Unique Random number generator in vb.net

//hey guys here is my new program in vb.NET which generates unique 20 random Numbers.
Public Class Form1
    Dim generator As New Random
    Dim arr(20) As Integer
    Dim num As Integer = 0
    Dim i As Integer = 1
    Dim k As Integer = 0
    Dim x As Boolean

    Public Sub btngenerate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btngenerate.Click

        numshow()


    End Sub


    Private Sub numshow()
        If k < 20 Then
            num = generator.Next(1, 21)
            x = check()
            If x = True Then

                arr(i) = num
                Label1.Text = arr(i).ToString
                i = i + 1
                k = k + 1
            Else

                numshow()
                num = 0
            End If


        Else
            MessageBox.Show("no more")
        End If

    End Sub
    Private Function check() As Boolean
        Dim l = i - 1
        For j As Integer = 1 To l

            If arr(j) = num Then
                Return False
            End If
        Next
        Return True

    End Function
End Class



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