am developin投票系統，但它顯示錯誤爲無效嘗試讀取時，沒有數據存在

Question

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click 
    Dim n As Integer = 0  
    str = "select vote from CANDIDATEDETAILS where PARTY='Red'"  
    cmd = New SqlCommand(str, con)  
    con.Open()  
    dr = cmd.ExecuteReader() 

    While dr.HasRows  
     dr.Read()  
     n = dr("vote").ToString()  
     n = n + 1 
    End While 

    str = "update CANDIDATEDETAILS set vote='" + n.ToString() + "' where PARTY='Rose'"  
    cmd = New SqlCommand(str, con)  
    cmd.ExecuteNonQuery()  
    MsgBox("insert sucessfully")  
    cmd.Dispose()  
    con.Close() 

    Label8.Text = n.ToString() 
End Sub 

Answer 1

使用SqlDataReader喜歡this example from MSDN並且總是讀完後關閉它。另外，如果數據庫中的「投票」字段是整數，則不需要將其轉換爲字符串。

While dr.Read() 
    n = dr("vote") 
    n = n + 1 
End While  
dr.Close() 

在你的代碼，while dr.HasRows將永遠是真實的，所以它會試圖讀取下一行，即使是沒有的。

作爲便箋，請閱讀this article about debugging in Visual Studio。它有很多很酷的提示。

Answer 2

當沒有數據存在？ ..試試這個..

If dr.HasRows 
    While dr.HasRows  
     dr.Read()  
     n = dr("vote").ToString()  
     n = n + 1 
    End While 

    str = "update CANDIDATEDETAILS set vote='" + n.ToString() + "' where PARTY='Rose'"  
    cmd = New SqlCommand(str, con)  
    cmd.ExecuteNonQuery()  
    MsgBox("insert sucessfully")  
    cmd.Dispose() 
End If  
con.Close()