从“DBNull”类型到“Short”类型的转换无效。 。网

Question

Imports System.Data.OleDb

Namespace wbdsproject

    Partial Class frmUserPrvMsg
        Inherits System.Web.UI.Page

        Dim topicDb As OleDbConnection
        Dim topicCmd As OleDbCommand
        Dim topicReader As OleDbDataReader
        Dim topicId As Integer
        Dim AuthorName, Username As String

    #Region " Web Form Designer Generated Code "

        'This call is required by the Web Form Designer.
        <System.Diagnostics.DebuggerStepThrough()> Private Sub InitializeComponent()

        End Sub


        Private Sub Page_Init(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Init
            'CODEGEN: This method call is required by the Web Form Designer
            'Do not modify it using the code editor.
            InitializeComponent()
        End Sub

    #End Region

        Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load, Me.Load
            'Put user code to initialize the page here
            Dim uid As Int16
            topicId = Request.QueryString("topicId")
            topicDb = New OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=E:\discussion\SampleForum.mdb")
            topicDb.Open()
            If Session("loginuser") = "" Then
                Response.Redirect("frmlogin.aspx")
            End If
            Username = Session("loginuser")
            topicCmd = New OleDbCommand("select uid from tbllog where uname='" & Username & "'", topicDb)
            uid = topicCmd.ExecuteScalar
            topicCmd = New OleDbCommand("Select uname,pMsgDate,pMsgSubject,pMsgDetail from tblprivatemessage, tbllog where tblprivatemessage.pMsgTo=" & uid & " and tbllog.uid=tblprivatemessage.pMsgFrom", topicDb)
            topicReader = topicCmd.ExecuteReader
            DataGrid2.DataSource = topicReader

            DataGrid2.DataBind()
            topicReader.Close()
        End Sub

        Private Sub DataGrid1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs)

        End Sub

        Private Sub btninbox_Click(ByVal sender As System.Object, ByVal e As System.Web.UI.ImageClickEventArgs) Handles btninbox.Click
            Response.Redirect("frmUserPrvMsg.aspx")
        End Sub

        Private Sub btnOutBox_Click(ByVal sender As System.Object, ByVal e As System.Web.UI.ImageClickEventArgs) Handles btnOutBox.Click
            Response.Redirect("OutBox.aspx")
        End Sub  
   End Class
End Namespace

.net 初学者，只是我无法理解未知问题。任何解决方案

Answer 1

你的命令

select uid from tbllog where uname='UserName'

正在返回 DBNull。

这个话题应该会有所帮助：DBNull check for Execute Scalar