Modernizing Legacy C# Code: Refactoring Strategies Every Developer Should Know

Legacy codebases are everywhere. They keep businesses running but often hold back agility, scalability, and developer productivity. As senior developers and architects, we have a responsibility to refactor wisely —not just for today's delivery, but for long-term maintainability. If we don't refactor or upgrade on time, we are compromising the efficiency and quality of the solution. Often, developers are held back because of a tight project timeline and budget. Although we cannot overcome those situations entirely, AI can help us to refactor swiftly and save time and cost both.

Let's explore how we can do the refactoring with AI tools like GitHub Copilot, ChatGPT, or similar AI tools.

Below are practical examples of how legacy C# code can be modernized using best practices that improve readability, scalability, and performance.

Example 1. Simplifying Loops with LINQ

Legacy code often relies on manual loops and conditional checks:

Before

  
public List<Order> GetOrders(int customerId)
{
    var orders = new List<Order>();
    foreach (var o in _dbContext.Orders)
    {
        if (o.CustomerId == customerId && o.IsActive)
        {
            orders.Add(o);
        }
    }
    return orders;
}

Refactored

public async Task<List<Order>> GetActiveOrdersAsync(int customerId) =>
    await _dbContext.Orders
        .Where(o => o.CustomerId == customerId && o.IsActive)
        .ToListAsync();

Benefits

Async-ready for non-blocking database operations
Clearer intent with fewer lines of code
Easier to maintain and extend

Example 2. Declarative Filtering

Legacy code often relies on manual loops and conditional checks:

Before

public List<int> GetEvenNumbers(List<int> numbers)
{
    var evens = new List<int>();
    foreach (var n in numbers)
    {
        if (n % 2 == 0)
        {
            evens.Add(n);
        }
    }
    return evens;
}

Refactored

public List<int> GetEvenNumbers(List<int> numbers) =>
    numbers.Where(n => n % 2 == 0).ToList();

Cleaner, fewer lines, easier to test.

Example 3. Modern String Handling

String concatenation can be error-prone and harder to read.

Before

public string GetFullName(string first, string last)
{
    return first + " " + last;
}

After

public string GetFullName(string first, string last) =>
    $"{first} {last}";

Benefits

Modern, readable syntax
Less risk of subtle bugs

Example 4. Pattern Matching for Switch Statements

Traditional switch statements can be verbose. C# pattern matching simplifies them:

Before

public string GetStatus(int code)
{
    switch (code)
    {
        case 200: return "OK";
        case 404: return "Not Found";
        case 500: return "Error";
        default: return "Unknown";
    }
}

After

public string GetStatus(int code) =>
    code switch
    {
        200 => "OK",
        404 => "Not Found",
        500 => "Error",
        _   => "Unknown"
    };

Concise and modern C# syntax.

Example 5. Proper Logging Practices

Many legacy systems still use Console.WriteLine for error handling, which is not suitable for production:

Before

public void Process()
{
    try
    {
        DoWork();
    }
    catch (Exception ex)
    {
        Console.WriteLine("Error: " + ex.Message);
    }
}

After

public void Process()
{
    try
    {
        DoWork();
    }
    catch (Exception ex)
    {
        _logger.LogError(ex, "Error while processing");
    }
}

Structured, production-grade logging, instead of Console.WriteLine.

Example 6. Blocking HTTP Calls → `IHttpClientFactory` (Best Practice)

Before

public string GetData()
{
    var client = new HttpClient();
    var response = client.GetAsync("https://api.example.com").Result;
    return response.Content.ReadAsStringAsync().Result;
}

After

Service Implementation:

public class ApiService
{
    private readonly IHttpClientFactory _httpClientFactory;

    public ApiService(IHttpClientFactory httpClientFactory)
    {
        _httpClientFactory = httpClientFactory;
    }

    public async Task<string> GetDataAsync()
    {
        var client = _httpClientFactory.CreateClient();
        var response = await client.GetAsync("https://api.example.com");
        response.EnsureSuccessStatusCode();
        return await response.Content.ReadAsStringAsync();
    }
}

Non-blocking, testable, and production-ready with proper resource management.

Key Takeaways

Modern C# features (LINQ, interpolation, pattern matching, async/await) make legacy systems cleaner and easier to maintain.
Proper logging improves observability and debugging.
Async-first and IHttpClientFactory ensure scalability in modern applications.
Small, incremental refactors reduce technical debt without costly rewrites.

Legacy code doesn’t have to slow you down. With thoughtful refactoring, you can transform existing systems into modern, maintainable, and future-proof solutions.

Example 1. Simplifying Loops with LINQ

Example 2. Declarative Filtering

Example 3. Modern String Handling

Example 4. Pattern Matching for Switch Statements

Example 5. Proper Logging Practices

Example 6. Blocking HTTP Calls → IHttpClientFactory (Best Practice)

Key Takeaways

Example 6. Blocking HTTP Calls → `IHttpClientFactory` (Best Practice)