编写高质量代码改善C#程序的157个建议[正确操作字符串、使用默认转型方法、却别对待强制转换与as和is]

String str1="str1"+9;
String str2="str2"+9.ToString();

private static void NewMethod1()
{
      string s1="abc";
      s1="123"+s1+"456";    ////以上两行代码创建了3个字符串对象对象，并执行了一次string.Contact方法            
}

private static void NewMethod2()
{
      string re=9+"456";    ////该方法发生了一次装箱，并调用一次string.Contact方法  
}

private static void NewMethod3()
{
       string re2="123"+"abc"+"456";   ///该代码等效于///string re2="123abc456";  
}

private static void NewMethod4()
{
      const string a="t";
      string re="abc"+a;   ///因为a是一个常量，所以该代码等效于string=re="abc"+"t";  最终等效于string re="abct";  
}  

        private static void NewMethod1()
        {
            string a = "t";
            a += "e";
            a += "s";
            a += "t";
        }

        private static void NewMethod2()
        {
            string a = "t";
            string b = "e";
            string c = "s";
            string d = "t";
            string result = a + b + c + d;
        }

        public static void NewMethod()
        {
            ////定义了四个变量
            string a = "t";
            string b = "e";
            string c = "s";
            string d = "t";
            StringBuilder sb = new StringBuilder(a);
            sb.Append(b);
            sb.Append(c);
            sb.Append(d);
            
            ///提示是运行时，所以没有使用以下代码
            //StringBuilder sb = new StringBuilder("t");
            //sb.Append("e");
            //sb.Append("s");
            //sb.Append("t");
            //string result = sb.ToString();
        }

        public static void NewMethod4()
        {
            string a = "t";
            string b = "e";
            string c = "s";
            string d = "t";
            string result = string.Format("{0}{1}{2}{3}", a, b, c, d);
        }

            int i = 0;
            float j = 0;
            j = i;  ///int 到float存在一个隐式转换
            i = (int)j; ///float到int必须存在一个显式转换

    public class Ip
    {
        IPAddress value;
        public Ip(string ip)
        {
            value = IPAddress.Parse(ip);
        }

        //重载转换运算符，implicit 关键字用于声明隐式的用户定义类型转换运算符。
        public static implicit operator Ip(string ip)
        {
            Ip iptemp = new Ip(ip);
            return iptemp;
        }

        //重写ToString方法
        public override string ToString()
        {
            return value.ToString();
        }
    }
    class Program
    {
        public static void Main(string[] args)
        {
            Ip ip = "192.168.1.1";   //通过Ip类的重载转换运算符，实现字符串到Ip类型的隐式转换
            Console.WriteLine(ip.ToString());
            Console.ReadLine();
        }
    }

            Animal animal = new Animal();
            Dog dog = new Dog();
            animal = dog;   /////隐式转换，因为Dog就是Animal
            ///dog=animal;  ////编译不通过
            dog = (dog)animal; /////必须存在一个显式转换

            FirstType firstType = new FirstType();
            SecondType secondType = new SecondType();
            secondType = (SecondType)firstType;

    public class FirstType
    {
        public string Name { get; set; }
    }

    public class SecondType
    {
        public string Name { get; set; }

        public static explicit operator SecondType(FirstType firstType)
        {
            SecondType secondType = new SecondType() { Name = "转型自：" + firstType.Name };
            return secondType;
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            FirstType firstType = new FirstType() { Name="First Type"};
            SecondType secondType = (SecondType)firstType;  ///此转换是成功的
            secondType = firstType as SecondType;           ///编译不通过
            Console.ReadLine();
        }
    }

        static void DoWithSomeType(object obj)
        {
            ///编译器首先判断的是，SeondType和ojbect之间有没有继承关系。
            ///因为在C#中，所有的类型都是继承自object的，所以这里编译没有什么问题。
            ///但编译器会自动产生代码来检查obj在运行时是不是SecondType，这样就绕过了操作转换符，导致转换失败。
            SecondType secondType = (SecondType)obj;
        }

        static void DoWithSomeType(object obj)
        {
            SecondType secondType = obj as SecondType;
        }

    public class FirstType 
    {
        public string Name { get; set; }
    }

    public class SecondType : FirstType
    {
    }
    class Program
    {
        static void Main(string[] args)
        {
            SecondType secondType = new SecondType() { Name="aehyok"};
            FirstType firstType = (FirstType)secondType;
            firstType = secondType as FirstType;
            Console.ReadLine();
        }
    }

        static void DoWithSomeType(object obj)
        {
            if (obj is SecondType)
            {
                SecondType secondType = obj as SecondType;
            }
        }