HTTP 400 on simple ASP.Net Core Razor page

Question

I'm trying to create a simple "Hello world" ASP.Net Core 3.1 app, but trying to POST a form, I keep getting:

This page isn’t working. If the problem continues, contact the site owner.

HTTP ERROR 400

Steps to reproduce:

1. MSVS2019 > New Project > ASP.Net Core Web Application, C#

2. DELETE the boilerplate, auto-generated code in Pages\*, substitute my own Index.cshtml and Index.cshtml.cs

3. Run program. I see my form.

4. Click "submit". I get HTTP 400.

Pages\Index.cshtml.cs

using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;

namespace HelloUploads.Pages
{
    public class IndexModel : PageModel
    { 
    
        public IActionResult OnGet()
        {
            return Page();
        }

        public IActionResult OnPost()
        {
            return RedirectToPage("./Index");
        }
    }
}

Index.cshtml

@page
@model HelloUploads.Pages.IndexModel
@{
    ViewData["Title"] = "Test";
}

<form method="post">
   <input type="submit" >
</form>

I'm sure the problem is simple. I've tried many variations, and I've been banging my head for several hours.

Q: What am I doing wrong? Any suggestions?

Answer 1

Your problem is probably anti-forgery tokens. Add @addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers to the page

@page
@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
@model HelloUploads.Pages.IndexModel
@{
    ViewData["Title"] = "Test";
}

<form method="post">
   <input type="submit" >
</form>

on the code behind add [ValidateAntiForgeryToken] attribute

[ValidateAntiForgeryToken]
public IActionResult OnPost()
{
   return RedirectToPage("./Index");
}

Hope it works!