RichTextBox选中项正当中

RichTextBox选中项居中
如题，并不是让整个文本居中
所以

C# code

 TextAlignment align = TextAlignment.Left;                switch ((this.cmbStyles.SelectedValue as ComboBoxItem).Tag.ToString())                {                    case "左对齐":                        align = TextAlignment.Left;                        break;                    case "居中":                        align = TextAlignment.Center;                        break;                    case "右对齐":                        align = TextAlignment.Right;                        break;                }

C# code

 TextAlignment align = TextAlignment.Left;            switch ((this.cmbStyles.SelectedValue as ComboBoxItem).Tag.ToString())            {                case "左对齐":                    align = TextAlignment.Left;                    break;                case "居中":                    align = TextAlignment.Center;                    break;                case "右对齐":                    align = TextAlignment.Right;                    break;            }            //将样式赋给富文本框的选中项            rtb_text.Selection.ApplyPropertyValue(Block.TextAlignmentProperty, align);