Action Methods — Return Types, Parameters and HTTP Verbs

Action methods are the public methods on a controller that handle HTTP requests. By convention, any public method on a controller class is an action method. An action method receives input through model binding (route values, query strings, form fields), performs work (calls services, validates data), and returns an IActionResult that tells ASP.NET Core what response to generate. Understanding the full vocabulary of action method signatures and return types lets you handle every request scenario cleanly.

Action Method Signatures #

public class PostsController(IPostService service) : Controller
{
    // ── GET — no parameters ────────────────────────────────────────────────
    // GET /posts
    public async Task<IActionResult> Index()
    {
        var posts = await service.GetAllAsync();
        return View(posts);
    }

    // ── GET — route parameter ──────────────────────────────────────────────
    // GET /posts/details/42
    public async Task<IActionResult> Details(int id)
    {
        var post = await service.GetByIdAsync(id);
        return post is null ? NotFound() : View(post);
    }

    // ── GET — optional query string parameters ─────────────────────────────
    // GET /posts?page=2&pageSize=10&search=asp.net
    public async Task<IActionResult> Search(
        int    page     = 1,
        int    pageSize = 10,
        string search   = "")
    {
        var results = await service.SearchAsync(search, page, pageSize);
        return View(results);
    }

    // ── GET — display the create form ─────────────────────────────────────
    [HttpGet]
    public IActionResult Create()
        => View(new CreatePostViewModel());

    // ── POST — handle form submission ─────────────────────────────────────
    [HttpPost]
    [ValidateAntiForgeryToken]
    public async Task<IActionResult> Create(CreatePostViewModel model)
    {
        if (!ModelState.IsValid)
            return View(model);   // re-display form with validation errors

        await service.CreateAsync(model.ToRequest());
        TempData["Success"] = "Post created successfully!";
        return RedirectToAction(nameof(Index));
    }

    // ── POST — delete with PRG pattern ────────────────────────────────────
    [HttpPost]
    [ValidateAntiForgeryToken]
    public async Task<IActionResult> Delete(int id)
    {
        await service.DeleteAsync(id);
        TempData["Success"] = "Post deleted.";
        return RedirectToAction(nameof(Index));
    }

    // ── Action returning JSON (for AJAX) ──────────────────────────────────
    [HttpGet]
    public async Task<IActionResult> GetTags(string prefix)
    {
        var tags = await service.SearchTagsAsync(prefix);
        return Json(tags);   // returns application/json
    }
}

Action Method Return Types #

Common Mistakes #

Mistake 1 — Two actions with the same name and no HTTP verb attribute (ambiguity) #

❌ Wrong — framework cannot determine which Create() to call:

public IActionResult Create() { }        // GET — no attribute
public IActionResult Create(Model m) { } // POST — no attribute — ambiguous!

✅ Correct — annotate both: [HttpGet] on the first, [HttpPost] on the second.

Mistake 2 — Not re-displaying the form when ModelState is invalid #

❌ Wrong — redirecting away on validation failure loses ModelState and error messages:

if (!ModelState.IsValid) return RedirectToAction(nameof(Create)); // errors lost!

✅ Correct — return View(model) to re-render the form with ModelState errors inline.