大家好，我有下拉列表的問題。我第一次將數據從數據庫綁定到 Dropdownlist 并且它工作正常。當我第二次提交表單（單擊注冊按鈕）時，它會自動返回空值。下拉列表第一次填充數據時第二次按下創建按鈕后，下拉列表為空控制器代碼：// GET method[HttpGet][Authorize]public IActionResult Register(string returnUrl = null){    ViewData["RoleId"] = new SelectList(_MedicalCenterOrdinanceContext.AspNetRoles,"Id","Name");    return View();}// POST method[HttpPost][Authorize][ValidateAntiForgeryToken]public async Task<IActionResult> Register([Bind("Username,NameAndTitle,Password,RoleId")] RegisterViewModel model, string returnUrl = null){    ViewData["ReturnUrl"] = returnUrl;    if (ModelState.IsValid)    {        var user = new ApplicationUser        {            UserName = model.Username,            NameAndTitle = model.NameAndTitle,        };        var result = await _userManager.CreateAsync(user, model.Password);        if (result.Succeeded)        {            var role = _userRoles.GetRole(model.RoleId);            var userAddedToRole = await _userManager.AddToRoleAsync(user, role.Name);            if (userAddedToRole.Succeeded)            {                _logger.LogInformation("User created a new account with password.");                TempData["RegisterUserPassed"] = "Succesfully created account";            }                     }        else        {            model.InvalidCredentials = "This Username already exist !";            TempData["RegisterUserFailed"] = "This Username already exist";        }    }    // Redisplay form    ModelState.Clear();    return View();}查看代碼：<div class="input-group">     <span class="input-group-addon"><i class="glyphicon glyphicon-book"></i></span>     <select asp-for="RoleId" class="form-control" asp-items="@ViewBag.RoleId"></select></div><br /><button type="submit" class="col-lg-12 col-md-12 col-sm-12 col-xs-12 btn btn-lg btn-primary">Create</button><h4 hidden class="col-lg-12 col-md-12 col-sm-12 col-xs-12 text-danger " text-danger">@Html.DisplayFor(model => model.InvalidCredentials)</h4></form></div>