A handler is a method invoked in response to a request or event. In ASP.NET Core, the name matters: a Razor Page uses page-model handlers such as OnGet and OnPost for HTTP requests, while a Blazor component connects an event such as a button click to a method with an @onclick attribute. section 23.2.2 section 33.4.1
For a Razor Page, follow the request from route to response:
- The route selects the page; the framework creates its page model and resolves constructor dependencies. OnGet handles a GET request, and OnGetAsync is its asynchronous counterpart. section 23.2.2 section 23.3
- A form POST invokes OnPost or OnPostAsync. Model binding supplies handler parameters from the request, including route values and form fields. A handler can update data and return a redirect rather than render the page immediately. section 23.4.3 section 8.2.5
- A page can have named alternatives. For example, asp-page-handler="Remove" on a POST form selects OnPostRemove; a handler query-string value of related selects OnGetRelatedAsync without the OnGet prefix or Async suffix. section 9.2.1 section 23.4.4
Edit a product price
An editor page loads a product in OnGetAsync and displays its price in a form. Submission sends a POST to the page’s URL; OnPostAsync receives the route id and submitted price, saves the change, then calls RedirectToPage(). The form includes an anti-forgery token: without the required field, the POST is rejected. section 23.4.3
Listing 23.17 The contents of the Editor.cshtml.cs file in the Pages folder
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;
using WebApp.Models;
namespace WebApp.Pages {
public class EditorModel : PageModel {
private DataContext context;
public Product? Product { get; set; }
public EditorModel(DataContext ctx) {
context = ctx;
}
public async Task OnGetAsync(long id) {
Product = await context.Products.FindAsync(id);
}
public async Task<IActionResult> OnPostAsync(long id,
decimal price) {
Product? p = await context.Products.FindAsync(id);
if (p != null) {
p.Price = price;
}
await context.SaveChangesAsync();
return RedirectToPage();
}
}
}
Compare the two page-model method signatures, their database operations, and the redirect returned after the POST. section 23.4.3
Blazor event handlers respond to events in a component rather than to a page’s HTTP method. With @onclick="IncrementCounter", a button click invokes the named method, which may receive MouseEventArgs. In the Blazor Server example, JavaScript forwards the event to the server; a changed component value is then reflected in the browser. Use a lambda when the handler also needs a value identifying which generated button was clicked. section 33.4.1
Listing 33.10 The contents of the Events.razor file in the Blazor folder
<div class="m-2 p-2 border">
<button class="btn btn-primary" @onclick="IncrementCounter">
Increment
</button>
<span class="p-2">Counter Value: @Counter</span>
</div>
@code {
public int Counter { get; set; } = 1;
public void IncrementCounter(MouseEventArgs e) {
Counter++;
}
}
The button attribute names the click handler; the method increments the value displayed beside it. section 33.4.1
Do not mistake a visible browser control for a working handler. An uncaught Blazor Server exception can leave controls usable in the browser while later event handlers stop responding; an error UI element can make that failure visible and offer a reload. section 34.4.2
