Circulate playing audio with MediaElement in WPF

I am developing a video player with WPF.(in VB)

I have already created a MediaElement ,ListBox, "Next" button,

then start playing through reading ListBox,

and use "Next" to skip to next audio/video.

In "MediaEnded" event, i just copy all code in "Next" button.

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Now, problem is coming,

Assume the Listbox has four audios(.mp3), "test1.mp3", "test2.mp3", ......

now playing is "test1.mp3", i push "Next" button, then now playing is "test2.mp3".

However, when i just let "test1.mp3" play completed, my player will not play "test2.mp3",

it plays "test3.mp3" or others in random.

This situation like "MediaEnded" event was processed for many times.

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Private Sub MediaElement1_MediaEnded(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs) Handles MediaElement1.MediaEnded

    nextmedia()

End Sub

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Private Sub nextmedia()

    Try
        'pi is play index, start from 1, 0 is non playing
        If pi <> 0 Then
            If pi = ListBox_temp.Items.Count Then

                Dim filename As String = ListBox_temp.Items.Item(0).ToString
                MediaElement1.Source = New Uri(filename开发者_如何学运维)
                pi = 1

            Else

                Dim filename As String = ListBox_temp.Items.Item(pi).ToString
                MediaElement1.Source = New Uri(filename)
                pi = pi + 1

            End If
        End If

    Catch ex As Exception

    End Try
    Window1.Title = "Video Sampler - " + CStr(pi) + ". " + CStr(ListBox1.Items.Item(pi - 1))
End Sub

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Who can help me....

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I haven't tested it, but try the following.

Try 
    pi = If(pi < ListBox_temp.Items.Count - 1, pi + 1, 0)
    Dim filename As String = ListBox_temp.Items.Item(pi).ToString 
    MediaElement1.Source = New Uri(filename) 
Catch ex As Exception
End Try

This increments pi unless the last ListBoxItem is selected, in which case it sets it to zero.