Why This Matters

.NET developers get strong Copilot support — C# is well-represented in training, and Visual Studio's integration is deep. Knowing C# 12+ features and nullable refs unlocks the best suggestions.

Concept Explained

C# nullable reference types, records, and pattern matching give Copilot rich context. ASP.NET attributes ([ApiController], [Route]) trigger pattern-aware suggestions. xUnit is the default test framework.

How It Works

Nullable reference types (enabled in csproj) let Copilot suggest null checks. Records and pattern matching enable functional-style suggestions. ASP.NET attributes drive controller suggestions. /tests detects xUnit from packages.

Step-by-Step Tutorial

1. Enable nullable refs

2. Use records

3. Use pattern matching

4. ASP.NET attributes

5. xUnit with /tests

Real-World Example

A .NET developer defined `public record User(string Id, string Name, string Email);` and an `[ApiController]` class. Copilot suggested a complete CRUD controller with proper DI, validation, and IActionResult returns. With nullable refs enabled, it also added null checks.

Example Prompts / Commands / Code

public record User(string Id, string Name, string Email);

// Copilot suggests:
public UserDto ToDto(User user) =>
    new(user.Id, user.Name, user.Email.ToLowerInvariant());

[ApiController]
[Route("api/[controller]")]
public class UsersController : ControllerBase {
    // Copilot suggests:
    //   [HttpGet]
    //   public async Task<ActionResult<IEnumerable<User>>> List() { ... }
    //
    //   [HttpGet("{id}")]
    //   public async Task<ActionResult<User>> Get(string id) { ... }
    //
    //   [HttpPost]
    //   public async Task<ActionResult<User>> Create([FromBody] User user) { ... }
}

Common Mistakes

• Not enabling nullable reference types — Copilot omits null checks.
• Using old-style classes instead of records.
• Forgetting [ApiController] — Copilot can't suggest controller patterns.
• Mixing xUnit and NUnit — Copilot may guess wrong.

Best Practices

• Enable nullable reference types in csproj.
• Use records for immutable data.
• Annotate controllers with [ApiController].
• Ensure xUnit in csproj for /tests.
• Use C# 12+ features (pattern matching, primary constructors).

Troubleshooting

Practical Exercise

Frequently Asked Questions

Further Reading

• Copilot in Visual Studio

Official References

• Copilot in Visual Studio

Related lessons: BE-39, BE-41