public static class StringLibrary
    {
　　　　　　//第一个参数指定该方法作用于哪个类型，并且该参数以 this 修饰符为前缀,当前作用于String类型，也可作用于自定义类型
        public static bool StartWithUpper(this String str)
        {
            if (String.IsNullOrEmpty(str))
                return false;

            char ch = str[0];
            return Char.IsUpper(ch);
        }
    }

[TestMethod]
        public void TestDoesNotStartWithUpper()
        {
            string[] words = { "alphabet", "Error", "zebra", "Abc", "αυτοκινητοβιομηχαν?α", "государство",
                   "1234", ".", ";", " " };

            foreach (var word in words)
            {
                bool result = word.StartWithUpper();
                Assert.IsFalse(result, $"Expected for ‘{word}‘: false; Actual:{result}");
            }
        }