Stardew Valley isn't open source, but it has been decompiled (.Net) and has a an open-source modding API inclding Harmony integration for patching the base game

Stardew Valley isn't open source, but it has been decompiled (.Net) and has a an open-source modding API inclding Harmony integration for patching the base game code and and a huge open-source modding community. ttps://github.com/pardeike/Harmony