## Summary
Phase 3 PR B of the message-interpolation cleanup. Handles the multi-line restructure sites flagged in `dev-docs/string-handling-message-interp-audit.md` (Phase 2). Phase 3.1 (PR #2436) handled trivial sweeps; this PR handles sites that needed an `if/else` hoist or switch restructure to eliminate `string.Format` while preserving exact message text.
Each site previously allocated an intermediate `string.Format(...)` result before passing to the message handler, despite Phase 1 making the handler accept interpolated string handlers natively.
## Sites fixed
- **`Projects/Server/Mobiles/Mobile.cs:7911`** - Title/guild header was using `string.Format` with a conditional template (`"[{1}]{2}"` vs `"[{0}, {1}]{2}"`). Split into `if (title.Length <= 0)` / `else` with direct `$"..."` interpolation.
- **`Projects/UOContent/Engines/ConPVP/DuelContext.cs:1337`** - View-ladder rank text used `string.Format(text, from == pm ? "You" : "They")`. Split into `if (from == pm)` / `else` with direct `$"..."` interpolation in each branch.
- **`Projects/UOContent/Engines/ConPVP/DuelContext.cs:1463`** - Showladder text reused a single format string for both `LocalOverheadMessage` ("You ... are ...") and `NonlocalOverheadMessage` ("`{pm.Name}` ... is ..."). Each call now uses an inline `$"..."` directly; no shared template.
- **`Projects/UOContent/Engines/ConPVP/Gumps/ConfirmSignupGump.cs:518`** - The signup confirmation message used a `switch` expression assigning a literal format string to `fmt`, then `string.Format(fmt, from.Female ? "Lady" : "Lord", timeUntil)`. Converted to a `switch` statement where each case calls `_registrar.PrivateOverheadMessage(...)` directly with an inline `$"..."`. Lady/Lord branching is hoisted to a `title` local.
## Exempted
- **`Projects/UOContent/Engines/ConPVP/Participant.cs:138`** - The `nonLocalOverhead` format string is a parameter passed in by callers of `Participant.Broadcast`. Investigation found 5 call sites in `DuelContext.cs` (lines 782, 802, 1187, 1196, and three at 1535/1564/1608) that pass distinct literal format strings. Refactoring would require changing all 5 callers and the method signature - out of scope for this PR. Marked with a `// Phase 3 audit:` comment per the audit's exemption convention.
## Summary
- **Add a self-referencing `InterpolationHandler` to `ValueStringBuilder`** that writes directly into the builder's buffer — zero intermediate allocation, works with `stackalloc`-backed builders
- **Replace all `System.Text.StringBuilder` usage** across the codebase with `ValueStringBuilder`
- **Convert `ValueStringBuilder.Create()` to `stackalloc`** at 10 sites where output length is provably bounded
- **Convert manual `Dispose()` to `using var`** where possible, and hoist loop-scoped builders outside loops with `Reset()`
- **Convert verbose `Append()` chains to `Append($"...")`** interpolation for readability
- **Add comprehensive documentation** for string handling patterns
## InterpolationHandler Design
`ValueStringBuilder` is a `ref struct`, which creates challenges for C#'s interpolated string handler pattern:
- **`ref` fields to ref structs are not allowed** (CS9050)
- **`[InterpolatedStringHandlerArgument("")]` passes struct receivers by value**, not by ref
- **`ISelfInterpolatedStringHandler` requires boxing** ref structs into interface fields
**Solution: Copy-and-reconcile pattern.** The handler receives a value copy of the builder. The copy shares the same underlying `char` buffer (`Span` points to the same `stackalloc`/pooled memory), so writes go to the original buffer. `Append()` reconciles by `this = handler._builder`, updating `_length` and any buffer references changed by `Grow()`.
This is safe because:
- The game loop is single-threaded — no concurrent access between handler construction and reconciliation
- If `Grow()` occurs in the copy, the original's stale buffer isn't accessed until `Append()` replaces it
- `Dispose()` correctly returns the reconciled buffer to the pool
## Changes by Category
### ValueStringBuilder (`Projects/Server/Buffers/ValueStringBuilder.cs`)
- Added nested `InterpolationHandler` ref struct with copy-and-reconcile pattern
- Added `Append([InterpolatedStringHandlerArgument("")] scoped ref InterpolationHandler)` method
- Removed `RawInterpolatedStringHandler` overloads (new handler replaces them)
- All `AppendFormatted` overloads delegate to existing `Append` methods (no code duplication)
- Alignment support via direct private field access (nested type privilege)
### StringBuilder → ValueStringBuilder (15 files)
Replaced all `new StringBuilder()` with `ValueStringBuilder.Create()` or `stackalloc`:
- ConPVP games: KingOfTheHill, DoubleDom, CTF, BombingRun, TourneyMatch
- ConPVP infrastructure: Tournament, Participant, TourneyParticipant
- ConPVP gumps: ArenaGump, TournamentBracketGump, AcceptTeamGump, ConfirmSignupGump
- Commands: Handlers, Logging, Add
- Other: TownCrier, SpeechLogGump, TestCenter
Key patterns:
- `sb = new StringBuilder()` reassignment → `sb.Reset()`
- `sb.AppendFormat("{0:N0}", value)` → `sb.Append($"{value:N0}")`
- `sb.Append(x).Append(y)` chains → separate statements (VSB returns void)
### Create() → stackalloc (10 files)
Converted heap-allocated builders to stackalloc where output is bounded:
- ClientVersion (32), MapSelection (160), HouseRaffleStone (48)
- HolySense (96), UnholySense (96), ClientVerification (192)
- AcceptTeamGump (64), ConfirmSignupGump (64)
- BaseWeapon (160), BaseArmor (128)
### Loop optimizations (2 files)
Hoisted `ValueStringBuilder` creation outside loops with `Reset()` per iteration:
- TourneyMatch.cs: `using var` inside for loop → stackalloc before loop
- ArenaGump.cs: `Create()` + `Dispose()` per iteration → stackalloc before loop
### Append chain → interpolation (5 files)
Converted multi-line `Append()` chains to `Append($"...")`:
- BountyMessage.cs: title switch (6 cases), paragraph (15→1 Append), description lines, closing
- AcceptTeamGump, ConfirmSignupGump, TournamentBracketGump: tournament type strings
- AdminGump: comment/tag formatting in loops
### Documentation
- `dev-docs/string-handling.md`: Full reference — construction, interpolation, disposal, decision guide
- `dev-docs/claude-skills/modernuo-string-handling.md`: Claude skill with quick reference
- `CLAUDE.md`: Added rule 17 (no StringBuilder), dev-docs table entry, skills table entry
- `dev-docs/code-standards.md`: Updated memory management section
## Test Plan
- [x] `dotnet build` — 0 errors, 0 warnings
- [x] `dotnet test` — 940/940 tests pass
- [x] 28 ValueStringBuilder tests covering all reconciliation scenarios:
- Stackalloc no-grow, stackalloc with grow (→pool transition)
- Heap no-grow, heap with grow, heap double grow
- Pre-existing content with and without grow
- Sequential multiple `Append($"...")` calls
- Mixed plain + interpolated Append
- Empty interpolation, literal-only, format specifiers
- Null string holes, ISpanFormattable types
- Dispose after stackalloc→pool grow
