如何修复“必需的”注解在.NET中不起作用

public class User
{
    [Required(ErrorMessage = "Username is required.")]
    public string Username { get; set; }

    [Required(ErrorMessage = "Email is required.")]
    [EmailAddress(ErrorMessage = "Invalid Email Address")]
    public string Email { get; set; }
}

[HttpPost]
public IActionResult Register(User user)
{
    if (!ModelState.IsValid)
    {
        return View(user);
    }
    // 用户注册逻辑
    return RedirectToAction("ThankYou");
}

<form asp-action="Register">
    <div asp-validation-summary="ModelOnly" class="text-danger"></div>
    <div class="form-group">
        <label asp-for="Username"></label>
        <input asp-for="Username" class="form-control" />
        <span asp-validation-for="Username" class="text-danger"></span>
    </div>
    <div class="form-group">
        <label asp-for="Email"></label>
        <input asp-for="Email" class="form-control" />
        <span asp-validation-for="Email" class="text-danger"></span>
    </div>
    <button type="submit" class="btn btn-primary">Register</button>
</form>