Controllers & Action Methods

1. What is a Controller?

A controller is a class that:

• Handles HTTP requests
• Calls the data/service layer
• Returns a response (view, JSON, file, status code, redirect, etc.)

Think: request coordinator

Rules

• Class name ends with Controller
• Inherits from Controller (MVC) or ControllerBase (API)
• Lives in the Controllers folder (by convention)

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2. Action Methods

Action methods are:

• public methods inside controllers
• Each handles one request
• Selected via routing

They usually return IActionResult.

public IActionResult Index()
{
    return Content("Hello");
}

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3. Attribute Routing

Routes are defined directly on controllers/actions.

[Route("home")]
public class HomeController : Controller
{
    [HttpGet("")]
    public IActionResult Index() => Content("Home");

    [HttpGet("about")]
    public IActionResult About() => Content("About");
}

Enable controllers:

builder.Services.AddControllers();
app.MapControllers();

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4. IActionResult (Key Concept)

IActionResult lets an action return different responses based on logic.

One action → many possible results

Examples

public IActionResult Details(int id)
{
    if (id <= 0)
        return BadRequest("Invalid ID");

    var item = repo.Find(id);
    if (item == null)
        return NotFound();

    return View(item);
}

This action might return:

• BadRequest if the ID is invalid
• NotFound if the item isn't found
• View if the item exists

Another example:

public IActionResult Download(int id)
{
    var file = fileService.GetFile(id);
    if (file == null)
        return NotFound();

    return File(file.Bytes, file.ContentType, file.FileName);
}

This action returns either a file download or a 404 "Not Found" error depending on the logic.

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5. Common Action Results (Know These)

ContentResult (raw text / HTML)

return Content("<h1>Hello</h1>", "text/html");

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JsonResult (JSON response)

return Json(new { id = 1, name = "Book" });

(Content-Type auto = application/json)

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File Results

return File("/sample.pdf", "application/pdf");          // from wwwroot
return PhysicalFile(@"C:\file.pdf", "application/pdf");
return File(bytes, "application/pdf");                  // in-memory

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Status Code Results

return Ok(); // 200
return BadRequest("Invalid"); // 400
return NotFound(); // 404
return Unauthorized(); // 401
return StatusCode(403); // custom

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Redirect Results

return Redirect("/home"); // URL
return RedirectToAction("Index", "Home"); // action
return LocalRedirect("/products/10"); // local only

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6. Typical Action with Validation

[HttpGet("book")]
public IActionResult GetBook(int id)
{
    if (id <= 0)
        return BadRequest("Invalid id");

    if (id > 1000)
        return NotFound();

    return File("/sample.pdf", "application/pdf");
}

One action, multiple IActionResults

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7. When to Use What (Mental Map)

Need	Use
Plain text / HTML	Content()
API data	Json()
File download	File()
Validation error	BadRequest()
Missing resource	NotFound()
Success (has data)	Ok()
Success (no content)	NoContent()
Created (new resource)	Created()
Unauthorized (auth required)	Unauthorized()
Forbidden (no permission)	StatusCode(403)
Server error	StatusCode(500)
Navigate elsewhere	Redirect*()

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