title: Check if combobox value is empty
body: <p>I have created a ComboBox with three values. I wanted that a message box opens when no item is selected so I tried this:</p>
  
  <pre><code>if (comboBox1.SelectedItem == null)
  {
      MessageBox.Show("Please select a value");
      return;
  }
  </code></pre>
  
  <p>That works fine but only if I click into the field in the combobox. When I dont touch it, the program will start without message box. Whats wrong?</p>