c# - Example AJAX call back to an ASP.NET Core Razor Page

Question

I've found examples of have multiple handlers on a page and the associated naming convention (ie OnPostXXX) and 'asp-post-hanlder' tag helper. But how can I call one of these methods from an AJAX call.

I have an older example with a typical MVC view and controller but how does this work with a Razor Page?

For example if I take the base application and modify the About.cshtml page to the following:

@page
@model AboutModel
@{
    ViewData["Title"] = "About";
}
<h2>@ViewData["Title"]</h2>
<h3>@Model.Message</h3>

    <input type="button" value="Ajax test" class="btn btn-default" onclick="ajaxTest();"  />

@section Scripts {
<script type="text/javascript">
    function ajaxTest() {
        console.log("Entered method");
        $.ajax({
            type: "POST",
            url: '/About', // <-- Where should this point?
            contentType: "application/json; charset=utf-8",
            dataType: "json",
        error: function (xhr, status, errorThrown) {
            var err = "Status: " + status + " " + errorThrown;
            console.log(err);
        }
        }).done(function (data) {
            console.log(data.result);
        })
    }
</script>
}

And on the model page About.cshtml.cs

public class AboutModel : PageModel
{
    public string Message { get; set; }

    public void OnGet()
    {
        Message = "Your application description page.";
    }

    public IActionResult OnPost() {
        //throw new Exception("stop");
        return new JsonResult("");
    }
}

The OnPost is not called from the Ajax call.

Answer 1

Razor Pages automatically generates and validates Antiforgery tokens to prevent CSRF attacks. Since you aren't sending any token within your AJAX callback, the request fails.

To solve this problem you will have to:

1. Register the Antiforgery-Service
2. Add the token to your request
3. Add the antiforgery token to your page either by adding a <form> or by directly using the @Html.AntiForgeryToken HtmlHelper

1. Register the Antiforgery-Service in your Startup.cs

public void ConfigureServices(IServiceCollection services)
{
  services.AddRazorPages();
  services.AddAntiforgery(o => o.HeaderName = "XSRF-TOKEN");
}

2. Modify your AJAX callback

In the AJAX callback we add additional code to send the XSRF-TOKEN with our request header.

$.ajax({
    type: "POST",
    url: '/?handler=YOUR_CUSTOM_HANDLER', // Replace YOUR_CUSTOM_HANDLER with your handler.
    contentType: "application/json; charset=utf-8",

    beforeSend: function (xhr) {
      xhr.setRequestHeader("XSRF-TOKEN",
        $('input:hidden[name="__RequestVerificationToken"]').val());
    },

    dataType: "json"
}).done(function (data) {
  console.log(data.result);
})

3. Add the antiforgery token to your page

You can accomplish this by adding a <form>:

<form method="post">
    <input type="button" value="Ajax test" class="btn btn-default" onclick="ajaxTest();" />
</form>

or by using the @Html.AntiForgeryToken:

@Html.AntiForgeryToken()
<input type="button" value="Ajax test" class="btn btn-default" onclick="ajaxTest();" />

In both cases Razor Pages will automatically add a hidden input field which contains the antiforgery token once the page is loaded:

<input name="__RequestVerificationToken" type="hidden" value="THE_TOKEN_VALUE" />