VB - 一次显示msgbox（r）

Question

I have the following code :

Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
        Dim mesaj As New Integer

        My.Computer.Network.DownloadFile("http://rotutorial.net/anunt.txt", "c:\classmate\msg1.txt", "", "", False, 60000, True)
        Dim readtext As New System.IO.StreamReader("c:\classmate\msg1.txt")
        Dim text As String
        text = readtext.ReadToEnd
        readtext.Close()
        Dim parti(10) As String
        parti = text.Split("_")

        Dim writetext1 As New System.IO.StreamReader("c:\classmate\msg.txt")
        Dim text1 As String
        Dim parti1(10) As String
        text1 = writetext1.ReadToEnd
        parti1 = text1.Split("_")

        writetext1.Close()
        Dim unic As New Integer
        unic = Val(parti(0))
        Dim unic1 As New Integer
        unic1 = Val(parti1(0))

        If unic <> unic1 Then
            If unic <> unic1 Then
                mesaj = MsgBox(parti(3), vbYesNo, "Mesaj")
            End If
            Dim writetext2 As New System.IO.StreamWriter("c:\classmate\msg.txt")
            Dim text2 As String
            text2 = text & "/" & text1
            writetext2.Write(text2)
            writetext2.Close()

            Timer1.Enabled = False
            Timer1.Enabled = True
        End If
        Timer1.Enabled = False
        Timer1.Enabled = True






    End Sub

The timer interval is set to 5000(5 seconds), but every time when the timer is ticking the msgbox appears on screen but the in the file msg.txt is writting once. So, The timer check if that unic is different from unic1, and if is different shows up a msg box, and it's writting the new line in msg.txt, but on next timer tick, even if the unic and unic1 are equals the msgbox shows up anyway, but it's more interesting because it doesn't write again in file, only shows up the msgbox. I don't understand this.

Sorry for my bad english, i'm from Romania.

Thank you!

Answer 1

A message box can be a bit dangerous when used for the right reasons in the wrong place. This is one of them. The problem is that it does the equivalent of calling DoEvents, in a loop, designed to keep the message loop operational. So that Windows messages, like Paint events and input events for the message box get dispatched and processed as normal. That prevents your UI from freezing.

That can however cause re-entrancy problems. Message box solves the most severe ones by disabling all the windows in your application. Note the beep you get when you click on one of those windows. That however does not block Timer messages. So after 5 seconds, your Timer1_Tick() method runs again. Displaying another message box. Wait long enough with dismissing them and your screen just fills up with message boxes. You'll likely have additional problems from the file manipulation code running multiple times.

The workaround is simple, just disable the Timer at the start of the method. Re-enable it at the end. A BackgroundWorker is also a common choice in code like this, it prevents a slow file download from freezing your UI.