Csharp DropDownListFor未显示选定值

我在.NET4.5上有一个ASP.NETMVC页面.

以下是我的代码模型类:

public class UserDetail
{
    public Int64 UserID { get; set; }
    [Required]
    [EmailAddress]
    public string LoginID { get; set; }
    [Required]
    public string FirstName { get; set; }
    [Required]
    public string LastName { get; set; }
    public bool Status { get; set; } = true;
    public DateTime LastLoginTime { get; set; }
    public DateTime CreatedOn { get; set; }
    public DateTime UpdatedOn { get; set; }
    // Columns for Mapping
    [Range(1,int.MaxValue, ErrorMessage = "Select a Valid Group Value")]
    public int GroupID { get; set; }
    public string Group { get; set; }
    [Range(1, int.MaxValue, ErrorMessage = "Select a Valid Role Value")]
    public int RoleID { get; set; }
    public string Role { get; set; }
}

public class User视图Model
{
    public UserDetail User { get; set; } = new UserDetail();
    public IEnumerable<SelectListItem> Groups { get; set; }
    public IEnumerable<SelectListItem> Roles { get; set; }
}

视图

@model MyApp.Models.User视图Model
<label for="ddl_role" class="font-weight-bold">Select Role</label>
                    @Html.DropDownListFor(x => x.User.RoleID, Model.Roles, "Choose...", new { id = "ddl_role", @class = "form-control form-control-sm" })

控制器操作方法

public ActionResult SomeName()
{
    var viewmodel = new User视图Model();
    viewmodel.Groups = Helper.GetSelectListItems(nameof(viewmodel.Groups));
    viewmodel.Roles = Helper.GetSelectListItems(nameof(viewmodel.Roles));
    viewmodel.User = Helper.GetData();

    return 视图(viewmodel);
}

viewmodel.User = Helper.GetData()提取UserDetail对象的数据，viewmodel.Groups、viewmodel.Roles都是SelectListItem的IEnumerable，它们包含文本和值字段.

但是当我加载视图时，下拉菜单总是显示‘Choose...’.作为选定值，即使Model.User.RoleID被限定为反映数据，SelectList也存在，但它不显示通过Model.User.RoleID设置的选定值

有什么暗示我可能遗漏了什么吗？