RichTextBoxPEXT.cs

    public static class ParagraphExtention
    {
        public static void Append(this Paragraph paragraph, string value = "", Brush background = null, Brush foreground = null, bool bold = false, bool italic = false, bool underline = false, bool waitUntilReturn = false)
        {
            Action append = () =>
            {
                Inline run = new Run(value);

                if (background != null) run.Background = background;
                if (foreground != null) run.Foreground = foreground;
                if (bold) run = new Bold(run);
                if (italic) run = new Italic(run);
                if (underline) run = new Underline(run);
                paragraph.Inlines.Add(run);

                paragraph.Inlines.Add(run);
            };
            if (paragraph.CheckAccess())
            {
                append();
            }
            else if (waitUntilReturn)
            {
                paragraph.Dispatcher.Invoke(append);
            }
            else
            {
                paragraph.Dispatcher.BeginInvoke(append);
            }
        }
    }

    public static class RichTextBoxExtensions
    {
        public static void CheckAppendText(this RichTextBox richtextBox, Paragraph msg, bool waitUntilReturn = false)
        {
            //Action append = () =>
            Action append = () =>
            {
                richtextBox.Document.CheckAppendText(msg);
            };
            if (richtextBox.CheckAccess())
            {
                append();
            }
            else if (waitUntilReturn)
            {
                richtextBox.Dispatcher.Invoke(append);
            }
            else
            {
                richtextBox.Dispatcher.BeginInvoke(append);
            }
        }
    }

    public static class FlowDocumentExtensions
    {
        public static void CheckAppendText(this FlowDocument fDoc, Paragraph msg, bool waitUntilReturn = false)
        {
            //Action append = () =>
            Action append = () =>
            {
                //Paragraph msgx = msg;
                // msgx.Inlines.Add(Environment.NewLine);

                fDoc.Blocks.Add(msg);
            };
            if (fDoc.CheckAccess())
            {
                append();
            }
            else if (waitUntilReturn)
            {
                fDoc.Dispatcher.Invoke(append);
            }
            else
            {
                fDoc.Dispatcher.BeginInvoke(append);
            }
        }
    }