Your code, as I think you know, will not compile as it is, since 'SelectedValue is mis-spelled.
'SelectedValue is going to return something "meaningful" only if you are doing data-binding and have a valid 'ValueMember linked to the Control [
^].
'SelectedItem.ToString() can be used to test the user's choice in the ComboBox, as can 'SelectedIndex.
Hiding a main form while you show another Form in 'Dialog mode is not a good idea; it's not the way users expect Windows Applications to behave.
I suggest:
private Form3 f3 = new Form3();
if (comboBox1.SelectedItem == null) return;
switch (comboBox1.SelectedItem.ToString())
{
case "choice 1":
f3.ShowDialog();
break;
case "choice 2":
break;
}
Or, (more efficient):
switch (comboBox1.SelectedIndex)
{
case -1:
break;
case 0:
f3.ShowDialog();
break;
case 1:
break;
}