使用新类型Nullable处理数据库表中null字段

 private DateTime? _updateTimestamp;
 /// <summary>
 /// 文件更新日期
 /// </summary>
  public Nullable<DateTime> UpdateTimestamp
 {
 get { return this._updateTimestamp; }
 set { this._updateTimestamp = value; }
 }
 /// <summary>
 /// 从DataReader中加载数据
 /// </summary>
 /// <param name="rdr"></param>
 public void Load(IDataReader rdr)
 {
 if (rdr.Read())
 {
 IsLoaded = true;
 this.FileId = (int)rdr["fileId"];
 if (!rdr["updatetimestamp"].Equals(DBNull.Value))
 {
 this.UpdateTimestamp = (DateTime)rdr["updatetimestamp"];
 }
 ……
 }
 }
}
//保存文件方法
public abstract int CreateFile(……，,DateTime? updatetimestamp, int downloadCount);