Private Sub tab_Modulverwaltung_Selected(sender As Object, e As _
  TabControlEventArgs) Handles tab_Modulverwaltung.Selected
 
        Select Case tab_Modulverwaltung.SelectedIndex
 
            Case 0 ' Registerkarte "Zuordnen"
 
                 ....
                 ....
 
            Case 1 ' Registerkarte "Mutieren"
 
                Call Update_grd_Alle_Module()
 
            Case 2 ' Registerkarte "Kategorien"
 
                ....
                ....
 
        End Select
 
    End Sub

Private Sub grd_Alle_Module_SelectionChanged(sender As Object, e As EventArgs) _
  Handles grd_Alle_Module.SelectionChanged
 
        If grd_Alle_Module.SelectedRows.Count <> 0 Then
            MsgBox(grd_Alle_Module.Rows.Count.ToString)
        End If
 
    End Sub

   Private Sub Update_grd_Alle_Module()
 
        Dim SQLcmd As New SqlCommand()
        Dim DataTable As New DataTable
 
        SQLcmd.CommandType = CommandType.Text
        SQLcmd.CommandText = "SELECT * FROM dbo.View_tblModule ORDER BY" & _
          "mod_kurzbez"
        DataTable = SQLDB.DataTransaction(SQLcmd).Table
 
        With grd_Alle_Module
            .AllowUserToAddRows = False
            .RowTemplate.Height = 18
            .RowTemplate.Resizable = DataGridViewTriState.False
 
            .SelectionMode = DataGridViewSelectionMode.FullRowSelect
            .MultiSelect = False
            .CellBorderStyle = DataGridViewCellBorderStyle.None
            .RowHeadersVisible = False
 
            .DataSource = DataTable
 
        End With
 
    End Sub