Cursor rule

1---
2description: CRITICAL - All number parsing/formatting must use CultureInfo.InvariantCulture to prevent locale bugs
3alwaysApply: true
4---
5 
6# Locale-Invariant Number Handling
7 
8**CRITICAL**: This app MUST work correctly regardless of the user's Windows region/locale setting (e.g., German uses `,` as decimal separator instead of `.`). Locale-sensitive number formatting has caused serious bugs TWICE in this project. Always use InvariantCulture.
9 
10## Rules
11 
12### Parsing numbers from strings (config, text boxes, files)
13 
14```csharp
15// BAD - uses current locale, breaks under German/French/etc.
16double.TryParse(value, out double result)
17float.TryParse(value, out float result)
18 
19// GOOD - always uses '.' as decimal separator
20double.TryParse(value, NumberStyles.Float, CultureInfo.InvariantCulture, out double result)
21float.TryParse(value, NumberStyles.Float, CultureInfo.InvariantCulture, out float result)
22```
23 
24### Formatting numbers to strings (config persistence, CSS/HTML generation)
25 
26```csharp
27// BAD - produces "1,50" under German locale
28value.ToString("F2")
29value.ToString()  // for doubles/floats being saved to config
30 
31// GOOD
32value.ToString("F2", CultureInfo.InvariantCulture)
33value.ToString(CultureInfo.InvariantCulture)
34```
35 
36### String interpolation with doubles into CSS/HTML
37 
38```csharp
39// BAD - produces "font-size: 14,5px" or "rgba(0,0,0,0,700)" under German locale
40$"font-size: {fontSize}px"
41$"rgba({r},{g},{b},{alpha:F3})"
42 
43// GOOD - FormattableString.Invariant forces '.' decimals
44FormattableString.Invariant($"font-size: {fontSize}px")
45FormattableString.Invariant($"rgba({r},{g},{b},{alpha:F3})")
46```
47 
48## Required import
49 
50```csharp
51using System.Globalization;
52```
53 
54## Why this matters
55 
56- Config values saved with `,` under one locale fail to parse under another
57- CSS/HTML with `,` decimals produces invalid styles (browsers expect `.`)
58- Font sizes, opacity values, and positions all break silently
59