ASP.NET Core 6 MVC：回发后保留DropDownList选中值

问题核心是Post请求重新生成SelectList时未指定选中项，导致下拉框无法保留用户之前的选择。以下是针对你现有代码的修复方案：

修复步骤（基于ViewBag的快速修改） #

修改StudentsController中的[HttpPost] Index方法，在创建SelectList时传入用户选中的分组ID作为默认选中值：

[HttpPost]
public async Task<IActionResult> Index(string groups)
{
    // 处理空值，避免转换出错，默认选中GroupId=1
    int Id = string.IsNullOrEmpty(groups) ? 1 : Convert.ToInt32(groups);
    
    // 重点：在SelectList构造中加入第四个参数（选中值）
    ViewBag.ListOfGroups = new SelectList(
        _context.Groups.OrderBy(k => k.Level).ThenBy(k => k.ShortName), 
        "GroupId", 
        "ShortName", 
        Id); // 传入选中的Id
    
    var applicationDbContext = _context.Students
        .Where(k => k.Group.GroupId == Id)
        .Include(s => s.Group)
        .OrderBy(k => k.Group.Level)
        .ThenBy(k => k.Group.ShortName)
        .ThenBy(k => k.Number);
        
    return View(await applicationDbContext.ToListAsync());
}

补充：优化HttpGet方法的默认选中（可选） #

如果希望首次加载页面时默认选中GroupId=1的选项，也可以在HttpGet的Index方法中加入选中值：

public async Task<IActionResult> Index()
{
    int defaultGroupId = 1;
    ViewBag.ListOfGroups = new SelectList(
        _context.Groups.OrderBy(k => k.Seviye).ThenBy(k => k.ShortName), 
        "GroupId", 
        "ShortName", 
        defaultGroupId);
        
    var applicationDbContext = _context.Students
        .Where(k => k.Group.GroupId == defaultGroupId)
        .Include(s => s.Group)
        .OrderBy(k => k.Group.Level)
        .ThenBy(k => k.Group.ShortName)
        .ThenBy(k => k.Number);
        
    return View(await applicationDbContext.ToListAsync());
}

更规范的强类型视图模型方案（推荐） #

长期来看，使用强类型ViewModel更符合ASP.NET Core的设计原则，避免ViewBag的弱类型问题：

1. 创建ViewModel类：

public class StudentListViewModel
{
    public int SelectedGroupId { get; set; }
    public IEnumerable<SelectListItem> GroupOptions { get; set; }
    public IEnumerable<Student> Students { get; set; }
}

2. 修改控制器方法：

public async Task<IActionResult> Index()
{
    var viewModel = new StudentListViewModel
    {
        SelectedGroupId = 1,
        GroupOptions = new SelectList(
            _context.Groups.OrderBy(k => k.Seviye).ThenBy(k => k.ShortName), 
            "GroupId", 
            "ShortName"),
        Students = await _context.Students
            .Where(k => k.Group.GroupId == 1)
            .Include(s => s.Group)
            .OrderBy(k => k.Group.Level)
            .ThenBy(k => k.Group.ShortName)
            .ThenBy(k => k.Number)
            .ToListAsync()
    };
    return View(viewModel);
}

[HttpPost]
public async Task<IActionResult> Index(StudentListViewModel viewModel)
{
    viewModel.GroupOptions = new SelectList(
        _context.Groups.OrderBy(k => k.Level).ThenBy(k => k.ShortName), 
        "GroupId", 
        "ShortName", 
        viewModel.SelectedGroupId);
        
    viewModel.Students = await _context.Students
        .Where(k => k.Group.GroupId == viewModel.SelectedGroupId)
        .Include(s => s.Group)
        .OrderBy(k => k.Group.Level)
        .ThenBy(k => k.Group.ShortName)
        .ThenBy(k => k.Number)
        .ToListAsync();
        
    return View(viewModel);
}

3. 修改视图代码：

@model Proje.Models.StudentListViewModel

<form asp-action="Index" method="post" >
    @Html.DropDownListFor(m => m.SelectedGroupId, Model.GroupOptions, "Select...", new {@class="form-control"})
    <button type="submit" class="btn btn-sm btn-danger">List</button>
</form>

<!-- 学生列表展示代码 -->
@foreach(var student in Model.Students)
{
    <!-- 输出学生信息 -->
}

内容的提问来源于stack exchange，提问作者Zafer Andaç Özer