ExceptionInspector throws NullReferenceException every frame when a patch method belongs to no mod (dynamic assembly)

從 Mods.AddMetadata 開始，接著追蹤 ExceptionInfo.GetReport 如何建立 mod 詳細資訊，以及 Columns.Column 如何使用其中繼資料。使用動態組件中的 Harmony 修補方法，以及一個經過該方法的例外來重現問題。開啟 inspector 時，沒有 mod 中繼資料的修補不再反覆擲出例外，即為完成。

Summary

When a captured exception has a stack frame whose method carries a Harmony patch from an assembly that no mod owns (for example a runtime-generated assembly), the inspector window throws a NullReferenceException on every frame. The log fills with thousands of errors and the window cannot be used.

Seen with Visual Exceptions 1.5.0.0 on RimWorld 1.7.5005, Harmony 3.

Cause

Mods.AddMetadata creates a ModInfo for every patch method, also when GetModMetaData(assembly) returns null:

static IEnumerable<ModInfo> AddMetadata(IEnumerable<MethodInfo> methods)
{
    return methods.Select(method =>
    {
        var assembly = method.DeclaringType.Assembly;
        var metaData = GetModMetaData(assembly);
        return new ModInfo { method = method, metaData = metaData };
    });
}

ExceptionInfo.GetReport then adds an ExceptionDetails.Mod with meta == null, and Columns.Column(this ExceptionDetails.Mod mod) dereferences it:

var name = mod.meta.Name.Truncate(200, truncatedModNamesCache);

Stack frames do not have this problem, because GetMetadataIfMod returns null for non-mod code and GetAllMethods skips those. Only patches are missing the check.

How it showed up

A player ran the Overclock mod with "Parallel pawn ticks (experimental)" on. That feature builds its prefix classes at runtime with AssemblyBuilder.DefineDynamicAssembly (types named IdPrefix_<n>_<method> and IdState_<n>) and patches thousands of mod methods with them. Those types are in no mod's loadedAssemblies.

During loading, another mod requested a graphic with a null texture path, which throws ArgumentNullException in ContentFinder<Texture2D>.Get. One frame of that exception had such a generated prefix on it. Once the inspector opened at the main menu:

Exception filling window for VisualExceptions.ExceptionInspector: System.NullReferenceException: Object reference not set to an instance of an object.
   at VisualExceptions.Columns.Column_Patch1(Mod mod)
   at VisualExceptions.ExceptionInspector.<>c.<DoWindowContents>b__9_2(Mod mod)
   at System.Linq.Enumerable.ListSelectIterator`2.MoveNext()
   at System.Linq.Enumerable.MaxFloat[TSource,TResult](IEnumerable`1 source, Func`2 selector)
   at VisualExceptions.Columns.MaxWidth(IEnumerable`1 drawers)
   at VisualExceptions.ExceptionInspector.DoWindowContents_Patch1(ExceptionInspector this, Rect inRect)
   at Verse.Window.InnerWindowOnGUI_Patch3(Window this, Int32 x)

This repeated 2,515 times in under a minute.

Steps to reproduce

1. Have any Harmony patch whose patch method is declared in a dynamic assembly, on a method of a mod.
2. Throw an exception that passes through that method and is caught by one of the handlers Visual Exceptions instruments.
3. Open the inspector.