Dotnet Exception Handling

.NET Exception Handling Quality Improvement

Purpose

Systematic investigation and remediation of .NET exception handling anti-patterns. Detects, documents, and fixes the 10 most common exception handling mistakes in .NET applications.

Use when: Preparing for production, code quality audits, security reviews, or onboarding to a .NET codebase.

Usage

# Investigation only (default)
/dotnet-exception-handling <path-to-dotnet-project>

# Filter by severity
/dotnet-exception-handling <project-path> --priority critical

# Auto-implement all fixes
/dotnet-exception-handling <project-path> --fix-all

Arguments:

• project-path: Directory containing .csproj or .sln (default: current directory)
• --priority: critical | high | medium | low | all (default: all)
• --fix-all: Implement fixes automatically (default: investigate only)

The 10 Common Mistakes

1. Catching Exception Too Broadly - Base Exception instead of specific types
2. Swallowing Exceptions Silently - Empty catch blocks hiding errors
3. Using throw ex; - Resets stack traces (use throw;)
4. Wrapping Everything in Try/Catch - Defensive coding clutter
5. Exceptions for Control Flow - Performance overhead for expected conditions
6. Forgetting to Await Async - Unhandled exceptions on background threads
7. Ignoring Background Task Exceptions - Fire-and-forget losing errors
8. Generic Exception Types - Vague new Exception() instead of specific types
9. Losing Inner Exceptions - Breaking exception chains
10. Missing Global Handler - No centralized error handling (stack traces exposed)

Detailed descriptions, detection patterns, and fix templates → see reference.md

Execution Workflow

Phase 1: Investigation (6 Steps)

Step 1: Project Detection

• Scan for .csproj, .sln files
• Identify project types (ASP.NET Core, worker services, libraries)
• Count C# files for scope estimation

Step 2: Parallel Analysis

• Deploy 5 specialized agents:
  • Background Worker Specialist
  • API Layer Specialist
  • Service Layer Specialist
  • Data Layer Specialist
  • Infrastructure Specialist

Step 3: Violation Detection

• Use rg -P (ripgrep PCRE mode) for pattern matching:

  # Mistake #1: Broad catches
  rg -P 'catch\s*\(Exception\b' --glob '*.cs'
  
  # Mistake #2: Empty catches
  rg -P 'catch[^{]*\{\s*(//[^\n]*)?\s*\}' --glob '*.cs'
  
  # Mistake #3: throw ex
  rg 'throw\s+ex;' --glob '*.cs'
  

• See reference.md for complete pattern list

Step 4: Severity Classification

• CRITICAL: Security (stack trace exposure, missing global handler)
• HIGH: Reliability (swallowed exceptions, broad catches)
• MEDIUM: Code quality (excessive try/catch)
• LOW: Style issues

Step 5: Findings Report

• Generate markdown with file:line references
• Code snippets + recommended fixes
• Priority-based roadmap

Step 6: Knowledge Capture

• Store in .claude/runtime/logs/EXCEPTION_INVESTIGATION_YYYY-MM-DD.md
• Update project memory

Phase 2: Development (If --fix-all)

Step 7: Orchestrate Default Workflow

• Create GitHub issue with findings
• Set up worktree for fixes
• Implement GlobalExceptionHandler, Result, etc.
• Write comprehensive tests (TDD)
• Three-agent review (reviewer, security, philosophy)

Step 8: Validation

• All tests pass
• Security: Zero stack traces
• Performance: <5ms p99 overhead

Quick Start Examples

Example 1: Investigation Only

/dotnet-exception-handling ./src/MyApi

Output: Investigation report with 23 violations (1 CRITICAL, 8 HIGH, 12 MEDIUM, 2 LOW)

Example 2: Fix Critical Only

/dotnet-exception-handling ./src/MyApi --priority critical --fix-all

Output: GitHub issue + PR implementing GlobalExceptionHandler + 15 tests

Example 3: Complete Fix

/dotnet-exception-handling ./src/MyApi --fix-all

Output: 23 violations fixed, 67 tests, PR ready (CI passing)

Core Architecture Patterns

GlobalExceptionHandler (IExceptionHandler)

Centralized exception-to-HTTP mapping for ASP.NET Core:

public class GlobalExceptionHandler : IExceptionHandler
{
    public async ValueTask<bool> TryHandleAsync(
        HttpContext httpContext,
        Exception exception,
        CancellationToken cancellationToken)
    {
        var (statusCode, title) = exception switch
        {
            ArgumentException => (400, "Invalid request"),
            NotFoundException => (404, "Not found"),
            ConflictException => (409, "Conflict"),
            _ => (500, "Internal server error")
        };

        httpContext.Response.StatusCode = statusCode;
        await httpContext.Response.WriteAsJsonAsync(new ProblemDetails
        {
            Status = statusCode,
            Title = title,
            Detail = statusCode >= 500
                ? "An error occurred"
                : exception.Message
        }, cancellationToken);

        return true;
    }
}

// Registration in Program.cs
builder.Services.AddExceptionHandler<GlobalExceptionHandler>();
app.UseExceptionHandler();

Benefits: Zero stack traces, consistent responses, no try/catch in controllers

Result Pattern

Railway-oriented programming for validation (no exceptions for expected conditions):

public Result<Order> ValidateOrder(CreateOrderDto dto)
{
    if (dto.Items.Count == 0)
        return Result<Order>.Failure("Order must have items");

    return Result<Order>.Success(new Order(dto));
}

// Controller usage
var result = await _service.ValidateOrder(dto);
return result.Match(
    onSuccess: order => Ok(order),
    onFailure: error => BadRequest(error)
);

Benefits: 100x faster than exceptions, explicit error handling, better composition

Complete implementations → see examples.md

Navigation Guide

When to Read Supporting Files

reference.md - Read when you need:

• Detailed descriptions of all 10 exception handling mistakes
• Complete detection patterns for ripgrep/grep
• Fix templates for each mistake type
• Security considerations (OWASP compliance, stack trace prevention)
• Severity classification reference
• Integration patterns (Azure SDK, EF Core, Service Bus)

examples.md - Read when you need:

• Before/after code examples for each mistake
• Complete working implementations (GlobalExceptionHandler, Result, DbContextExtensions)
• Real-world scenarios (order processing, payment systems)
• Unit and integration testing patterns
• Copy-paste ready code

patterns.md - Read when you need:

• Architecture decision trees (global handler vs try/catch, Result vs exceptions)
• Background worker exception handling patterns
• Azure SDK exception translation patterns
• EF Core concurrency and transaction patterns
• Performance benchmarks (Result vs exceptions)
• Anti-patterns to avoid

Workflow Integration

This skill orchestrates two canonical workflows:

1. Investigation Workflow (Phase 1): Scope → Explore → Analyze → Classify → Report → Capture
2. Default Workflow (Phase 2, if --fix-all): Requirements → Architecture → TDD → Implementation → Review → CI/CD

References

• Article: Top 10 Exception Handling Mistakes in .NET
• Microsoft: Best Practices for Exceptions
• ASP.NET Core: Handle Errors in Web APIs
• Investigation Workflow: investigation-workflow skill/recipe
• Default Workflow: default-workflow skill/recipe

Version History

• v1.0.0 (2026-02-10): Initial implementation based on CyberGym investigation (52 violations fixed, 87 tests)

Known Limitations

• Requires .NET 6+ for IExceptionHandler
• Result pattern targets C# 7.0+ (struct readonly, expression-bodied members)
• Patterns specific to ASP.NET Core (may not apply to class libraries)