How to store richtextbox to sqlserver database in WPF using entity framework

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C#

WPF

RichTextBox

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I'm a bit new to C# and WPF and experiencing a very frustrating issue storing a richtextbox with formatting to my database using entity framework. I've attached some code below.

Please help me debug it, I'm getting the error on the ConvertRtbToBytes Function that... 'string' does not contain definition for 'Document' and no accessible extension method 'Document' accepting a first argument of type string could be found (are you missing a using directive or an assembly reference?)

I am also sure the general code is not Okay, any help will be massively appreciated.

Here is my code.

What I have tried:

namespace simpleton   
 public partial class AddNewOE : Window   
   {   
      //db connection           
      simpletonDBEntities _db = new simpletonDBEntities();  
      public AddNewOE()  
      {  
         InitializeComponent();  
       }  
  
       private void insertobBtn_Click(object sender, RoutedEventArgs e)  
       {  
   
           db_entry newdb_entries = new db_entry()   
          {   
             ReportDetails = ConvertRtbToBytes(rtfText)  
           };               
            _db.ob_entries.Add(newOb_entries);           
           _db.SaveChanges();   
        }  
       public static byte[] ConvertRtbToBytes(string richTextBox)   
       {   
            byte[] data = null; string rtfText; //string to save to db   
           TextRange tr = new TextRange(richTextBox.Document.ContentStart, richTextBox.Document.ContentEnd);   
           using (MemoryStream ms = new MemoryStream())   
           {               
                   tr.Save(ms, DataFormats.Rtf);              
                   rtfText = Encoding.ASCII.GetString(ms.ToArray());  
             }   
            return data;  
         }  
     }  
  
}