考虑了一下，将字段实现给贴了出来，但是说实话，我琢磨不定哪一个是最佳方案（原因如下）。
public class User
{
internal string _username;
public string Username { get { return this._username; } set { this._username = value; } }
internal int? _id;
public int? ID { get { return this._id; } set { this._id = value; } }
}
{
internal string _username;
public string Username { get { return this._username; } set { this._username = value; } }
internal int? _id;
public int? ID { get { return this._id; } set { this._id = value; } }
}