How to know if a value is selected in combobox code example

Example 1: c# if combobox selected index

//detect if no selected item on ComboBox is chosen?
if( ComboBox.SelectedItem == null ) {
   // do something
}

Example 2: c# if combobox selected index

Item m = comboBox.Items[comboBox.SelectedIndex];
  //Or
  Item m = comboBox.SelectedItem;