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.