C# 两个Task同步访问共享资源

using System;
using System.Diagnostics;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp5
{
    class Program
    {
        static Random r = new Random();
        static string Message;

        static void MethodA()
        {
            for (int i = 0; i < 5; i++)
            {
                Thread.Sleep(r.Next(2000));
                Message += "A";
                Console.Write(".");
            }
        }


        static void MethodB()
        {
            for (int i = 0; i < 5; i++)
            {
                Thread.Sleep(r.Next(2000));
                Message += "B";
                Console.Write(".");
            }
        }
        static void Main(string[] args)
        {
            Console.WriteLine("Please wait for the tasks to complete");
            Stopwatch watch = Stopwatch.StartNew();
            Task a = Task.Factory.StartNew(MethodA);
            Task b = Task.Factory.StartNew(MethodB);

            Task.WaitAll(new Task[] { a, b });
            Console.WriteLine() ;
            Console.WriteLine($"Results:{Message}");
            Console.WriteLine($"{watch.ElapsedMilliseconds:#.##0} elapsed millseconds");
        }
    }
}