Question

我是编程新手，遇到了以下错误：

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:

我使用New创建了一个新对象，但仍然遇到了同样的错误。

以下是代码：

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

Answer 1

你需要告诉我们它所引发的代码行，但我猜是

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

是dv.DataItem一个DataRowView吗？如果没有，它返回Nothing，你没有检查。否则，你做得很好，所以其他地方也不多。

暂且不说：这段代码

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

可能只是

Dim dv As DetailsView = DirectCast(sender, DetailsView)

无需创建新的，然后立即扔掉它。（不是100％确定我的语法，但不要调用新的）

你调用的对象是空的

1 个答案: