C# Code Cleanup

Systematically identify and remove dead code, update deprecated APIs, and apply modern C# patterns to maintain a clean, maintainable codebase.

When to Use This Skill

Use this skill when you need to:

• Remove unused using directives and dead code
• Update deprecated .NET API usage
• Apply modern C# features (6.0+)
• Fix ReSharper/Rider warnings
• Modernize async/await patterns
• Optimize LINQ usage
• Clean up before code review

Trigger phrases: "cleanup C#", "remove dead code C#", "modernize C#", "fix ReSharper", ".NET refactor", "cleanup dotnet"

What This Skill Does

Cleanup Areas

1. Dead Code Removal

   • Unused using directives
   • Unused private methods and fields
   • Unreachable code
   • Redundant code

2. Style Compliance

   • ReSharper/Rider warnings
   • StyleCop rules
   • Naming conventions

3. Modernization

   • Nullable reference types
   • Pattern matching
   • Records
   • File-scoped namespaces

Instructions

Step 1: Run Analysis Tools

# Build and check for warnings
dotnet build

# Run code analysis
dotnet format --verify-no-changes

# Check for nullable warnings
dotnet build /warnaserror:nullable

Step 2: Identify Dead Code

// Unused using directives
// Remove any using statements not referenced

// Unused private members
// Remove private fields, properties, methods that are never accessed

// Unreachable code
// Remove code after return statements or in impossible branches

Step 3: Modernize Patterns

Null-Conditional and Null-Coalescing (C# 6+)

// Before
string name = user != null ? user.Name : "Unknown";
// After
string name = user?.Name ?? "Unknown";

// Before
if (handler != null)
{
    handler(this, args);
}
// After
handler?.Invoke(this, args);

// Null-coalescing assignment (C# 8+)
// Before
if (list == null)
{
    list = new List<string>();
}
// After
list ??= new List<string>();

String Interpolation (C# 6+)

// Before
string message = string.Format("Hello, {0}! You have {1} messages.", name, count);
// After
string message = $"Hello, {name}! You have {count} messages.";

// Raw string literals (C# 11+)
string json = """
    {
        "name": "John",
        "age": 30
    }
    """;

Expression-Bodied Members (C# 6+)

// Before
public string FullName
{
    get { return $"{FirstName} {LastName}"; }
}
// After
public string FullName => $"{FirstName} {LastName}";

// Before
public override string ToString()
{
    return $"{Name}: {Value}";
}
// After
public override string ToString() => $"{Name}: {Value}";

Pattern Matching (C# 7+)

// Before
if (obj is string)
{
    string s = (string)obj;
    Console.WriteLine(s.Length);
}
// After
if (obj is string s)
{
    Console.WriteLine(s.Length);
}

// Switch expressions (C# 8+)
// Before
string GetDescription(Status status)
{
    switch (status)
    {
        case Status.Active: return "Active";
        case Status.Inactive: return "Inactive";
        case Status.Pending: return "Pending";
        default: return "Unknown";
    }
}
// After
string GetDescription(Status status) => status switch
{
    Status.Active => "Active",
    Status.Inactive => "Inactive",
    Status.Pending => "Pending",
    _ => "Unknown"
};

// Property patterns (C# 8+)
if (person is { Age: >= 18, Name: not null })
{
    // Adult with name
}

Records (C# 9+)

// Before
public class User
{
    public string Name { get; init; }
    public string Email { get; init; }

    public User(string name, string email)
    {
        Name = name;
        Email = email;
    }

    // Plus Equals, GetHashCode, ToString...
}
// After
public record User(string Name, string Email);

// Record with additional members
public record User(string Name, string Email)
{
    public string DisplayName => $"{Name} <{Email}>";
}

File-Scoped Namespaces (C# 10+)

// Before
namespace MyApp.Services
{
    public class UserService
    {
        // ...
    }
}
// After
namespace MyApp.Services;

public class UserService
{
    // ...
}

Global Using Directives (C# 10+)

// GlobalUsings.cs
global using System;
global using System.Collections.Generic;
global using System.Linq;
global using System.Threading.Tasks;
global using Microsoft.Extensions.Logging;

Required Members (C# 11+)

// Before
public class User
{
    public User(string name, string email)
    {
        Name = name;
        Email = email;
    }

    public string Name { get; set; }
    public string Email { get; set; }
}
// After
public class User
{
    public required string Name { get; init; }
    public required string Email { get; init; }
}

Step 4: Optimize LINQ

// Avoid multiple enumerations
// Before
if (items.Count() > 0)
{
    var first = items.First();
}
// After
var itemList = items.ToList();
if (itemList.Count > 0)
{
    var first = itemList[0];
}

// Use Any() instead of Count() > 0
// Before
if (items.Count() > 0)
// After
if (items.Any())

// Use FirstOrDefault instead of Where().FirstOrDefault()
// Before
items.Where(x => x.Id == id).FirstOrDefault()
// After
items.FirstOrDefault(x => x.Id == id)

// Use method syntax for complex queries
// Before
(from u in users
 where u.IsActive
 select u.Name).ToList()
// After
users.Where(u => u.IsActive).Select(u => u.Name).ToList()

Step 5: Async/Await Best Practices

// Use ConfigureAwait(false) in library code
// Before
var result = await GetDataAsync();
// After (library code)
var result = await GetDataAsync().ConfigureAwait(false);

// Avoid async void (except event handlers)
// Before
public async void ProcessData()
// After
public async Task ProcessDataAsync()

// Use Task.WhenAll for parallel operations
// Before
var result1 = await GetData1Async();
var result2 = await GetData2Async();
// After
var results = await Task.WhenAll(GetData1Async(), GetData2Async());

// Use CancellationToken
public async Task<Data> GetDataAsync(CancellationToken cancellationToken = default)
{
    return await _client.GetAsync(url, cancellationToken);
}

// Avoid .Result and .Wait() - use async all the way
// Before
var result = GetDataAsync().Result;
// After
var result = await GetDataAsync();

Step 6: Clean Up Dependencies

# Remove unused NuGet packages
dotnet list package

# Check for outdated packages
dotnet list package --outdated

# Remove unused project references
dotnet list reference

Tools

• Visual Studio/Rider: Built-in code analysis
• ReSharper: Advanced code inspection
• StyleCop: Style checking
• Roslynator: Additional analyzers
• SonarLint: Quality analysis

Quality Checklist

• Unused using directives removed
• Unused private members removed
• Deprecated APIs updated
• Modern C# patterns applied
• Nullable reference types enabled/addressed
• LINQ optimized
• Async patterns correct
• Build succeeds without warnings
• Tests still pass

Common Issues and Solutions

Issue: Nullable reference type warnings

Solution: Enable nullable context and add appropriate null checks or annotations:

#nullable enable
public string? NullableName { get; set; }
public string NonNullName { get; set; } = string.Empty;

Issue: Async void methods

Solution: Change to async Task and ensure callers await:

// Before
public async void OnButtonClick() { ... }
// After
public async Task OnButtonClickAsync() { ... }

Issue: Multiple LINQ enumerations

Solution: Materialize the collection once:

var list = query.ToList();
if (list.Any())
{
    ProcessItems(list);
}

Related Skills

• code-review-quality - Code quality assessment
• security-review - Security analysis

---

Version: 1.0.0 Last Updated: December 2025 Based on: AI Templates code_cleanup/csharp_cleanup.md

Iterative Refinement Strategy

This skill is optimized for an iterative approach:

1. Execute: Perform the core steps defined above.
2. Review: Critically analyze the output (coverage, quality, completeness).
3. Refine: If targets aren't met, repeat the specific implementation steps with improved context.
4. Loop: Continue until the definition of done is satisfied.