System.IO.Stream st = new System.IO.MemoryStream();
richTextBox1.SaveFile(st, RichTextBoxStreamType.RichText );
byte[] texttodataBase= new byte[st.Length];
st.Seek(0,System.IO.SeekOrigin.Begin );
st.Read(texttodataBase ,0 , (int)st.Length);
// save data to database....
/******************************************/
byte[] textfromdataBase;
//read from database to textfromdataBase
System.IO.Stream st1 = new System.IO.MemoryStream();
st1.Write(textfromdataBase,0 , textfromdataBase.Length);
st1.Seek(0,System.IO.SeekOrigin.Begin);
richTextBox1.LoadFile(st1, RichTextBoxStreamType.RichText);