Remote validation on file input in .NET Core MVC

Question

I have implemented remote validation on a text property without issue, and the remote validation fires correctly, however I am trying to also add validation for an image upload before the form is submitted. Ideally I would like the file to be validated remotely after selecting the local file. Is there any way to get this working? I cannot see what event actually triggers the remote validation, but I guess it is something that isn't fired in an input element for a file. Any suggestions? Thanks

Have tried the following:

ViewModel:

[Remote(action: "ValidatePhoto", controller: "Photos", ErrorMessage = "Photo width and height must be at least 300 pixels")]
public IFormFile Photo { get; set; }

PhotosController:

public IActionResult ValidatePhoto(IFormFile Photo)
{
    if (Validation.MeetsMinimumImageDimensions(Photo))
    {
        return Json(true);
    }
    return Json(false);
}

View:

<input id="photoUpload" asp-for="Photo" type="file" accept="image/*" name="Photo" style="display:none" />
<span id="submitError" asp-validation-for="Photo" class="text-danger"></span>

Answer 1

For built-in Remote valition, it will Get or Post methods to send request with content in fields. It will not submit file object with form-data.

Try to implment your own ajax request to back-end method. like below:

    @section Scripts {
        @{await Html.RenderPartialAsync("_ValidationScriptsPartial");}

        <script type="text/javascript">
            $(document).ready(function () {
                document.querySelector("form input[type=file]").onchange = function (event) {
                    var path = $(this).attr("data-val-remote-url");
                    var field = $(this).attr("id");
                    var errormsg = $(this).attr("data-val-remote");
                    var formdata = new FormData();
                    var file = document.getElementById(field).files[0];
                    formdata.append(field, file)
                    $.ajax({
                        url: path,
                        type: 'POST',
                        data: formdata,
                        processData: false,
                        contentType: false,
                        success: function (data) {
                            if (data == false) {
                                var msg = $("[data-valmsg-for=" + field + "]").first();
                                msg.html(errormsg)
                            }
                        }
                    });
                    return false;
                };
            });
        </script>
    }

With this method, it will send two requests to the action with Get method for built-in validation and Post method for our custom request, try change your validation method like

    public IActionResult ValidatePhoto(IFormFile Photo)
    {
        if (Request.Method == HttpMethods.Get)
        {
            return Json(true);
        }
        if (Validation.MeetsMinimumImageDimensions(Photo))
        {
            return Json(true);
        }
        return Json(false);
    }