Convert Generic list to data

I am getting an error when I try to use a data class to retrieve data that I store in a list

it says Cannot implicitly convert type 'System.Collections.Generic.List<MessageBoard.Data.MessageData>' to 'MessageBoard.Data.MessageData

Data.Message m = new Data.Message();
                Data.MessageData md = m.GetEditThread(ID);
                //List<Data.MessageData> md = new List<Data.MessageData>();
                //List<Data.MessageData> md = m.GetEditThread(ID);
                lblPostID.Text = md.id;
                radtxtBody = md.Body;
                radtxtSubject = md.Subject;

I have my sql statement and my reader to add each item to the stubs in Message and my stubs in message data

This is message data

    public class MessageData
    {
         public MessageData()
        {
 
        }
        public int id { get; set; }
        public string Subject { get; set; }
        public string Body { get; set;}
        public int File_Id {get; set;}
        public bool ForPublic {get;set;}
        public bool ForPrivate {get;set;}
        public bool ForKKOnly {get;set;}
        public int UserID {get;set;}
        public int PriorityID {get;set;}
    }

This is Message

    class Message
    {
        public List<MessageData> GetEditThread(int ID)
        {
            List<MessageData> list = new List<MessageData>();

            
            string sql;
            sql = "Select id,Subject, Body,File_Id,ForPublic,ForKKOnly, ForPrivate,UserID,PriorityID FROM MsgPost Where id = '" + ID + "'";


            using (SqlConnection conn = new SqlConnection(DBConnectionString.Get(Database.Task)))
            {
                SqlCommand cmd = new SqlCommand(sql, conn);
                cmd.CommandType = System.Data.CommandType.Text;

                conn.Open();

                SqlDataReaderHelper reader = new SqlDataReaderHelper(cmd.ExecuteReader());
                if (reader.IsValid)
                {
                    while (reader.Read())
                    {
                        list.Add(new MessageData()
                        {
                            ID = reader.GetInt("ID", 0),
                            Subject = reader.GetString("Subject", ""),
                            Body = reader.GetString("Body", ""),
                            File_Id = reader.GetInt("File_Id", 0),
                            ForPublic = reader.GetBool("ForPublic", false),
                            ForPrivate = reader.GetBool("ForPrivate", false),
                            ForKKOnly = reader.GetBool("ForKKOnly", false),
                            UserID = reader.GetInt("UserID", 0),
                            PriorityID = reader.GetInt("PriorityID", 0),
                        });
                    }
                }
                conn.Close();
            }
            return list;
        }

    
    }