继承自定义PageModel后，如何在父模型中实现重定向？

为了给布局传递导航栏所需变量（如用户名），我创建了继承自PageModel的LayoutModel类，代码如下：

public class LayoutModel : PageModel
{
  public NavbarDataModel Navbar { get; set; }
  public readonly FeaturesConfiguration _config;

  public LayoutModel(IHttpContextAccessor httpContextAccessor, IOptionsSnapshot<FeaturesConfiguration> options) : base()
  {
    Navbar = new NavbarDataModel();
    Navbar.UserName = httpContextAccessor.HttpContext.Session.GetString("username");

    if (string.IsNullOrEmpty(Navbar.UserName))
    {
        RedirectToPage("Login"); // 此处重定向不生效，因为没有返回值，但这是预期的行为
    }

    _config = options.Value;
  }
}

当前重定向未生效，若在父模型中创建IActionResult类型的OnGet()方法返回重定向，会与子类的OnGet()冲突导致程序报错。请问能否在父模型中完成该校验，无需在所有子类中重复实现？

可以通过以下几种方式实现父类统一校验，避免子类重复编写逻辑：

方法1：使用页面过滤器（Page Filter） #

创建一个实现IAsyncPageFilter的过滤器类，在过滤器中统一处理登录校验和导航栏数据注入，所有继承LayoutModel的页面会自动应用该逻辑，完全解耦页面模型与校验逻辑：

public class NavbarAuthFilter : IAsyncPageFilter
{
    private readonly IHttpContextAccessor _httpContextAccessor;
    private readonly IOptionsSnapshot<FeaturesConfiguration> _options;

    public NavbarAuthFilter(IHttpContextAccessor httpContextAccessor, IOptionsSnapshot<FeaturesConfiguration> options)
    {
        _httpContextAccessor = httpContextAccessor;
        _options = options;
    }

    public async Task OnPageHandlerSelectionAsync(PageHandlerSelectedContext context)
    {
        // 仅对继承LayoutModel的页面生效
        if (context.HandlerInstance is LayoutModel layoutModel)
        {
            var username = _httpContextAccessor.HttpContext.Session.GetString("username");
            layoutModel.Navbar = new NavbarDataModel { UserName = username };
            layoutModel._config = _options.Value;

            if (string.IsNullOrEmpty(username))
            {
                context.Result = new RedirectToPageResult("Login");
            }
        }
    }

    public Task OnPageHandlerExecutionAsync(PageHandlerExecutingContext context, PageHandlerExecutionDelegate next)
    {
        return next();
    }
}

在Program.cs中注册过滤器：

builder.Services.AddMvc(options =>
{
    options.Filters.Add<NavbarAuthFilter>();
});

方法2：父类重写OnPageHandlerExecuting方法 #

利用Razor Pages的生命周期方法OnPageHandlerExecuting，它会在页面处理方法（如OnGet）执行前触发，在父类中重写该方法完成校验：

public class LayoutModel : PageModel
{
    public NavbarDataModel Navbar { get; set; }
    public FeaturesConfiguration _config;
    private readonly IHttpContextAccessor _httpContextAccessor;
    private readonly IOptionsSnapshot<FeaturesConfiguration> _options;

    public LayoutModel(IHttpContextAccessor httpContextAccessor, IOptionsSnapshot<FeaturesConfiguration> options)
    {
        _httpContextAccessor = httpContextAccessor;
        _options = options;
    }

    public override void OnPageHandlerExecuting(PageHandlerExecutingContext context)
    {
        Navbar = new NavbarDataModel();
        Navbar.UserName = _httpContextAccessor.HttpContext.Session.GetString("username");
        _config = _options.Value;

        if (string.IsNullOrEmpty(Navbar.UserName))
        {
            context.Result = new RedirectToPageResult("Login");
            return;
        }

        base.OnPageHandlerExecuting(context);
    }
}

所有继承LayoutModel的页面会自动执行该校验逻辑，一旦设置context.Result，后续页面处理方法将不再执行，完全不会与子类OnGet冲突。

方法3：父类提供虚OnGet方法（需子类配合） #

如果偏好使用OnGet方法，可将父类OnGet设为虚方法，子类重写时调用基类方法即可：

public class LayoutModel : PageModel
{
    public NavbarDataModel Navbar { get; set; }
    public FeaturesConfiguration _config;
    private readonly IHttpContextAccessor _httpContextAccessor;
    private readonly IOptionsSnapshot<FeaturesConfiguration> _options;

    public LayoutModel(IHttpContextAccessor httpContextAccessor, IOptionsSnapshot<FeaturesConfiguration> options)
    {
        _httpContextAccessor = httpContextAccessor;
        _options = options;
    }

    public virtual IActionResult OnGet()
    {
        Navbar = new NavbarDataModel();
        Navbar.UserName = _httpContextAccessor.HttpContext.Session.GetString("username");
        _config = _options.Value;

        if (string.IsNullOrEmpty(Navbar.UserName))
        {
            return RedirectToPage("Login");
        }

        return Page();
    }
}

子类重写示例：

public class HomeModel : LayoutModel
{
    public HomeModel(IHttpContextAccessor httpContextAccessor, IOptionsSnapshot<FeaturesConfiguration> options) : base(httpContextAccessor, options)
    {
    }

    public override IActionResult OnGet()
    {
        var baseResult = base.OnGet();
        // 若基类返回重定向，直接返回；否则执行子类逻辑
        if (baseResult is RedirectToPageResult)
        {
            return baseResult;
        }

        // 子类自定义逻辑
        return Page();
    }
}

此方式需要子类主动调用基类方法，相对前两种灵活性稍弱。

内容的提问来源于stack exchange，提问作者Robert