实体框架代码优先的空外键

User

Country

 public class User{
    public int CountryId { get; set; }
    public Country Country { get; set; }
}

public class Country{
    public List<User> Users {get; set;}
    public int CountryId {get; set;}
}

A foreign key value cannot be inserted because a corresponding primary key value does not exist. [ Foreign key constraint name = Country_Users ]\"}

public class User
{
    public int Id { get; set; }
    public int? CountryId { get; set; }
    public virtual Country Country { get; set; }
}

public class User
{
    public int Id { get; set; }
    public int? CountryId { get; set; }
    [ForeignKey(\"CountryId\")]
    public virtual Country Country { get; set; }
}

    modelBuilder.Entity<Country>()
        .HasMany(c => c.Users)
        .WithOptional(c => c.Country)
        .HasForeignKey(c => c.CountryId)
        .WillCascadeOnDelete(false);