400错误，因为Razor Pages被设计为可以自动防御跨站点请求伪造(CSRF/XSRF)攻击.您无需编写任何其他代码.由于我注意到您已经在页面上添加了FormTagHelper，因此Razor Pages中会自动包含防伪令牌的生成和验证.因此，只需修改ajax以在标头中包含令牌:

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

并配置防伪服务以查找X-CSRF-TOKEN标头:

  services.AddAntiforgery(o => o.HeaderName ="XSRF-TOKEN"); 

下面的文章供您参考:

在ASP中处理Ajax请求.NET核心剃刀页面

然后要发布到ajax中的OnPostUploadImage函数，请将url修改为 @ Url.Page("，"UploadImage"):

  $.ajax({url:'@ Url.Page("，"UploadImage")'，beforeSend:函数(xhr){xhr.setRequestHeader("XSRF-TOKEN"，$('input:hidden [name ="__ RequestVerificationToken"]').val());}，contentType:false，processData:否，类型:"post"，成功:功能(){alert('由jQuery上传');}}); 

I am trying to upload Image using Ajax method in asp.net core Razor pages, I am main form in will all input fields are kept and with the form for Fileupload i am also added addition button which is for file upload using Ajax, When i hit the

<input type="submit" value="Upload Image" asp-page-handler="OnPostUploadImage" id="btnUploadImage" />

i want it to call OnPostUploadImage method in pageModel file but it alway goes to default OnPost method. when i rename the OnPost to OnPost2 nothing happend..

How can i call OnPostUploadImage() on button btnUploadImage click event.

When i hit click btnUploadImage it generates following error on browser console

Error in FF

XML Parsing Error: no root element found Location: https://localhost:44364/Admin/News/NewsCreate?handler=OnPostUploadImage Line Number 1, Column 1:

Error in Chrome

jquery.min.js:2 POST https://localhost:44364/Admin/News/NewsCreateMultipleSubmit?handler=OnPostUpLoadImage 400 (Bad Request)

event though path looks fine but it cant find it as per error message

 @page
    @model BookListRazor.Pages.Admin.News.NewsCreateModel
    @{
        ViewData["Title"] = "News Create";
        Layout = "~/Pages/Shared/_LayoutAdmin.cshtml";
    }


     <div class="border container" style="padding:30px;">
            <form method="post" enctype="multipart/form-data">
                <div class="text-danger" asp-validation-summary="ModelOnly"></div>
                <input hidden asp-for="News.NewsImage" />
                <input id="fileName" hidden value="" />

                <div class="form-group row">
                    <div class="col-2">
                        <label asp-for="News.NewsHeading"></label>
                    </div>
                    <div class="col-10">
                        <input asp-for="News.NewsHeading" class="form-control" />

                    </div>
                    <span asp-validation-for="News.NewsHeading" class="text-danger"></span>
                </div>


                <div class="form-group row">
                    <div class="col-2">
                        <label asp-for="News.NewsImage"></label>
                    </div>
                    <div class="col-10">
                        @*<input asp-for="News.NewsImage" type="file" class="form-control" id="NewsImage">*@
                        @*Photo property type is IFormFile, so ASP.NET Core automatically creates a FileUpload control  *@
                        <div class="custom-file">
                            <input asp-for="NewsImageForUpload" class="custom-file-input form-control">
                            <label class="custom-file-label">Click here to change photo</label>
                                                <input type="submit" value="Upload Image" asp-page-handler="OnPostUploadImage" id="btnUploadImage" />

                        </div>
                    </div>
                    <span id="imageStatus" class="text-danger"></span>
                    <span asp-validation-for="NewsImageForUpload" class="text-danger"></span>
                </div>


                <div class="form-group row">
                    <div class="col-3 offset-3">
                        <input id="btnSave" type="submit" value="Create" class="btn btn-primary form-control" />
                    </div>
                    <div class="col-3">
                        <a asp-page="Index" class="btn btn-success form-control">Back to List</a>
                    </div>
                </div>
            </form>
            <script src="~/lib/jquery/dist/jquery.min.js"></script>
            <script src="https://cdn.ckeditor.com/4.14.0/full/ckeditor.js"></script>
            <script src="https://cdnjs.cloudflare.com/ajax/libs/jqueryui/1.12.1/jquery-ui.min.js"></script>
            <script>

                $(document).ready(function () {

                    $("#btnSave").addClass("disable-button");

                    $('.custom-file-input').on("change", function () {
                        var fileName = $(this).val().split("\\").pop();
                        $(this).next('.custom-file-label').html(fileName);
                        $("#fileName").val(fileName);
                        $("#btnSave").removeClass("disable-button");
                    });

                    if ($("#fileName").val() == "") {
                        //alert("Select Image...");;
                    }
                });

            </script>
        </div>
        @section Scripts{
            <partial name="_ValidationScriptsPartial" />

            <script>
 $(function () {
                $('#btnUploadImage').on('click', function (evt) {
                    console.log("btnUploadImage");
                    evt.preventDefault();
                    console.log("btnUploadImage after evt.preventDefault()");
                    $.ajax({
                        url: '@Url.Page("", "OnPostUploadImage")',
                        //data: new FormData(document.forms[0]),
                        contentType: false,
                        processData: false,
                        type: 'post',
                        success: function () {
                            alert('Uploaded by jQuery');
                        }
                    });
                });
            });
            </script>
            }

.cs file CODE

public async Task<IActionResult> OnPost()
        {
            if (ModelState.IsValid)
            {
                return Page();
            }
            else
            {

                return Page();
            }
        }

        public IActionResult OnPostUploadImage()
        {

            //Some code here
        }

400 error because Razor Pages are designed to be automatically protected from cross-site request forgery (CSRF/XSRF) attacks. You don’t have to write any additional code. Antiforgery token generation and validation is automatically included in Razor Pages since i notice you have add FormTagHelper on page . So just modify your ajax to include token in header :

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

And configure the antiforgery service to look for the X-CSRF-TOKEN header :

services.AddAntiforgery(o => o.HeaderName = "XSRF-TOKEN");

Below article is for your reference :

Handle Ajax Requests in ASP.NET Core Razor Pages

And to post to OnPostUploadImage function in ajax , modify url to @Url.Page("", "UploadImage"):

$.ajax({
    url: '@Url.Page("", "UploadImage")',
    beforeSend: function (xhr) {
        xhr.setRequestHeader("XSRF-TOKEN",
        $('input:hidden[name="__RequestVerificationToken"]').val());
    },
    contentType: false,
    processData: false,
    type: 'post',
    success: function () {
        alert('Uploaded by jQuery');
    }
});