Status Codes & Errors

Connecting to C#

When you write a C# method, it usually returns a value. But what if it fails? You might throw an exception: throw new Exception("Not found");

In REST APIs, we don't just throw exceptions. We return a specific HTTP Status Code so the client knows exactly what went wrong without crashing.

Common Status Codes (Cheat Sheet)

• 200 OK: The request was successful.
• 201 Created: A new resource was successfully created (used after POST).
• 400 Bad Request: The client sent invalid data.
• 401 Unauthorized: You need to log in.
• 404 Not Found: The URL or data doesn't exist.
• 500 Internal Server Error: The server crashed (a bug in your C# code).

Example: Returning Status Codes in ASP.NET Core

ASP.NET Core provides built-in helper methods to return these codes:

[HttpGet("{id}")]
public IActionResult GetUser(int id)
{
    var user = _database.Users.Find(id);

    if (user == null)
    {
        // Returns HTTP 404
        return NotFound("User does not exist"); 
    }

    if (id < 0)
    {
        // Returns HTTP 400
        return BadRequest("ID cannot be negative"); 
    }

    // Returns HTTP 200 with the user object
    return Ok(user); 
}

Handling Server Errors (Try-Catch)

[HttpPost]
public IActionResult CreateUser(User user)
{
    try
    {
        _database.Users.Add(user);
        _database.SaveChanges();
        return CreatedAtAction(nameof(GetUser), new { id = user.Id }, user);
    }
    catch (Exception ex)
    {
        // Log the error, and return 500
        return StatusCode(500, "An internal server error occurred.");
    }
}