Can you explain why you forbid the empty char ?
As far as i can see you can set it in the exposed child element (maskEditBoxElement) and it is handled correctly if set as empty.
I need it as empty, so i'm wondering if i'll run into problems that i currently can not see.
public char PromptChar{ get { return this.maskEditBoxElement.PromptChar; } set { if (value == char.MinValue) throw new ArgumentException("Specified Character is not valid for this property!"); this.maskEditBoxElement.PromptChar = value; }}