# Common Compile Failures
**Last Updated:** 2026-04-05

Record concise error fingerprints and verified fixes.

## Entry Template
- **Date:** YYYY-MM-DD
- **Error:** `CSXXXX`
- **Fingerprint:** file + line + short symptom
- **Root cause:** one sentence
- **Fix:** one to three concrete steps
- **Prevention link:** guardrail/pattern doc entry added

## Starter Examples
1. **`CS0115` no suitable method found to override**
   - Root cause: incorrect NT8 override signature.
   - Fix: replace with exact official signature; recompile.

2. **`CS0507` cannot change access modifiers when overriding**
   - Root cause: used `public override`/`private override` instead of `protected override`.
   - Fix: change to `protected override`; recompile.

3. **`CS0246` type or namespace not found**
   - Root cause: missing using/namespace or undefined domain type.
   - Fix: add correct namespace reference or confirmed type definition.

4. **`CS1503` argument type mismatch**
   - Root cause: wrong overload or argument ordering.
   - Fix: align call with exact method signature and parameter types.

5. **C# 6+ syntax in C# 5 project**
   - Root cause: use of `$""`, `?.`, `nameof`, expression-bodied members, etc.
   - Fix: rewrite using C# 5 compatible constructs.