dotnet-editorconfig

What this skill does

# dotnet-editorconfig

Comprehensive guide to configuring .NET code analysis rules via `.editorconfig` and global AnalyzerConfig files. Covers code style rules (IDE*), code quality rules (CA*), severity levels, `AnalysisLevel`, `EnforceCodeStyleInBuild`, directory hierarchy precedence, and `.globalconfig` files.

**Scope boundary:** This skill covers *configuring and tuning* analyzer rules. For *adding analyzer packages to a project*, see [skill:dotnet-add-analyzers]. For *authoring custom analyzers*, see [skill:dotnet-roslyn-analyzers]. For *project-level build configuration* (Directory.Build.props, solution layout), see [skill:dotnet-project-structure].

Cross-references: [skill:dotnet-add-analyzers] for adding analyzer packages and AnalysisLevel setup, [skill:dotnet-roslyn-analyzers] for authoring custom analyzers, [skill:dotnet-project-structure] for Directory.Build.props and solution layout, [skill:dotnet-csharp-coding-standards] for naming and formatting conventions enforced by EditorConfig rules.

---

## EditorConfig Overview

`.editorconfig` is the standard configuration file for controlling code style and analysis rule behavior in .NET projects. The .NET compiler (Roslyn) reads `.editorconfig` to determine:

- **Code style preferences** -- naming, formatting, expression-level patterns (IDE* rules)
- **Code quality rule severity** -- suppress, demote, or escalate CA* and IDE* diagnostics
- **Formatting rules** -- indentation, spacing, newlines

### Directory Hierarchy and Precedence

EditorConfig files apply hierarchically. The compiler searches upward from the source file to the filesystem root, merging settings from each `.editorconfig` found. **Closest file wins** -- a setting in `src/MyApp/.editorconfig` overrides the same setting in the repo root `.editorconfig`.

```
repo-root/
  .editorconfig              # Shared baseline (root = true)
  src/
    .editorconfig            # Overrides for production code
    MyApp.Api/
      .editorconfig          # API-specific overrides (if needed)
  tests/
    .editorconfig            # Relaxed rules for test projects
```

Set `root = true` in the topmost file to stop upward traversal. Without this, the editor traverses above the repo root into user or system-level EditorConfig files, producing non-reproducible behavior.

```ini
# repo-root/.editorconfig
root = true

[*.cs]
indent_style = space
indent_size = 4
```

### File Glob Patterns

EditorConfig sections use glob patterns to scope settings to specific files:

| Pattern | Matches |
|---------|---------|
| `[*.cs]` | All C# files |
| `[*.{cs,vb}]` | C# and Visual Basic files |
| `[**/test/**/*.cs]` | C# files under any `test` directory |
| `[Program.cs]` | Exact file name |

---

## Code Style Rules (IDE*)

IDE rules control code style preferences enforced by the Roslyn compiler and IDE. They are configured with `dotnet_style_*`, `csharp_style_*`, and `dotnet_diagnostic.IDE*.severity` entries.

### Key IDE Rule Categories

| Range | Category | Examples |
|-------|----------|----------|
| IDE0001-IDE0009 | Simplification | IDE0001 (simplify name), IDE0003 (remove `this.` qualification), IDE0005 (remove unnecessary using) |
| IDE0010-IDE0039 | Expression preferences | IDE0016 (throw expression), IDE0017 (object initializer), IDE0018 (inline variable), IDE0028 (collection initializer), IDE0034 (simplify default), IDE0039 (use local function) |
| IDE0040-IDE0069 | Modifier and access preferences | IDE0040 (add accessibility modifiers), IDE0044 (add readonly), IDE0062 (make local function static) |
| IDE0070-IDE0090+ | Pattern matching and modern syntax | IDE0071 (simplify interpolation), IDE0078 (use pattern matching), IDE0090 (simplify `new` expression) |
| IDE0100-IDE0180 | Additional simplification | IDE0130 (namespace match folder), IDE0160/IDE0161 (block vs file-scoped namespace) |
| IDE0200-IDE0260 | Lambda and method preferences | IDE0200 (remove unnecessary lambda), IDE0230 (use UTF-8 string literal) |
| IDE1005-IDE1006 | Naming rules | IDE1006 (naming rule violation) |

### Configuring Code Style Preferences

```ini
[*.cs]
# Expression-level preferences
csharp_style_expression_bodied_methods = when_on_single_line:suggestion
csharp_style_expression_bodied_properties = true:suggestion
csharp_style_expression_bodied_constructors = false:silent

# Pattern matching
csharp_style_prefer_pattern_matching = true:suggestion
csharp_style_prefer_switch_expression = true:suggestion
csharp_style_prefer_not_pattern = true:suggestion

# Null checking
csharp_style_prefer_null_check_over_type_check = true:suggestion
dotnet_style_coalesce_expression = true:suggestion
dotnet_style_null_propagation = true:suggestion

# var preferences
csharp_style_var_for_built_in_types = false:suggestion
csharp_style_var_when_type_is_apparent = true:suggestion
csharp_style_var_elsewhere = false:suggestion

# Namespace style (.NET 6+)
csharp_style_namespace_declarations = file_scoped:warning

# Using directives
csharp_using_directive_placement = outside_namespace:warning
dotnet_sort_system_directives_first = true
```

### IDE Rule Severity via dotnet_diagnostic

Each IDE rule can have its severity set independently:

```ini
[*.cs]
# Enforce removal of unnecessary usings as a build warning
dotnet_diagnostic.IDE0005.severity = warning

# Enforce file-scoped namespaces as a build error
dotnet_diagnostic.IDE0161.severity = error

# Demote new-expression simplification to suggestion
dotnet_diagnostic.IDE0090.severity = suggestion

# Disable this. qualification rule entirely
dotnet_diagnostic.IDE0003.severity = none
```

---

## Code Quality Rules (CA*)

CA rules detect design, performance, security, reliability, and usage issues. They are shipped with the .NET SDK and controlled by `AnalysisLevel`. For a complete CA rule category table and `AnalysisLevel` setup guidance, see [skill:dotnet-add-analyzers].

The main CA categories are: Design (CA1000s), Globalization (CA1300s), Interoperability (CA1400s), Maintainability (CA1500s), Naming (CA1700s), Performance (CA1800s), Reliability (CA2000s), Security (CA2100s, CA3xxx, CA5xxx), and Usage (CA2200s).

### CA Rule Severity Configuration

```ini
[*.cs]
# Suppress rules not applicable to your project type
dotnet_diagnostic.CA1062.severity = none          # Nullable handles parameter validation
dotnet_diagnostic.CA2007.severity = none          # ConfigureAwait not needed in ASP.NET Core apps

# Escalate important rules
dotnet_diagnostic.CA1822.severity = warning       # Mark members as static
dotnet_diagnostic.CA1848.severity = warning       # Use LoggerMessage delegates
dotnet_diagnostic.CA2016.severity = warning       # Forward CancellationToken

# Error-level for security rules
dotnet_diagnostic.CA2100.severity = error         # SQL injection review
dotnet_diagnostic.CA5350.severity = error         # Weak cryptographic algorithms
```

---

## Severity Levels

The five severity levels control how a diagnostic is reported:

| Severity | Build Output | IDE Squiggles | Error List | Fails Build (`TreatWarningsAsErrors`) |
|----------|-------------|---------------|------------|---------------------------------------|
| `error` | Yes (error) | Red | Error tab | Always |
| `warning` | Yes (warning) | Green | Warning tab | Yes (with `TreatWarningsAsErrors`) |
| `suggestion` | No | Gray dots | Message tab | No |
| `silent` | No | No | No | No (code fix available, not shown in build or Error List) |
| `none` | No | No | No | No (rule fully disabled) |

### Bulk Severity Configuration

Set default severity for entire categories:

```ini
[*.cs]
# Set all design rules to warning
dotnet_analyzer_diagnostic.category-Design.severity = warning

# Set all performance rules to error
dotnet_analyzer_diagnostic.category-Performance.severity = error

# Set all naming rules to suggestion
dotnet_analyzer_diagnostic.category-Naming.severity = suggestion
```

Valid category names for `dotnet_analyzer_diagnostic.category-{Category}.severity` include: `Design`,