i want to make textbox be readonly when H is selected

public static readonly DependencyProperty DiscTypeProperty = DependencyProperty.Register("DiscTypeDP", typeof(string), typeof(ItemReceipt));
        public string DiscType
        {
            get
            {
                return GetValue<string>(DiscTypeProperty);
            }
            set
            {

                //if (IsLoaded)
                //{
                if (DiscLevel == "H")
                {
                    if (DiscType == null || DiscType != value)
                    {
                        SetValue<string>(DiscTypeProperty, value);
                        DiscValue = 0;
                    }
                }
                else
                {
                    SetValue<string>(DiscTypeProperty, value);
                }
                //}

            }
        }