Question

这是我的代码，在第144行

peopleEntering.Add(new PersonEntered(personName, dateEntered, doorTypeEnum));

是我得到的错误：对象引用未设置为对象的实例。为什么？

  protected void btnUpload_Click(object sender, EventArgs e)
    {
        if (FileUpload1.PostedFile.FileName == string.Empty)
        {

            return;
        }
        else
        {
            string[] FileExt = FileUpload1.FileName.Split('.');
            string FileEx = FileExt[FileExt.Length - 1];
            if (FileEx.ToLower() == "csv")
            {
                FileUpload1.SaveAs(Server.MapPath(" " + FileUpload1.FileName));
            }
            else
            {

                return;
            }
        }
        CSVReader reader = new CSVReader(FileUpload1.PostedFile.InputStream);
        string[] headers = reader.GetCSVLine();
        DataTable dt = new DataTable();

        foreach (string strHeader in headers)
            dt.Columns.Add(strHeader);
        string[] data;
        while ((data = reader.GetCSVLine()) != null)
            dt.Rows.Add(data);
        foreach (DataRow row in dt.Rows)
        {
            string dateAndTime = row["Date and Time"].ToString();
            string personName = row["Description #2"].ToString();
            string doorType = row["Card number"].ToString();

            DateTime dateEntered = Convert.ToDateTime(dateAndTime);
            DoorType doorTypeEnum;

            bool personExists = false;
            foreach (object name in listboxOfNames.Items)
            {
                if (name.ToString() == personName)
                {
                    personExists = true;
                    break;
                }
            }
            if (!personExists)
            {
                listboxOfNames.Items.Add(personName);

            }
            switch (doorType)
            {
                case "A02 - Rear Entrance":
                    doorTypeEnum = DoorType.RearEntranceDoor;
                    break;
                case "B12 - Exterior Main Floor Man Trap":
                    doorTypeEnum = DoorType.ExteriorMainFloorDoor;
                    break;
                case "B12 - Interior Main Floor Man Trap":
                    doorTypeEnum = DoorType.InteriorMainFloorDoor;
                    break;
                case "C13 - Rear Break Room Door":
                    doorTypeEnum = DoorType.RearBreakRoomDoor;
                    break;
                case "B02 - Exterior Basement Man Trap":
                    doorTypeEnum = DoorType.ExteriorBasementDoor;
                    break;
                case "B02 - Interior Basement Man Trap":
                    doorTypeEnum = DoorType.InteriorBasementDoor;
                    break;
                case "D01 - Managed Services Main door":
                    doorTypeEnum = DoorType.ManagedServicesDoor;
                    break;
                case "D01 - Managed Services Big Door":
                    doorTypeEnum = DoorType.ManagedServicesBigDoor;
                    break;
                default:
                    doorTypeEnum = DoorType.None;
                    break;
            }
            peopleEntering.Add(new PersonEntered(personName, dateEntered, doorTypeEnum));
        }


        for (int i = 0; i < peopleEntering.Count; i++)
        {
            DateTime startDate = new DateTime();
            DateTime endDate = new DateTime();
            string personName = peopleEntering[i].PersonName;
            if ( peopleEntering[i].DoorEntered == DoorType.RearEntranceDoor)
            {
                startDate = peopleEntering[i].DateOfEntry;
                for ( int j = i + 1; j < peopleEntering.Count; j++)
                {
                    if (peopleEntering[j].DoorEntered == DoorType.ExteriorBasementDoor && peopleEntering[j].PersonName == personName)
                    {
                        endDate = peopleEntering[j].DateOfEntry;
                    }
                }

            }
            workSpans.Add(new WorkSpan(personName, startDate, endDate));

        }

        csvReaderGv.DataSource = dt.Rows;
        csvReaderGv.DataBind();
    }




            }

Answer 1

在该特定行上NullReferenceException的唯一可能来源是peopleEntering是null。该语句中的其他所有内容都是新表达式或本地/字段读取。查看已发布的方法，看起来这是方法中的字段与本地。您是否记得将其初始化为非空值（假设类型为List<PersonEntered>）

private List<PersonEntered> peopleEntering = new List<PersonEntered>();

Answer 2

因为您使用的是ASP.NET（如prev. question所示）：

我假设您已经初始化了List一次，可能会使用IsPostBack检查。

private List<PersonEntered> peopleEntering;

protected void Page_Load(Object sender, EventArgs e)
{
    if(!IsPostBack)
    {
         peopleEntering = GetPeropleEntering(); // a method that retrieves them from database
    }
}

在这种情况下，您必须在每次回发时重新加载列表（删除IsPostBack检查），或者您必须将其存储在某个地方以保留其在回发中的状态。 Session是一种方式。请记住，所有变量（包括所有控件）都放在页面生命周期的末尾。

以下是其他8个人：http://msdn.microsoft.com/en-us/magazine/cc300437.aspx

我得到一个我无法弄清楚的错误。你调用的对象是空的。我哪里做错了？

2 个答案: