[Discussion] Nullable Analysis and `NullReferenceException`s

[BinToss]

Questions

• @BinToss: Was nullability analysis disabled as a "we'll fix it later" kind of thing?
• @BinToss: Is it because Anego Studio's fork of CairoSharp isn't nullable-aware?
• @BinToss: I'm assuming Vintagestory.API.Common.IBlockEntityContainer.CheckInventoryClearedMidTick was added to mitigate race-condition NullReferenceExceptions. What other null de-reference protections (aside from nullable types) are planned?

If community developers have similar questions or suggested changes, please reply to this Issue! A Maintainer or I will will append them to this post with a a quote and link to the reply.

---

Problem

anegostudios/vsapi has many source files with the #nullable disable preprocessor directive, thereby disabling nullability analysis for large swaths of the project. This means symbols that should be null-aware are instead null-_un_aware—They appear to be never-null, but are actually maybe-null. This leads to a high risk of null-dereferences (NullReferenceException) throughout the library. Because of this, API users are also at risk of null-dereferences.

Solution

• #nullable disable preprocessor directives should be removed or used very sparingly.
• Maybe-null cases should be handled with either a null-conditional operator (?., ?[]) or the occasional not-null assertion (!) where it makes sense e.g. in classes whose static members are null until Initialize is called (consider using a lazy singleton, instead!).

Side effects may include encouraging mod developers to write nullable-aware code.

Background

I've been seeing several mods with NullReferenceException stack traces pointing to GuiElementItemSlotGridBase.ComposeSlotOverlays. And so I began investigating them.

Mario90900/Toolsmith#35
dizzyd/packrat#2
dizzyd/packrat#3

At first, I thought it was isolated to ToolSmith's Reflection-based IL patch of the method, but the ComposeSlotOverlays NullReferenceException reportedly occurs even without ToolSmith's IL patches. So, I read the body of ComposeSlotOverlays, found that the stack traces' :Line### indicated the wrong lines (bad symbols?), and then wondered why nullability analysis is explicitly disabled in

vsapi/Client/UI/Elements/Impl/Interactive/Inventory/GuiElementItemSlotGridBase.cs

Line 10 in 409af70

#nullable disable

...and many other source files.

---

And so...
(See Questions)