c# - Razor Pages OnPost 方法中的 Page() 方法有什么作用？

Question

ASP.NET 核心 2 我有以下页面模型代码

public class CreateModel : PageModel
{
    private readonly IItemService _itemService;
    public ItemCreateViewModel ItemModel { get; set; }

    public CreateModel(IItemService itemService)
    {
        _itemService = itemService;
    }
    // all related data downloaded from db, so all rendered correctly
    public async Task OnGet(ItemCreateViewModel itemModel)
    {
        ItemModel = await _itemService.GetCreateViewModel();
    }

    public async Task<IActionResult> OnPost(ItemCreateViewModel itemModel)
    {
        if (!ModelState.IsValid)
        {
            return Page();
        }
        var item = _itemService.CreateItem(itemModel);
        var image = await ImagesController.ProcessFormImage(itemModel.ImageUpload.UploadPhoto, item.Id, ModelState);
        if (!ModelState.IsValid)
        {
            //trouble is somewhere here 
            return Page();
        }
        await _itemService.SaveItemImage(image);
        _itemService.CreateItem(itemModel);
        return RedirectToPage("./Index");
    }
}

当我尝试发布错误的图像时，!ModelState.IsValid 返回 true 并调用 Page()。我得到 NullReferenceException:对象引用未设置为对象的实例。 在 Model.ItemModel.Categories 中，其中 Categories 是通常下载的几个选择列表之一来自 _itemService.GetCreateViewModel() 的数据库。 我不明白 Page() 发生了什么。为什么不使用表单中的旧数据？如何重新呈现出现验证错误的页面(我在 cshtml 中有 asp-validation-for)？我尝试了 this.ItemModel = itemModel，但我没有再获得 Null 引用，但没有看到任何验证错误，并且之前的选择列表值未保存。

Answer 1

return Page 与在正常的 MVC Controller 操作中调用 return View() 相同。

When the submitted form has validation errors (that are passed to the server), the OnPostAsync handler method calls the Page helper method. Page returns an instance of PageResult. Returning Page is similar to how actions in controllers return View. PageResult is the default return type for a handler method. A handler method that returns void renders the page.

更新页面模型属性以允许 View 绑定(bind)到模型。

public class CreateModel : PageModel {
    private readonly IItemService _itemService;

    public CreateModel(IItemService itemService) {
        _itemService = itemService;
    }

    [BindProperty]
    public ItemCreateViewModel ItemModel { get; set; }

    // all related data downloaded from db, so all rendered correctly
    public  async Task<IActionResult>  OnGetAsync() {
        ItemModel = await _itemService.GetCreateViewModel();
        return Page();
    }

    public async Task<IActionResult> OnPostAsync() {
        if (!ModelState.IsValid) {
            return Page();
        }
        var item = _itemService.CreateItem(ItemModel);
        var image = await ImagesController.ProcessFormImage(ItemModel.ImageUpload.UploadPhoto, item.Id, ModelState);
        if (!ModelState.IsValid) {                
            return Page();
        }

        await _itemService.SaveItemImage(image);
        _itemService.CreateItem(ItemModel);
        return RedirectToPage("./Index");
    }
}

引用 Introduction to Razor Pages in ASP.NET Core