单例下并发的情况下访问私有变量的一个典型问题

public class dssd
    {
        static void Main(string[] args)
        {
            var t = Task.Factory.StartNew(() =>
            {
                Thread.Sleep(1000);
                var obj2 = ImportProvider.Instance;
                
                Arg ds = new Arg() { TypeSS = 2 };
                obj2.Import(ds);
                        
            });

            var obj = ImportProvider.Instance;

            obj.Import(new Arg() { TypeSS = 1 });
                                
            Console.Read();
        }    
    }


    public interface IImportProvider
    {
        string Import(Arg args);
    }

    public class ImportProvider : IImportProvider
    {
        #region 单例
        public static readonly IImportProvider _instance = new ImportProvider();

        private ImportProvider() { }

        public static IImportProvider Instance { get { return _instance; } }
        #endregion

        public string Import(Arg args)
        {
            Yaya = new List<int>() { args.TypeSS };
            HSSFWorkbook workBook = new HSSFWorkbook() { dddd =args.TypeSS};

            if (args.TypeSS==1)
            {
                Thread.Sleep(5000);
                Console.WriteLine(Yaya[0]);               
            }        
            
            return "ssss";
        }

        private List<int> Yaya = new List<int>();

    }

   


    public class HSSFWorkbook
    {
        public int dddd { get; set; }    
    }

    public class Arg
    {
        public int TypeSS { get; set; }

    }

public class dssd
    {


        static void Main(string[] args)
        {
            List<int> Yaya = new List<int>();

            var t = Task.Factory.StartNew(() =>
            {
                Thread.Sleep(1000);
                var obj2 = ImportProvider.Instance;
                
                Arg ds = new Arg() { TypeSS = 2 };
                obj2.Import(ds, Yaya);

            });

            var obj = ImportProvider.Instance;

            obj.Import(new Arg() { TypeSS = 1 }, Yaya);                      
          
            Console.Read();
        }

    }


    public interface IImportProvider
    {
        string Import(Arg args, List<int> ww);
    }

    public class ImportProvider : IImportProvider
    {
        #region 单例
        public static readonly IImportProvider _instance = new ImportProvider();

        private ImportProvider() { }

        public static IImportProvider Instance { get { return _instance; } }
        #endregion

        public string Import(Arg args,List<int> ww)
        {
            ww = new List<int>() { args.TypeSS };
            HSSFWorkbook workBook = new HSSFWorkbook() { dddd =args.TypeSS};

            if (args.TypeSS==1)
            {
                Thread.Sleep(5000);
                Console.WriteLine(ww[0]);               
            }        
            
            return "ssss";
        }

    }

   


    public class HSSFWorkbook
    {
        public int dddd { get; set; }    
    }

    public class Arg
    {
        public int TypeSS { get; set; }

    }