對象引用未設置到對象

我是新來編程的一個實例，所遇到以下錯誤：對象引用未設置到對象

System.NullReferenceException was unhandled by user code 
    Message=Object reference not set to an instance of an object. 
    Source=App_Web_hnmuvsif 
    StackTrace: 
     at Default3.DetailsView1_DataBound(Object sender, EventArgs e) in C:\projects\FPOS_v2\photo_view.aspx.vb:line 54 
     at System.Web.UI.WebControls.BaseDataBoundControl.OnDataBound(EventArgs e) 
     at System.Web.UI.WebControls.DataBoundControl.OnDataSourceViewSelectCallback(IEnumerable data) 
     at System.Web.UI.DataSourceView.Select(DataSourceSelectArguments arguments, DataSourceViewSelectCallback callback) 
     at System.Web.UI.WebControls.DataBoundControl.PerformSelect() 
     at System.Web.UI.WebControls.BaseDataBoundControl.DataBind() 
     at System.Web.UI.WebControls.DetailsView.DataBind() 
     at System.Web.UI.WebControls.BaseDataBoundControl.EnsureDataBound() 
     at System.Web.UI.WebControls.DetailsView.EnsureDataBound() 
     at System.Web.UI.WebControls.CompositeDataBoundControl.CreateChildControls() 
     at System.Web.UI.Control.EnsureChildControls() 
     at System.Web.UI.Control.PreRenderRecursiveInternal() 
     at System.Web.UI.Control.PreRenderRecursiveInternal() 
     at System.Web.UI.Control.PreRenderRecursiveInternal() 
     at System.Web.UI.Control.PreRenderRecursiveInternal() 
     at System.Web.UI.Control.PreRenderRecursiveInternal() 
     at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) 
    InnerException:

我已經使用新創建一個新的對象，但仍然得到同樣的錯誤。

下面是代碼：

If IsPostBack Then 
    Session("selectedPhotoID") = DetailsView1.DataKey.Value.ToString() 
    DataList1.DataBind() 
End If 

DetailsView1.Fields(0).Visible = False 
DetailsView1.Fields(2).ShowHeader = False 

DetailsView1.Fields(3).ShowHeader = False 
DetailsView1.Fields(3).ItemStyle.HorizontalAlign = HorizontalAlign.Center 
DetailsView1.Fields(3).ItemStyle.Font.Bold = True 

DetailsView1.HorizontalAlign = HorizontalAlign.Center 

'The code below loops through and selects the photo rather then selecting the first 

If Not String.IsNullOrEmpty(Session("selectedPhotoID")) Then 
    Dim dv As New DetailsView 
    dv = DirectCast(sender, DetailsView) 

    If dv IsNot Nothing Then 
     Dim myDataRowView As DataRowView = TryCast(dv.DataItem, DataRowView) 

     For iRowIndex As Integer = 0 To myDataRowView.DataView.Count - 1 

      If myDataRowView.DataView(iRowIndex).Row(0).ToString() = Session("selectedPhotoID") Then 

       'found record 

       dv.PageIndex = iRowIndex 

       Exit For 

      End If 

     Next 

    End If 
End If

來源

2011-02-12 AJsStack

哪一行是拋出的異常？ – BoltClock 2011-02-12 22:14:33

參見http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-in-net – 2011-02-12 22:29:07

您的需要告訴我們的，它被扔在代碼行，但我的猜測是，

Dim myDataRowView As DataRowView = TryCast(dv.DataItem, DataRowView)

是dv.DataItem一個DataRowView的？如果沒有，它將返回Nothing，並且你沒有檢查它。否則，你的檢查工作做得非常好，所以沒有其他可能的地方。

順便說一句：這個代碼

Dim dv As New DetailsView 
dv = DirectCast(sender, DetailsView)

可能只是

Dim dv As DetailsView = DirectCast(sender, DetailsView)

無需創建一個新的，然後立即把它扔掉。（不是100％確定我的語法，但不要叫New）

來源

2011-02-12 22:23:14

對象引用未設置到對象

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