数据类型转换

Question

    /// <summary>
        /// 
        /// </summary>
        public Guid? UserID
        {      
            get { return this._UserID; }
            set {  this._UserID =value; }
        }
value下面有红线，求正解

Answer 1

private Guid? _UserID ;

Answer 2

属性定义错误
  private Guid? _UserID;
            public Guid? UserID
            {
                get { return this._UserID; }
                set { this._UserID = value; }
            }