Getting Combobox Selected Value in WPF

Question

hi
i want to get selected value of a combobox that created in WPF,
i use this code  :


private void cmb_StuffGroup_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
string str;
ComboBoxItem item = (ComboBoxItem)cmb_StuffGroup.SelectedItem;
str = item.Content.ToString();
MessageBox.Show(str, "ITEM");
}


but when i run this code it stop with InvalidCastExeption in line 4.
whats wrong with this code??
thanks