private void checkToolBarButtons()
    {
        // this method checks if the button should be highlighted or not.
        Func<Button, object> check = (Button button) =>
        {
            try
            {
                // the button has two costum properties that allow to determine which text property should be
                // applied to the selected text.
                string propString = ToolBarButton.GetDocumentProperty(button);
                // e.g. "TextDecorations"
                DependencyProperty dependency = this.getPropertyByString(propString);
                // e.g. TextDecorationsProperty
                string propertyValue = ToolBarButton.GetDocumentPropertyValue(button);
                // e.g. "Underline"

                if (dependency != null)
                {
                    TextRange selectionRange = new TextRange(this.richTextBox.Selection.Start, this.richTextBox.Selection.End);
                    object selectedProperty = selectionRange.GetPropertyValue(dependency);

                    if (selectedProperty.GetType() == typeof(TextDecorationCollection) && ((TextDecorationCollection)selectedProperty).Count > 0)
                    {
                        if (selectedProperty.Equals(TextDecorations.Underline))
                        {
                            // this code is never reached
                            selectedProperty = "Underline";
                        }
                        else if (selectedProperty.Equals(TextDecorations.Strikethrough))
                        {
                            // this code is never reached
                            selectedProperty = "Strikethrough";
                        }
                    }

                    if (selectedProperty.ToString() == propertyValue)
                    {
                        button.Background = new SolidColorBrush(Colors.Yellow);
                    }
                    else
                    {
                        button.Background = new SolidColorBrush(Colors.LightGray);
                    }
                }
            }
            catch (Exception ex)
            {

            }
            return null;
        };

        foreach (FrameworkElement ctrl in toolBar.Children)
        {
            if (ctrl.GetType() == typeof(StackPanel))
            {
                foreach (Button button in ((Panel)ctrl).Children)
                {
                    check(button);
                }
            }
            else if (ctrl.GetType() == typeof(Button))
            {
                check((Button)ctrl);
            }
        }
    }

    /// <summary>
    /// converts a string to a DependencyProperty
    /// </summary>
    /// <param name="propertyString"></param>
    /// <returns></returns>
    private DependencyProperty getPropertyByString(string propertyString)
    {
        switch (propertyString)
        {
            case ("FontStyleProperty"): return FontStyleProperty;
            case ("FontWeightProperty"): return FontWeightProperty;
            case ("TextDecorations"): return TextBlock.TextDecorationsProperty;
            case ("TextAlignment"): return TextBlock.TextAlignmentProperty;
            default:
                break;
        }

        return null;
    }