public class MyViewModel
{
    public int SelectedItemId { get; set; }
    public IEnumerable<SelectListItem> Items { get; set; }
}

public ActionResult Index()
{
    var model = new MyViewModel
    {
        Items = new List<SelectListItem>
        {
            new SelectListItem { Value = "1", Text = "选项1" },
            new SelectListItem { Value = "2", Text = "选项2" },
            new SelectListItem { Value = "3", Text = "选项3" }
        }
    };
    return View(model);
}

@model MyViewModel

@using (Html.BeginForm("Submit", "Home", FormMethod.Post, new { id = "myForm" }))
{
    @Html.DropDownListFor(m => m.SelectedItemId, Model.Items, "请选择...")
    <input type="submit" value="提交" />
}

$(function() {
    $('#myForm').submit(function(e) {
        e.preventDefault();
        
        $.ajax({
            url: $(this).attr('action'),
            type: $(this).attr('method'),
            data: $(this).serialize(),
            success: function(response) {
                alert('提交成功');
            },
            error: function() {
                alert('提交失败');
            }
        });
    });
});

[HttpPost]
public ActionResult Submit(MyViewModel model)
{
    // 处理提交的数据
    int selectedId = model.SelectedItemId;
    // 其他处理逻辑...
    
    return Json(new { success = true });
}

$.get('/Home/GetDropdownData', function(data) {
    var dropdown = $('#SelectedItemId');
    dropdown.empty();
    dropdown.append($('<option>').val('').text('请选择...'));
    $.each(data, function(index, item) {
        dropdown.append($('<option>').val(item.Value).text(item.Text));
    });
});

$('#ParentDropdown').change(function() {
    var parentId = $(this).val();
    $.get('/Home/GetChildItems', { parentId: parentId }, function(data) {
        var childDropdown = $('#ChildDropdown');
        childDropdown.empty();
        $.each(data, function(index, item) {
            childDropdown.append($('<option>').val(item.Value).text(item.Text));
        });
    });
});