C# MessageBox with ComboBox doesn't works

Question

The last MessageBox with the error message doesn't work; the program just crashes. I want to use the MessageBox to show the user that they forgot to select one of the items: Mountenbike, Rennrad, or Faltrad.

if (typen.SelectedItem.ToString() == "Mountenbike")
{
    mb = new Mountenbike(artikelNr, name, ekPreis, vkPreis);
    mbCounter++;
    MessageBox.Show("Fertig");
}
else if (typen.SelectedItem.ToString() == "Rennrad")
{
    rr = new Rennrad(artikelNr, name, ekPreis, vkPreis);
    rrCounter++;
    MessageBox.Show("Fertig");
}
else if (typen.SelectedItem.ToString() == "Faltrad")
{
    fr = new Faltrad(artikelNr, name, ekPreis, vkPreis);
    frCounter++;
    MessageBox.Show("Fertig");
}
else if (typen.SelectedItem == null)
{
    MessageBox.Show("Error - Please enter xyz");
}

Answer 1

You are probably getting an exception because you can't instantiate the method ToString() on a null reference, assuming by your last else if statement that the value of typen.SelectedItem can potentially be null if nothing is selected.

You should check whether the value of typen.SelectedItem is null first before instantiating any methods against it. That can be done simply as:

if (typen.SelectedItem == null)
{
    MessageBox.Show("Error - Please enter xyz");
    return;
}

if (typen.SelectedItem.ToString() == "Mountenbike")
{
    mb = new Mountenbike(artikelNr, name, ekPreis, vkPreis);
    mbCounter++;
    MessageBox.Show("Fertig");
}
else if (typen.SelectedItem.ToString() == "Rennrad")
{
    rr = new Rennrad(artikelNr, name, ekPreis, vkPreis);
    rrCounter++;
    MessageBox.Show("Fertig");
}
else if (typen.SelectedItem.ToString() == "Faltrad")
{
    fr = new Faltrad(artikelNr, name, ekPreis, vkPreis);
    frCounter++;
    MessageBox.Show("Fertig");
}