级联DropDownList已实现，选中值无法更新数据库的解决方法

1. 修复JavaScript的HTML拼接错误 #

你的级联下拉框JS代码中，拼接<option>标签时误用了HTML转义字符"，导致生成的选项value属性被错误追加无效字符（比如原本应为value="1"，实际变成value="1""），提交时模型绑定无法识别这个无效数值，直接导致StoreWid无法正确赋值。

修改后的JS代码：

<script>
    $('#AmmoWid').change(function () {
        $('#StoreWid').empty();
        $.ajax({
            url: "@Url.Action("GetStore")",
            type: "Get",
            data: { id: $('#AmmoWid option:selected').val() },
            dataType: "json",
            success: function (result) {
                $.each(result, function (i, Store) {
                    // 修正引号拼接逻辑，直接使用双引号转义
                    $('#StoreWid').append('<option value="' + Store.value + '">' + Store.text + '</option>');
                })
            }
        })
    })
</script>

2. 对齐表单字段与模型绑定的名称 #

从你的View代码（比如@Html.LabelFor(model => model.TblUsed.DateWid)）能看出，View的模型是一个包含TblUsed属性的父ViewModel，但Post方法直接接收TblUsed类型参数，导致表单提交的字段名（如TblUsed.StoreWid）和模型绑定预期的字段名（StoreWid）不匹配，这也是绑定失败的核心原因之一。

解决方式二选一： #

方式A：修改Post方法接收ViewModel

如果你的View使用的是包含TblUsed的ViewModel（比如CreateUsedViewModel），调整Post方法如下：

[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Create([Bind("TblUsed.RowWid,TblUsed.DateWid,TblUsed.AmmoWid,TblUsed.QtyUsed,TblUsed.StoreWid")] CreateUsedViewModel vm)
{
    if (ModelState.IsValid)
    {
        _context.Add(vm.TblUsed);
        await _context.SaveChangesAsync();
        return RedirectToAction(nameof(Index));
    }
    // 验证失败后重新填充下拉选项，避免页面下拉框为空
    var strRowWid = "RowWid";
    var strDt = "Dt";
    var dateNow = DateOnly.FromDateTime(DateTime.Now);
    ViewBag.DateWid = new SelectList(_context.TblDays.OrderByDescending(x => x.Dt).Where(n => n.Dt <= dateNow), strRowWid, strDt);
    ViewBag.ammo = _context.VwAmmos.OrderBy(n => n.Caliber)
        .Select(n => new SelectListItem
        {
            Value = n.RowWid.ToString(),
            Text = $"{n.WeaponType} {n.Caliber} {n.Brand} {n.Style}",
        }).ToList();
    return View(vm);
}

方式B：调整表单字段名称匹配模型

改用强类型辅助方法生成下拉框，自动对齐字段名称：

<div class="form-group">
    @Html.LabelFor(model => model.TblUsed.DateWid, htmlAttributes: new { @class = "control-label"})
    <div>
        @Html.DropDownListFor(model => model.TblUsed.DateWid, ViewBag.DateWid as SelectList, "--select date--", new { @class = "form-control form-select-sm form-select" })
    </div>
</div>

<div class="form-group">
    @Html.LabelFor(model => model.TblUsed.AmmoWid, htmlAttributes: new { @class = "control-label"})
    <div>
        @Html.DropDownListFor(model => model.TblUsed.AmmoWid, ViewBag.ammo as List<SelectListItem>, "--select Ammo--", new { @class = "form-control form-select-sm form-select" })
    </div>
</div>

<div class="form-group">
    @Html.LabelFor(model => model.TblUsed.StoreWid, htmlAttributes: new { @class = "control-label"})
    <div>
        @Html.DropDownListFor(model => model.TblUsed.StoreWid, new SelectList(Enumerable.Empty<SelectListItem>()), "--select Store--", new { @class = "form-control form-select-sm form-select" })
    </div>
</div>

同时修改Post方法的Bind属性，添加前缀匹配：

[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Create([Bind(Prefix = "TblUsed", Include = "RowWid,DateWid,AmmoWid,QtyUsed,StoreWid")] TblUsed tblUsed)
{
    if (ModelState.IsValid)
    {
        _context.Add(tblUsed);
        await _context.SaveChangesAsync();
        return RedirectToAction(nameof(Index));
    }
    // 重新填充下拉选项
    var strRowWid = "RowWid";
    var strDt = "Dt";
    var dateNow = DateOnly.FromDateTime(DateTime.Now);
    ViewBag.DateWid = new SelectList(_context.TblDays.OrderByDescending(x => x.Dt).Where(n => n.Dt <= dateNow), strRowWid, strDt);
    ViewBag.ammo = _context.VwAmmos.OrderBy(n => n.Caliber)
        .Select(n => new SelectListItem
        {
            Value = n.RowWid.ToString(),
            Text = $"{n.WeaponType} {n.Caliber} {n.Brand} {n.Style}",
        }).ToList();
    // 根据View的模型结构返回对应数据，示例为包含TblUsed的匿名对象
    return View(new { TblUsed = tblUsed });
}

3. 验证失败后重新填充下拉选项 #

当Post方法因验证失败返回View时，必须重新填充ViewBag中的下拉选项，否则页面上的下拉框会变为空，影响用户再次提交。上述代码已包含该逻辑。

内容的提问来源于stack exchange，提问作者user3536476