从模型显示时在DropDownListFor中设置默认值

public class User
{
    public string Country { get; set; }
}

public class UserController : Controller
{
    public ActionResult Index()
    {
        var model = new User { Country = "United States" };
        ViewBag.Countries = new SelectList(new[]
        {
            new { ID = "US", Name = "United States" },
            new { ID = "CA", Name = "Canada" },
            new { ID = "MX", Name = "Mexico" }
        }, "ID", "Name");

        return View(model);
    }
}

@model YourNamespace.User

@Html.DropDownListFor(m => m.Country, (SelectList)ViewBag.Countries)

public ActionResult Index()
{
    var model = new User { Country = "US" }; // 设置默认值为 "US"
    ViewBag.Countries = new SelectList(new[]
    {
        new { ID = "US", Name = "United States" },
        new { ID = "CA", Name = "Canada" },
        new { ID = "MX", Name = "Mexico" }
    }, "ID", "Name");

    return View(model);
}