Combobox force Validation when no value is Selected

Hello,

I have a combo box in WPF project page. I have several items in it and one of the items is '(None)'.
Since my combo box is a compulsory field I would like to force validation on the combo box and wish to throw new Validation error if '(None)' is selected which gives red border around the combo box, just like on text box validation.

Your guidance will be of great help as I am very new to WPF.

Thank you.