從'DBNull'類型到'Short'類型的轉換無效。在.NET .NET

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 

初學者，只需我運行無法理解unknoen problem.any soutions

Answer 1

你的命令

select uid from tbllog where uname='UserName' 

正在返回的DBNull。

本主題應該會有所幫助：DBNull check for Execute Scalar