public static class SelectedItemExtension
  {
    public static object GetSelectedItem(this ComboBox item)
    {
      return getObject(item.SelectedItem);
    }
 
    private static object getObject(object item)
    {
      if(item is ObjectWrapper)
      {
        return ((ObjectWrapper)item).Basis;
      }
      else
      {
        return item;
      }
    }
  }
 
  public partial class frmTest : Form
  {
    public frmTest()
    {
      comboBox1.Items.Add(new ObjectWrapper(new BasisItem("Hallo")));
      comboBox1.Items.Add(new ObjectWrapper(new BasisItem("Welt")));
    }
 
    private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
    {
      BasisItem beispiel = comboBox1.GetSelectedItem();
    }
  }
 
  class BasisItem
  {
    public BasisItem(string value)
    {
      this.value = value;
    }
 
    string value; 
 
    public override string ToString()
    {
      return value;
    }
  }