C#學(xué)習(xí)基礎(chǔ)概念二十五問續(xù)2第1/2頁

2020-01-24 03:49:45

字體：大中小

供稿：網(wǎng)友

6.sealed 修飾符是干什么的？
答：
sealed 修飾符表示密封
用于類時，表示該類不能再被繼承，不能和 abstract 同時使用，因?yàn)檫@兩個修飾符在含義上互相排斥
用于方法和屬性時，表示該方法或?qū)傩圆荒茉俦焕^承，必須和 override 關(guān)鍵字一起使用，因?yàn)槭褂?nbsp;sealed 修飾符的方法或?qū)傩钥隙ㄊ腔愔邢鄳?yīng)的虛成員
通常用于實(shí)現(xiàn)第三方類庫時不想被客戶端繼承，或用于沒有必要再繼承的類以防止濫用繼承造成層次結(jié)構(gòu)體系混亂
恰當(dāng)?shù)睦?nbsp;sealed 修飾符也可以提高一定的運(yùn)行效率，因?yàn)椴挥每紤]繼承類會重寫該成員
示例：
using System;
using System.Collections.Generic;
using System.Text;
namespace Example06
{
    class Program
    {
        class A
        {
            public virtual void F()
            {
                Console.WriteLine("A.F");
            }
            public virtual void G()
            {
                Console.WriteLine("A.G");
            }
        }
        class B : A
        {
            public sealed override void F()
            {
                Console.WriteLine("B.F");
            }
            public override void G()
            {
                Console.WriteLine("B.G");
            }
        }
        class C : B
        {
            public override void G()
            {
                Console.WriteLine("C.G");
            }
        }
        static void Main(string[] args)
        {
            new A().F();
            new A().G();
            new B().F();
            new B().G();
            new C().F();
            new C().G();
            Console.ReadLine();
        }
    }
}
結(jié)果：
類 B 在繼承類 A 時可以重寫兩個虛函數(shù)，如圖所示：

由于類 B 中對 F 方法進(jìn)行了密封，類 C 在繼承類 B 時只能重寫一個函數(shù)，如圖所示：

控制臺輸出結(jié)果，類 C 的方法 F 只能是輸出類B 中對該方法的實(shí)現(xiàn)：
A.F
A.G
B.F
B.G
B.F
C.G

7.override 和 overload 的區(qū)別？
答：
override 表示重寫，用于繼承類對基類中虛成員的實(shí)現(xiàn)
overload 表示重載，用于同一個類中同名方法不同參數(shù)（包括類型不同或個數(shù)不同）的實(shí)現(xiàn)
示例：
using System;
using System.Collections.Generic;
using System.Text;
namespace Example07
{
    class Program
    {
        class BaseClass
        {
            public virtual void F()
            {
                Console.WriteLine("BaseClass.F");
            }
        }
        class DeriveClass : BaseClass
        {
            public override void F()
            {
                base.F();
                Console.WriteLine("DeriveClass.F");
            }
            public void Add(int Left, int Right)
            {
                Console.WriteLine("Add for Int: {0}", Left + Right);
            }
            public void Add(double Left, double Right)
            {
                Console.WriteLine("Add for int: {0}", Left + Right);
            }
        }
        static void Main(string[] args)
        {
            DeriveClass tmpObj = new DeriveClass();
            tmpObj.F();
            tmpObj.Add(1, 2);
            tmpObj.Add(1.1, 2.2);
            Console.ReadLine();
        }
    }
}
結(jié)果：
BaseClass.F
DeriveClass.F
Add for Int: 3
Add for int: 3.3

8.什么是索引指示器？
答：
實(shí)現(xiàn)索引指示器（indexer）的類可以象數(shù)組那樣使用其實(shí)例后的對象，但與數(shù)組不同的是索引指示器的參數(shù)類型不僅限于int
簡單來說，其本質(zhì)就是一個含參數(shù)屬性
示例：

using System;
using System.Collections.Generic;
using System.Text;
namespace Example08
{
    public class Point
    {
        private double x, y;
        public Point(double X, double Y)
        {
            x = X;
            y = Y;
        }
        //重寫ToString方法方便輸出
        public override string ToString()
        {
            return String.Format("X: {0} , Y: {1}", x, y);
        }
    }
    public class Points
    {
        Point[] points;
        public Points(Point[] Points)
        {
            points = Points;
        }
        public int PointNumber
        {
            get
            {
                return points.Length;
            }
        }
        //實(shí)現(xiàn)索引訪問器
        public Point this[int Index]
        {
            get
            {
                return points[Index];
            }
        }
    }
    //感謝watson hua(http://huazhihao.cnblogs.com/)的指點(diǎn)
    //索引指示器的實(shí)質(zhì)是含參屬性，參數(shù)并不只限于int
    class WeatherOfWeek
    {
        public string this[int Index]
        {
            get
            {
                //注意case段使用return直接返回所以不需要break
                switch (Index)
                {
                    case 0:
                        {
                            return "Today is cloudy!";
                        }
                    case 5:
                        {
                            return "Today is thundershower!";
                        }
                    default:
                        {
                            return "Today is fine!";
                        }
                }
            }
        }
        public string this[string Day]
        {
            get
            {
                string TodayWeather = null;
                //switch的標(biāo)準(zhǔn)寫法
                switch (Day)
                {
                    case "Sunday":
                        {
                            TodayWeather = "Today is cloudy!";
                            break;
                        }
                    case "Friday":
                        {
                            TodayWeather = "Today is thundershower!";
                            break;
                        }
                    default:
                        {
                            TodayWeather = "Today is fine!";
                            break;
                        }
                }
                return TodayWeather;
            }
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            Point[] tmpPoints = new Point[10];
            for (int i = 0; i < tmpPoints.Length; i++)
            {
                tmpPoints[i] = new Point(i, Math.Sin(i));
            }
            Points tmpObj = new Points(tmpPoints);
            for (int i = 0; i < tmpObj.PointNumber; i++)
            {
                Console.WriteLine(tmpObj[i]);
            }

            string[] Week = new string[] { "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Staurday"};
            WeatherOfWeek tmpWeatherOfWeek = new WeatherOfWeek();
            for (int i = 0; i < 6; i++)
            {
                Console.WriteLine(tmpWeatherOfWeek[i]);
            }
            foreach (string tmpDay in Week)
            {
                Console.WriteLine(tmpWeatherOfWeek[tmpDay]);
            }
            Console.ReadLine();
        }
    }
}
結(jié)果：
X: 0 , Y: 0
X: 1 , Y: 0.841470984807897
X: 2 , Y: 0.909297426825682
X: 3 , Y: 0.141120008059867
X: 4 , Y: -0.756802495307928
X: 5 , Y: -0.958924274663138
X: 6 , Y: -0.279415498198926
X: 7 , Y: 0.656986598718789
X: 8 , Y: 0.989358246623382
X: 9 , Y: 0.412118485241757
Today is cloudy!
Today is fine!
Today is fine!
Today is fine!
Today is fine!
Today is thundershower!
Today is cloudy!
Today is fine!
Today is fine!
Today is fine!
Today is fine!
Today is thundershower!
Today is fine!