主题：我想产生不重复的20位数字字符串

提供个思路，以下代码可以生成16个不重复的字符：

Dim j As String = ""
For i As Integer = 1 To 16
    Do
        Dim k As String = Rand.NextString(1)
        If j.Contains(k) = False Then
            j = j & k
            Exit Do
        End If
    Loop
Next
Output.show(j)