StringBuilder是不是线程安全的？

static int maxNum = 1000000;
        static StringBuilder sbIsThreadSafe = new StringBuilder();
        static void Main(string[] args)
        {
            //迭代次数
            int iterationNum = 3;
            CodeTimer.Initialize();           
            CodeTimer.Time("StringBuilder是否是线程安全的呢？", iterationNum, new Action(SBIsThreadSafe));
            //Console.Write(sbIsThreadSafe.ToString());
            Console.Read();
        }
        private static void SBIsThreadSafe()
        {
            Parallel.For(1, maxNum / 2, (i) =>
            {
                sbIsThreadSafe.Append(i.ToString());
            });
            Parallel.For(maxNum / 2 + 1, maxNum, (i) =>
            {
                sbIsThreadSafe.Append(i.ToString());
            });
        }