删除下拉列表中的条目导致“值”不能为空.参数名称:item "
本文关键字：参数 quot item 不能 下拉列表 删除 | 更新日期: 2023-09-27 18:16:29
我有两个表单:一个用于为用户分配角色，另一个用于从用户删除角色。视图和控制器非常相似。它们是(表单本身):
AssignRole.cshtml
@using (Html.BeginForm("AssignRole", "User", new { ReturnUrl = ViewBag.ReturnUrl }, FormMethod.Post, new { @class = "form-horizontal", role = "form" }))
{
@Html.AntiForgeryToken()
@Html.ValidationSummary(true, "", new { @class = "text-danger" })
@Html.HiddenFor(m => m.UserID)
<div class="form-group">
@Html.LabelFor(m => m.UserName, new { @class = "col-md-2 control-label" })
<div class="col-md-10">
@Html.TextBoxFor(m => m.UserName, new { @class = "form-control", @readonly = "readonly" })
@Html.ValidationMessageFor(m => m.UserName, "", new { @class = "text-danger" })
</div>
</div>
<div class="form-group">
@Html.LabelFor(m => m.RoleName, new { @class = "col-md-2 control-label" })
<div class="col-md-10">
@Html.DropDownListFor(m => m.RoleName, new SelectList(Model.UnassignedRoles, "Value", "Text"), Resources.DropdownSelect, new { @class = "form-control" })
</div>
</div>
<div class="form-group">
<div class="col-md-offset-2 col-md-10">
<input type="submit" value="@Resources.Assign" class="btn btn-default" />
</div>
</div>
}
RemoveRole.cshtml
@using (Html.BeginForm("RemoveRole", "User", new { ReturnUrl = ViewBag.ReturnUrl }, FormMethod.Post, new { @class = "form-horizontal", role = "form" }))
{
@Html.AntiForgeryToken()
@Html.ValidationSummary(true, "", new { @class = "text-danger" })
@Html.HiddenFor(m => m.UserID)
<div class="form-group">
@Html.LabelFor(m => m.UserName, new { @class = "col-md-2 control-label" })
<div class="col-md-10">
@Html.TextBoxFor(m => m.UserName, new { @class = "form-control", @readonly = "readonly" })
@Html.ValidationMessageFor(m => m.UserName, "", new { @class = "text-danger" })
</div>
</div>
<div class="form-group">
@Html.LabelFor(m => m.RoleName, new { @class = "col-md-2 control-label" })
<div class="col-md-10">
@Html.DropDownListFor(m => m.RoleName, new SelectList(Model.AssignedRoles, "Value", "Text"), Resources.DropdownSelect, new { @class = "form-control" })
</div>
</div>
<div class="form-group">
<div class="col-md-offset-2 col-md-10">
<input type="submit" value="@Resources.Remove" class="btn btn-default" />
</div>
</div>
}
最后，这里是控制器和它们使用的动作:
UserController.cs
//
// GET: /User/AssignRole
[Authorize(Roles = "Admin")]
[HttpGet]
public ActionResult AssignRole(string userID)
{
var user = context.Users.Where(u => u.Id == userID).FirstOrDefault();
var vm = new UserAssignRoleViewModel();
vm.UserID = user.Id;
vm.UserName = user.UserName;
List<IdentityRole> unassignedRoles = new List<IdentityRole>();
foreach (var role in context.Roles)
{
if (this.UserManager.IsInRole(vm.UserID, role.Name) == false)
{
unassignedRoles.Add(role);
}
}
vm.UnassignedRoles = unassignedRoles.OrderBy(r => r.Name).Select(rr => new SelectListItem { Value = rr.Name.ToString(), Text = rr.Name }).ToList();
return View(vm);
}
//
// POST: /User/AssignRole
[Authorize(Roles = "Admin")]
[HttpPost]
public ActionResult AssignRole(UserAssignRoleViewModel vm)
{
this.UserManager.AddToRole(vm.UserID, vm.RoleName);
ViewBag.ResultMessage = Resources.RoleAssignedSuccessfully;
List<IdentityRole> unassignedRoles = new List<IdentityRole>();
foreach (var role in context.Roles)
{
if (this.UserManager.IsInRole(vm.UserID, role.Name) == false)
{
unassignedRoles.Add(role);
}
}
vm.UnassignedRoles = unassignedRoles.OrderBy(r => r.Name).Select(rr => new SelectListItem { Value = rr.Name.ToString(), Text = rr.Name }).ToList();
return View(vm);
}
//
// GET: /User/RemoveRole
[Authorize(Roles = "Admin")]
[HttpGet]
public ActionResult RemoveRole(string userID)
{
var user = context.Users.Where(u => u.Id == userID).FirstOrDefault();
var vm = new UserRemoveRoleViewModel();
vm.UserID = user.Id;
vm.UserName = user.UserName;
vm.AssignedRoles = context.Roles.OrderBy(r => r.Name).ToList().Select(rr => new SelectListItem { Value = rr.Name.ToString(), Text = rr.Name }).ToList();
List<IdentityRole> assignedRoles = new List<IdentityRole>();
foreach (var role in context.Roles)
{
if (this.UserManager.IsInRole(vm.UserID, role.Name) == true)
{
assignedRoles.Add(role);
}
}
vm.AssignedRoles = assignedRoles.OrderBy(r => r.Name).Select(rr => new SelectListItem { Value = rr.Name.ToString(), Text = rr.Name }).ToList();
return View(vm);
}
//
// POST: /User/RemoveRole
[Authorize(Roles = "Admin")]
[HttpPost]
public ActionResult RemoveRole(UserRemoveRoleViewModel vm)
{
if (this.UserManager.IsInRole(vm.UserID, vm.RoleName))
{
this.UserManager.RemoveFromRole(vm.UserID, vm.RoleName);
ViewBag.ResultMessage = Resources.RoleUnassignedSuccessfully;
List<IdentityRole> assignedRoles = new List<IdentityRole>();
foreach (var role in context.Roles)
{
if (this.UserManager.IsInRole(vm.UserID, role.Name) == true)
{
assignedRoles.Add(role);
}
}
}
else
{
ViewBag.ResultMessage = Resources.ThisUserDoesNotBelongToSelectedRole;
}
return View (vm);
}
问题是:
下拉菜单每次都要重新填充，要么给用户分配角色，要么删除角色。在分配角色方面一切都很顺利;它只在下拉框中显示未分配的角色，当您添加角色时，从POST操作返回时，它会显示刷新的下拉框，而不包含您刚刚分配的角色。
但是在删除角色方面，一旦您从用户中删除角色(它正确地执行了)，从POST操作返回到视图时，它会抛出异常
Value不能为空。参数名称:items
行
@Html.DropDownListFor(m => m.RoleName, new SelectList(Model.AssignedRoles, "Value", "Text"), Resources.DropdownSelect, new { @class = "form-control" })
我的猜测是，因为在POST Action方法RemoveRole中，我没有以任何方式改变UserRemoveRoleViewModel的RoleName属性，并且回到视图下拉框已经重新填充，m => m.RoleName崩溃，因为它正在寻找已经删除的角色，它不再在列表中了。希望我解释得足够清楚。
问题是我完全不知道如何解决这个问题。任何帮助吗?
看起来第二个方法，对于移除，缺少了用角色列表重新初始化模型对象的行。因为ASP。. NET MVC是无状态的，这个重新初始化必须在每个请求上完成，即使返回的视图是相同的。
添加这一行:
vm.AssignedRoles = assignedRoles.OrderBy(r => r.Name).Select(rr => new SelectListItem { Value = rr.Name.ToString(), Text = rr.Name }).ToList();
在foreach以RemoveRole结尾之后，错误应该会消失。
问题是您从未将assigndroles集合分配给您的视图属性(assigndroles)。
同样，您可以使用LINQ语句来创建列表，从而使代码更简洁。
在您的GET操作中，您填充视图模型的assigndroles属性。在POST操作中，您无法做到这一点。所以当它到达视图代码时，它是空的，助手不能从一个空集合中创建一个下拉列表。您需要在POST操作中填充它。(请注意，它不会在GET和POST操作之间保持填充状态。)