy===
ILSpy is generally recommended as it is the best-maintained and most reliable decompiler. The core project only has binaries for Windows, but there is an Avalonia-based port as well as a CLI application for Mono framework for OS X and Linux.
# Download ILSpy (Download latest release) and extract it to a directory of your choosing. Optionally create a desktop shortcut;
# Either: associate the .dll extension with ILSpy:
## Navigate to Assembly-CSharp.dll in ../Rimworld***_Data/Managed/, relative to your Rimworld installation and with *** being a version number (See Note on MacOS below);
## Right-click "Open with" and select a standard program. Navigate to your ILSpy installation and double-click ILSpy.exe, tick the checkbox and accept;
## Double-click Assembly-CSharp.dll,
# Or: open ILSpy and open a .dll:
## Open ILSpy;
## Go to File -> Open or press Ctrl+O, navigate to ../Rimworld***_Data/Managed/, relative to your Rimworld installation and with *** being a version number (See Note on MacOS below);
## Select Assembly-CSharp.dll and confirm,
# Click the "+" next to Assembly-CSharp (***), you will now see a list including the items Rimworld and Verse;
# Take your time to look through the source code, to make yourself familiar. If you ever need the source code, open ILSpy again:
## Ctrl+Shift+F or Ctrl+E opens the search bar which can be used to search through all loaded assemblies;
## Ctrl+F opens a search bar for the currently opened decompiled class.
If using CLI application:
# Despite the instructions saying .NET 5.0 SDK is needed, you may also need .NET Core 3.1;
# It seems that it's possible to simply build from a release tarball, even if the instructions suggest to use a git checkout;
# After you've built ILSpy, find the ilspycmd tool (should be located at ICSharpCode.Decompiler.Console/bin/Release/netcoreapp3.1);
# Run a command like ilspycmd RimWorld/RimWorld*_Data/Managed/Assembly-CSharp.dll -p -o <output directory> (use proper paths to RimWorld and for the output directory);
# The given output directory now contains decompiled sources.
===dnSpy===
dnSpy is an alternative with a Visual Studio editor feel. At the time of this writing, however, the original project has been archived for more than two years and none of its forks have reached a similar level of adoption. Decompilation glitches can occur.
# Download dnSpy and extract it somewhere.
# Open dnSpy.exe. Once it's open, click "open" on the top ribbon (or press Ctrl+O).
# Navigate to ../Rimworld***_Data/Managed/, relative to your Rimworld installation and with *** being a version number.
# Ctrl+Shift+K to open the search bar.
# Explore the assembly and look through the source code to your heart's desire.
===Rider / dotPeek===
Rider is a cross-platform IDE with a built-in decompiler. If you're using Rider as your IDE (Setup Instructions), you can view the source of any RimWorld class or method by right-clicking its name and clicking Go To > Definition.
The developer of Rider also offers a free standalone decompiler in the form of dotPeek. dotPeek is preferred by some for reading IL Code for the purpose of Harmony transpilers, but suffers similar glitches and inconsistency as dnSpy when decompiling back to C# code.
===MonoDevelop===
MonoDevelop is capable of decompiling DLLs, albeit using clumsy initial settings. It is Linux only, otherwise you have to download Xamarin Studio which doesn't have a decompiler.
# Download MonoDevelop and install it;
# Either: associate the .dll extension with MonoDevelop:
## Navigate to Assembly-CSharp.dll in ../Rimworld***_Data/Managed/, relative to your Rimworld installation and with *** being a version number (See Note on MacOS below);
## Right-click "Open with" and select MonoDevelop as standard program;
## Double-click Assembly-CSharp.dll,
# Or: open MonoDevelop and open a .dll:
## Open MonoDevelop;
## Go to File -> Open, navigate to ../Rimworld***_Data/Managed/, relative to your Rimworld installation and with *** being a version number (See Note on MacOS below);
## Select Assembly-CSharp.dll and confirm,
# Very important: search for a dropdown called "Visibility" and change it from "Only public members" to "All members";
# Very important: search for a dropdown called "Language" and change it from "Summary" to "C#";
# Click the "+" next to Assembly-CSharp (***), you will now see a list including the items Rimworld and Verse;
# Take your time to look through the source code, to make yourself familiar. If you ever need the source code, open Assembly-CSharp.dll again.
===MacOS Directories===
For Macs, directories are similar but in: ../RimWorldMac.app/Contents/Resources/Data/Managed. 
For Steam installed RimWorld, find your app here: ~/Library/Application Support/Steam/steamapps/common/RimWorld/RimWorldMac.app.
=How to make use of a decompiler=
=Tips when decompiling=
# Right-click any Type or Method and hit "analyse" to obtain more context on that item. 'Used by' and 'Uses' provide a lot of contextual clues which is required to know how things work.
# RimWorld often uses reflection to instantiate Workers and MakeThings. This means no decompiler will cleanly find what/where an instance of a class is created. Hint: if you find yourself using 'new Pawn()', you're doing it wrong.
# If you're going in circles trying to find things like "where is X assigned", odds are you'll need to look at the XML for it. The XML contains the data, C# does things with it.
=See also=
* Writing custom code shows a broad overview of C# coding.
Category:Modding tutorials