Modernizing Legacy .NET Applications with GitHub Copilot: A Practical Guide

public string GetStatus(bool isActive)
{
    if (isActive == true)
    {
        return "Active";
    }
    else
    {
        return "Inactive";
    }
}

GitHub Copilot may suggest a cleaner version:

public string GetStatus(bool isActive)
{
    return isActive ? "Active" : "Inactive";
}

While the functionality remains the same, the updated code is simpler and easier to read.

Step 3: Update Deprecated APIs

Older applications often rely on APIs or libraries that are no longer recommended.

GitHub Copilot can help identify outdated code and suggest modern alternatives.

Examples include:

• Updated HTTP client usage

• New configuration APIs

• Modern dependency injection patterns

• Improved asynchronous programming

• Current logging practices

Always verify suggested changes against official documentation before applying them.

Step 4: Generate Unit Tests

Many legacy applications have limited or no automated testing.

Before making significant changes, begin adding unit tests.

Suppose you have the following method:

public int Multiply(int a, int b)
{
    return a * b;
}

Copilot can help generate a simple unit test:

[TestMethod]
public void Multiply_ReturnsExpectedResult()
{
    var result = new Calculator().Multiply(4, 5);

    Assert.AreEqual(20, result);
}

Adding tests helps ensure that existing functionality continues to work after refactoring.

Step 5: Improve Code Quality

As you modernize the application, look for opportunities to improve readability and maintainability.

GitHub Copilot may recommend:

• Better variable names

• Smaller methods

• Cleaner conditional logic

• Consistent formatting

• Improved exception handling

• Modern C# language features

These incremental improvements make the codebase easier for the entire team to maintain.

Practical Example

Imagine your company has an ASP.NET application built several years ago.

The project contains:

• Hundreds of controllers

• Repeated validation logic

• Older authentication code

• Minimal test coverage

Using GitHub Copilot, developers can:

• Refactor duplicated methods

• Suggest cleaner implementations

• Generate missing unit tests

• Improve naming conventions

• Recommend updated APIs

• Reduce repetitive coding tasks

Instead of spending weeks rewriting routine code manually, the team can focus on validating business logic and delivering new features.

Best Practices

To modernize legacy applications successfully, follow these recommendations:

• Understand the codebase before making changes.

• Modernize the application in small phases.

• Review every AI-generated suggestion carefully.

• Run automated tests after each update.

• Use source control to track every change.

• Keep dependencies up to date.

• Perform code reviews before merging changes.

• Benchmark performance before and after modernization.

These practices help reduce risk and maintain application stability throughout the modernization process.

Conclusion

Modernizing a legacy .NET application is a gradual process that requires careful planning, testing, and continuous improvement. GitHub Copilot can significantly reduce the effort involved by assisting with code refactoring, updating deprecated APIs, generating unit tests, and improving overall code quality.

While AI can accelerate development, developers should always review generated code, validate business logic, and ensure changes align with project requirements. By combining GitHub Copilot with modern development practices, teams can successfully transform legacy .NET applications into cleaner, more maintainable, and future-ready solutions without the need for a complete rewrite.