Public Class Form1
 
    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) _
      Handles Button1.Click
 
        Dim laenge As String = TextBox3.Text.ToString
 
        Dim Wert As Integer = Nothing
 
        If Not Integer.TryParse(laenge, Wert) Then
            MessageBox.Show("Es wurde kein sinnvoller Wert eingegeben!")
            TextBox3.Focus
            Exit Sub
        End If
 
        Select Case Wert
            Case Is < 300
                MessageBox.Show("Geben Sie bitte einen Wert >=300 ein!")
            Case Is >= 300, Is <= 2500
                TextBox4.Focus()
            Case Is > 2500
                MessageBox.Show("Geben Sie bitte einen Wert <=2500 ein!")
        End Select
 
    End Sub
 
    Private Sub TextBox3_KeyPress(sender As Object, e As _
      System.Windows.Forms.KeyPressEventArgs) Handles TextBox3.KeyPress
        Select Case Asc(e.KeyChar)
            Case 48 To 57, 32
 
            Case Else
                e.Handled = True
        End Select
    End Sub
 
End Class

EALA FREYA FRESENA