[blauesau]

So zum Beispiel:

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim von As Integer = txtVon.Text
        Dim bis As Integer = txtBis.Text
        Dim i As Integer
        Dim j As Integer
        txtOutput.Text = ""
        For i = von To bis
            If isPrime(i) Then
                txtOutput.Text += CType(i, String) + ", "
            End If
        Next

    End Sub

    Function isPrime(ByVal zahl As Integer)
        Dim x As Integer
        Dim p As Boolean = True
        For x = 2 To Math.Sqrt(zahl)
            If zahl Mod x = 0 Then
                p = False
            End If
        Next
        If zahl = 1 Then
            p = False
        End If
        Return p
    End Function