Enum combo rows reflect and allocate on every frame

What

Three widget entry points call Enum.GetNames / Enum.GetValues inside the draw path, so every enum combo allocates two fresh arrays per frame, per row, for the life of the application. In an immediate-mode library this is the hot path by definition — a settings panel with a handful of enum rows at 60 fps allocates thousands of arrays a second, all immediately garbage.

Site	Calls
`ImGui.Widgets/Combo.cs:23,25`	`Enum.GetValues(typeof(TEnum))`, `Enum.GetNames(typeof(TEnum))`
`ImGui.Widgets/EnumCombo.cs:44`	`Enum.GetValues<T>()`
`ImGui.Widgets/PropertyGridRows.cs:342-343`	`Enum.GetNames<TEnum>()`, `Enum.GetValues<TEnum>()`

Combo<TEnum> is the worst of the three, because it uses the non-generic overload:

Array possibleValues = Enum.GetValues(typeof(TEnum));
int currentIndex = Array.IndexOf(possibleValues, selectedValue);
...
selectedValue = (TEnum)possibleValues.GetValue(currentIndex)!;

Array.IndexOf(Array, object) boxes selectedValue and every element it compares against, and GetValue boxes again on selection. So a single Combo<TEnum> costs two array allocations plus a boxing allocation per element scanned, every frame.

Combo<TString> (Combo.cs:50) has the same shape for a different reason — it rebuilds the display array with LINQ and calls ToString on every element each frame:

string[] possibleValuesNames = [.. possibleValues.Select(e => e.ToString(CultureInfo.InvariantCulture))];

Suggested fix

Cache per closed generic type in a static holder, which for enums is free after first use because the values never change:

private static class EnumCache<T> where T : struct, Enum
{
    public static readonly T[] Values = Enum.GetValues<T>();
    public static readonly string[] Names = Enum.GetNames<T>();
}

Then Combo<TEnum> can drop to the generic overload and index without boxing. PropertyGrid.Enum and EnumCombo read the same cache.

The Combo<TString> overload needs a different treatment since its collection is a caller argument — either document that the caller should hoist the name array, or key a small cache on the collection instance.

Notes

Behaviour should be unchanged. Worth keeping the existing index >= 0 guard in PropertyGrid.Enum, which is what makes a value outside the declared members (a cast integer, a [Flags] combination) render as no selection rather than throw.
No public API change.

Triage

Priority: Medium — no incorrect behavior, but a real per-frame allocation cost in an immediate-mode library where this code runs every frame a combo is visible.
Effort: Small — the fix is a per-closed-type static cache plus swapping Combo<TEnum> to the generic Enum.GetValues<T>()/boxing-free path. No public API change, no blockers.

Plan

Add a private generic cache class (e.g. EnumCache<T> where T : struct, Enum) holding Values/Names computed once per closed type, and read it from all three sites: Combo<TEnum> (Combo.cs), EnumCombo (EnumCombo.cs), and PropertyGridRows.cs's enum row. Switch Combo<TEnum> from the non-generic Enum.GetValues(typeof(TEnum))/Array.IndexOf(Array, object) path to the generic, boxing-free equivalent.

Acceptance criteria

No behavior change: existing Combo/EnumCombo/PropertyGrid.Enum tests continue to pass unmodified, including the index >= 0 guard that lets an out-of-range value (cast int, [Flags] combination) render as "no selection."
No allocation from Enum.GetValues/Enum.GetNames/boxing on the second and subsequent draws of a given enum type (verify by inspection or a simple allocation-count check in a widget test).
Combo<TString> is out of scope for this issue (its collection is caller-supplied, so it needs a different treatment — document as a separate concern rather than folding it in here).

No blockers or dependencies.

Generated by Claude Code

Triage

Category: Improvement
Priority: Medium
Suggested assignment: none — no specific team/area indicated
Possible duplicate: none found

Generated by Claude Code

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Enum combo rows reflect and allocate on every frame #385

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Enum combo rows reflect and allocate on every frame #385

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matt-edmondson commented on Sep 9, 2026

matt-edmondson commented on Sep 13, 2026

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