How to implement custom helper class for Rich text box?

In this article I am going to discuss how to implement a custom helper class for Rich text box in WPF?  I hope this will helpful for any custom work requirement. Please check below code for custom implementation.

 /// <summary>
    /// Helper class for creating the custom dependency property for richtextbox to allow to bind the string property to richtextbox's document.
    /// </summary>
    public class RichTextBoxHelper : DependencyObject
    {
        public static readonly DependencyProperty DocumentTextProperty = DependencyProperty.RegisterAttached(
            "DocumentText",
            typeof(string),
            typeof(RichTextBoxHelper),
            new FrameworkPropertyMetadata
        {
            BindsTwoWayByDefault = true,
            PropertyChangedCallback = (obj, e) =>
                {
                    var richTextBox = (RichTextBox)obj;
 
                    // Parse the Text to a document
                    var text = GetDocumentText(richTextBox);
                    var doc = new FlowDocument();
                    var range = new TextRange(doc.ContentStart, doc.ContentEnd);
                    if (text != null)
                    {
                        range.Load(new MemoryStream(Encoding.UTF8.GetBytes(text)), DataFormats.Text);
                    }
 
                    // Set the document      
                    richTextBox.Document = doc;
 
                    // When the document changes update the source      
                    range.Changed += (obj2, e2) =>
                    {
                        if (richTextBox.Document == doc)
                        {
                            MemoryStream buffer = new MemoryStream();
                            range.Save(buffer, DataFormats.Text);
                            SetDocumentText(richTextBox, Encoding.UTF8.GetString(buffer.ToArray()));
                        }
                    };
                }
        });
 
        public static string GetDocumentText(DependencyObject obj)
        {
            return (string)obj.GetValue(DocumentTextProperty);
        }
 
        public static void SetDocumentText(DependencyObject obj, string value)
        {
            obj.SetValue(DocumentTextProperty, value);
        }
    }

Add this helper class for custom implementation.