代码如下:

/// <summary>
  /// 计算本周起始日期（礼拜一的日期）
  /// </summary>
  /// <param name="someDate">该周中任意一天</param>
  /// <returns>返回礼拜一日期，后面的具体时、分、秒和传入值相等</returns>
  public static DateTime CalculateFirstDateOfWeek(DateTime someDate)
  {
   int i = someDate.DayOfWeek - DayOfWeek.Monday;
   if (i == -1) i = 6;// i值 > = 0 ，因为枚举原因，Sunday排在最前，此时Sunday-Monday=-1，必须+7=6。
   TimeSpan ts = new TimeSpan(i, 0, 0, 0);
   return someDate.Subtract(ts);
  }
  /**//// <summary>
  /// 计算本周结束日期（礼拜日的日期）
  /// </summary>
  /// <param name="someDate">该周中任意一天</param>
  /// <returns>返回礼拜日日期，后面的具体时、分、秒和传入值相等</returns>
  public static DateTime CalculateLastDateOfWeek(DateTime someDate)
  {
   int i = someDate.DayOfWeek - DayOfWeek.Sunday;
   if(i != 0) i = 7 - i;// 因为枚举原因，Sunday排在最前，相减间隔要被7减。
   TimeSpan ts = new TimeSpan(i, 0, 0, 0);
   return someDate.Add(ts);
  }
  /**//// <summary>
  /// 判断选择的日期是否是本周（根据系统当前时间决定的‘本周'比较而言）
  /// </summary>
  /// <param name="someDate"></param>
  /// <returns></returns>
  public static bool IsThisWeek(DateTime someDate)
  {
   //得到someDate对应的周一
   DateTime someMon = CalculateFirstDateOfWeek(someDate);
   //得到本周一
   DateTime nowMon = CalculateFirstDateOfWeek(DateTime.Now);
   TimeSpan ts = someMon - nowMon;
   if (ts.Days < 0)
    ts = -ts;//取正
   if (ts.Days >= 7)
   {
    return false;
   }
   else
   {
    return true;
   }
  }

来源：https://www.jb51.net/article/41147.htm