Lỗi Invalid token 'char' in class, struct, or interface member declaration

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Các bạn cho mình xin ý kiến với lỗi này với ạ "Invalid token 'char' in class, struct, or interface member declaration"
Code

using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Web;
using System.Web.Services;
using System.Web.Services.Protocols;


namespace Maychu
{
    /// <summary>
    /// Summary description for LtMang
    /// </summary>
    [WebService(Namespace = "http://tempuri.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    [ToolboxItem(false)]
    // To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
    // [System.Web.Script.Services.ScriptService]
    public class LtMang : System.Web.Services.WebService
    {
        private BT double id;
        private BT char name;
        private BT int year;
        private BT char symbol;
        private BT char authors;

        [WebMethod]
        public char ID(int id)
        {
            return id;
        }
        [WebMethod]

        [WebMethod]
        public char NAME(char name)
        {
            return name;
        }
        [WebMethod]
        public int YEAR(int year)
        {
            return year;
        }
        [WebMethod]
        public char SYMBOL(char symbol)
        {
            return symbol;
        }
        [WebMethod]
        public char AUTHORS(char authors)
        {
            return authors;
        }
        
    }
}