I have just started using WPF forms instead of Windows Forms forms. In a Windows Forms form I could just do:
ComboBox.SelectedValue.toString();
Actually you can do it the following way as well.
Suppose your ComboBox name is comboBoxA. Then its value can be gotten as:
comboBoxA
string combo = comboBoxA.SelectedValue.ToString();
I think it's now supported since your question is five years old.