---
description: CRITICAL - All number parsing/formatting must use CultureInfo.InvariantCulture to prevent locale bugs
alwaysApply: true
---

# Locale-Invariant Number Handling

**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.

## Rules

### Parsing numbers from strings (config, text boxes, files)

```csharp
// BAD - uses current locale, breaks under German/French/etc.
double.TryParse(value, out double result)
float.TryParse(value, out float result)

// GOOD - always uses '.' as decimal separator
double.TryParse(value, NumberStyles.Float, CultureInfo.InvariantCulture, out double result)
float.TryParse(value, NumberStyles.Float, CultureInfo.InvariantCulture, out float result)
```

### Formatting numbers to strings (config persistence, CSS/HTML generation)

```csharp
// BAD - produces "1,50" under German locale
value.ToString("F2")
value.ToString()  // for doubles/floats being saved to config

// GOOD
value.ToString("F2", CultureInfo.InvariantCulture)
value.ToString(CultureInfo.InvariantCulture)
```

### String interpolation with doubles into CSS/HTML

```csharp
// BAD - produces "font-size: 14,5px" or "rgba(0,0,0,0,700)" under German locale
$"font-size: {fontSize}px"
$"rgba({r},{g},{b},{alpha:F3})"

// GOOD - FormattableString.Invariant forces '.' decimals
FormattableString.Invariant($"font-size: {fontSize}px")
FormattableString.Invariant($"rgba({r},{g},{b},{alpha:F3})")
```

## Required import

```csharp
using System.Globalization;
```

## Why this matters

- Config values saved with `,` under one locale fail to parse under another
- CSS/HTML with `,` decimals produces invalid styles (browsers expect `.`)
- Font sizes, opacity values, and positions all break silently