For each Next 循环问题

Private Sub Button6_Click(sender As Object, e As EventArgs) Handles Button6.Click
        Dim ctl1 As Control
        Dim i As Integer = 4
        Dim a(i) As String
        i=1
        For Each ctl1 In Me.Controls
            If Mid(ctl1.Name, 1, 2) = "Bu" Then
               if i<5 then
                    a(i) = ctl1.Name
                    TextBox1.Text = TextBox1.Text & vbCrLf & "a" & i & "=" & a(i)
                    i = i + 1
                end if
            End If
        Next ctl1
    End Sub