我们通过一个方法判断 （1.str == ""  2.str == String.Empty  3.str.Length == 0）

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static void Main()
        {
            string str = "csdn";
            System.Diagnostics.Stopwatch st = new System.Diagnostics.Stopwatch();
            st.Start();
            for (int i = 0; i < 100000000; i++)
            {
                if (str == "")
                { }
            }
            st.Stop();
            Console.WriteLine("str == '' : " + st.ElapsedMilliseconds);
            st = new System.Diagnostics.Stopwatch();
            st.Start();
            for (int i = 0; i < 100000000; i++)
            {
                if (str == String.Empty)
                { }
            }
            st.Stop();
            Console.WriteLine("str == String.Empty : " + st.ElapsedMilliseconds);
            st = new System.Diagnostics.Stopwatch();
            st.Start();
            for (int i = 0; i < 100000000; i++)
            {
                if (str.Length == 0)
                { }
            }
            st.Stop();
            Console.WriteLine("str.Length == 0 : " + st.ElapsedMilliseconds);
            Console.ReadKey();
        }

F5 用行
str == '': 1951  (微妙为单位)
str =='': 1867
str =='': 522