Error Handling

ModelState

Use this when:

• You're returning the same view after validation fails (e.g. form errors)
• You want to show inline validation errors next to form fields
• You want to use @Html.ValidationSummary() or @Html.ValidationMessageFor(...) in Razor
• Doesn't persist across redirects
• Works only for the current request

if (!ModelState.IsValid)
    return View(model); // stays on same page

TempData

Use this when:

• You need to pass error/success messages across redirects
• You're doing Post-Redirect-Get (PRG) pattern
• You want to show a status banner or alert box on the next view
• Persists for one request after redirect
• Uses cookies or session under the hood
• Works in .NET Core (via ITempDataDictionary)

TempData["ErrorMessage"] = "Something went wrong.";
return RedirectToAction("List");

@if (TempData["ErrorMessage"] != null)
{
    <div class="alert alert-danger">
        @TempData["ErrorMessage"]
    </div>
}