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[];

            return Char.IsUpper(ch);

        }

    }

[TestMethod]

        public void TestDoesNotStartWithUpper()

        {

            string[] words = { "alphabet", "Error", "zebra", "Abc", "αυτοκινητοβιομηχανία", "государство",

                   "", ".", ";", " " };

            foreach (var word in words)

            {

                bool result = word.StartWithUpper();

                Assert.IsFalse(result, $"Expected for '{word}': false; Actual:{result}");

            }

        }