C#中的接口实现多态

〇、引言

接口，属于方法的抽象，它只定义方法，而不包含任何实现，既然没有实现，接口存在的意义是什么呢？

接口实际上是一种契约或者约定，例如，插头有三脚的，有两脚的，但他们都有一个相同的功能，就是绝缘与通电，这也是全世界的插头所共同遵守的规则。此时，我可以定义一个约定，说：凡是设计插头的都要遵守以下约定：提供绝缘的办法，提供通电的办法。对应到C#中，就是在接口中定义两个函数，一个为绝缘，一个为通电。所有插头的类都要继承这个接口，这样就会强迫所有类实现绝缘与通电两个方法，而具体如何实现，则因不同插头而不同。

一、接口实现多态。

多态是面向对象编程中十分重要的一个性质，它的设计思想是：只有对象自己知道具体如何执行特定的操作，通过规定执行这些操作的通用方式，还可以促进了代码重用。例如每种文档都有自己的打印方式，但是不应该定义switch来选择不同的文档不同输出，而应该利用多态性，在基类中设计Print(); 当有需要打印任务时候就调用Print()。

多态可以使用类的继承很容易的实现，但是多态也可以利用接口实现。代码如下：

1、首先定义一个接口，功能是返回一组字符串，代表着信息。

 1 // <copyright file="IListable.cs" company="TNT">
 2 // Copyright (c) TNT. All rights reserved.
 3 // </copyright>
 4 // <author>Li Mingjian</author>
 5 // <date> 2014-11-14 </date>
 6 // <summary>显示一行数据的接口</summary>
 7 namespace InterfaceMultiState
 8 {
 9     using System;
10     using System.Collections.Generic;
11     using System.Linq;
12     using System.Text;
13 
14     /// <summary>
15     /// A interface of show a row data
16     /// </summary>
17     /// <see cref=""></see>
18     /// <remarks>
19     /// <c>创建人：Li 
20     /// 创建日期：2014-11-14
21     /// 版本：1.0</c>
22     /// </remarks>
23     interface IListable
24     {
25         // 返回一行中的每一列数据
26         string[] Columnvalues
27         {
28             get;
29         }
30     }
31 }

View Code

然后，将会继续写两个类：Contact与Publishion，显然一个是关于人员的，一个是关于书籍的，这两个类都继承接口IListable，当需要输出信息时，直接利用接口的多态，将类的实例转型为接口类型，统一输出。接口实例便实现多态，在找对应的输出。

2、定义Contact类，其从抽象类PdaItem抽象而来，同时继承接口IListable：

PdaItem如下：

 1 // <copyright file="PdaItem.cs" company="TNT">
 2 // Copyright (c) TNT. All rights reserved PdaItem.cs.
 3 // </copyright>
 4 // <author>Li Mingjian</author>
 5 // <date> 2014-11-14 </date>
 6 // <summary>联系人抽象类</summary>
 7 namespace InterfaceMultiState
 8 {
 9     using System;
10     using System.Collections.Generic;
11     using System.Linq;
12     using System.Text;
13 
14     /// <summary>
15     /// A abstract class of contact or others
16     /// </summary>
17     /// <see cref=""></see>
18     /// <remarks>
19     /// <c>创建人：Li 
20     /// 创建日期：2014-11-14
21     /// 版本：1.0</c>
22     /// </remarks>
23     public abstract class PdaItem
24     {
25         /// <summary>
26         /// <c>Initializes a new instance of the PdaItem class.</c>
27         /// </summary>
28         /// <param name="name">The name</param>
29        public PdaItem(string name)
30        {
31        } // PadItem
32 
33        /// <summary>
34        /// Gets or sets virtual attribute:Name
35        /// </summary>
36        public virtual string Name { get; set; }
37     } // class
38 } // namespace InterfaceMultiState

PadItem

Contact类如下：

 1 // <copyright file="Contact.cs" company="TNT">
 2 // Copyright (c) TNT. All rights reserved Contact.cs.
 3 // </copyright>
 4 // <author>Li Mingjian</author>
 5 // <date> 2014-11-14 </date>
 6 // <summary>联系人抽象类</summary>
 7 namespace InterfaceMultiState
 8 {
 9     using System;
10     using System.Collections.Generic;
11     using System.Linq;
12     using System.Text;
13 
14     /// <summary>
15     /// A Contact class with set and show member information
16     /// </summary>
17     /// <see cref=""></see>
18     /// <remarks>
19     /// <c>创建人：Li 
20     /// 创建日期：2014-11-14
21     /// 版本：1.0</c>
22     /// </remarks>
23     internal class Contact : PdaItem, IListable
24     { 
25         /// <summary>
26         /// Initializes a new instance of the Contact class.
27         /// </summary>
28         /// <param name="firsteName">The firstname</param>
29         /// <param name="lastName">The lastname</param>
30         /// <param name="address">The address</param>
31         /// <param name="phone">The phone</param>
32         public Contact(string firsteName, string lastName, string address, string phone)
33             : base(null)
34         {
35             this.FirstName = firsteName;
36             this.LastName = lastName;
37             this.Address = address;
38             this.Phone = phone; 
39         } // public Contact()
40 
41         /// <summary>
42         /// Gets Header include: First Name. Last Name. Phone. Address
43         /// </summary>
44         public static string[] Headers
45         {
46             get
47             {
48                 return new string[]
49                 {
50                     "First Name          ", "Last Name          ",
51                     "Phone           ",
52                     "Address                                     "
53                 }; // return
54             }
55         } // Headers
56 
57         /// <summary>
58         /// Gets or sets firstname
59         /// </summary>
60         public string FirstName { get; set; }
61 
62         /// <summary>
63         /// Gets or sets lastname
64         /// </summary>
65         public string LastName { get; set; }
66 
67         /// <summary>
68         /// Gets or sets address
69         /// </summary>
70         public string Address { get; set; }
71 
72         /// <summary>
73         /// Gets or sets phone
74         /// </summary>
75         public string Phone { get; set; }
76 
77         /// <summary>
78         /// Gets information of member
79         /// </summary>
80         public string[] Columnvalues
81         {
82             get
83             {
84                 return new string[]
85                 {
86                     this.FirstName,
87                     this.LastName,
88                     this.Phone,
89                     this.Address
90                 }; // return
91             } // get
92         } // Columbalues
93     } // class Contact
94 } // namespace InterfaceMultiState

Contact

3、定义publishtion类，继承接口IListable

 1 // <copyright file="Publication.cs" company="TNT">
 2 // Copyright (c) TNT. All rights reserved Publication.cs.
 3 // </copyright>
 4 // <author>Li Mingjian</author>
 5 // <date> 2014-11-14 </date>
 6 // <summary>联系人抽象类</summary>
 7 namespace InterfaceMultiState
 8 {
 9     using System;
10     using System.Collections.Generic;
11     using System.Linq;
12     using System.Text;
13 
14     /// <summary>
15     /// A Publication class with set and show books information
16     /// </summary>
17     /// <see cref=""></see>
18     /// <remarks>
19     /// <c>创建人：Li 
20     /// 创建日期：2014-11-14
21     /// 版本：1.0</c>
22     /// </remarks>
23     internal class Publication : IListable
24     {
25         /// <summary>
26         /// Initializes a new instance of the Publication class
27         /// </summary>
28         /// <param name="title">The title of book</param>
29         /// <param name="author">The author</param>
30         /// <param name="year">The year of published</param>
31         public Publication(string title, string author, int year)
32         {
33             this.Title = title;
34             this.Author = author;
35             this.Year = year;
36         }
37 
38         /// <summary>
39         /// Gets Header include: Title Author Year
40         /// </summary>
41         public static string[] Headers
42         {
43             get
44             {
45                 return new string[]
46                 {
47                     "Title                  ",
48                     "Author             ",
49                     "Year"
50                 }; // return
51             }
52         } // Headers
53 
54         /// <summary>
55         /// Gets or sets title
56         /// </summary>
57         public string Title { get; set; }
58 
59         /// <summary>
60         /// Gets or sets author
61         /// </summary>
62         public string Author { get; set; }
63 
64         /// <summary>
65         /// Gets or sets year
66         /// </summary>
67         public int Year { get; set; }
68 
69         /// <summary>
70         /// Gets information of book
71         /// </summary>
72         public string[] Columnvalues
73         {
74             get
75             {
76                 return new string[]
77                 {
78                     this.Title,
79                     this.Author,
80                     this.Year.ToString(),
81                 }; // return
82             } // get
83         } // Columbalues
84     } // class
85 } // namespace InterfaceMultiState

Publishtion

4、编写一个输出类，用来控制输出，这个类提供了一个对外的静态函数

1 public static void List(string[] headers, IListable[] items)

用来根据传入的参数进行输出，注意第二个参数的类型，是一个接口类型。此类的重点是如何输出items.Columnvalues

 1 // <copyright file="ConsoleListControl.cs" company="TNT">
 2 // Copyright (c) TNT. All rights reserved ConsoleListControl.cs.
 3 // </copyright>
 4 // <author>Li Mingjian</author>
 5 // <date> 2014-11-14 </date>
 6 // <summary>联系人抽象类</summary>
 7 namespace InterfaceMultiState
 8 {
 9     using System;
10     using System.Collections.Generic;
11     using System.Linq;
12     using System.Text;
13 
14     /// <summary>
15     /// out put string[] to console
16     /// </summary>
17     /// <see cref=""></see>
18     /// <remarks>
19     /// <c>创建人：Li 
20     /// 创建日期：2014-11-14
21     /// 版本：1.0</c>
22     /// </remarks>
23     internal class ConsoleListControl
24     {
25         /// <summary>
26         /// out put information to console
27         /// </summary>
28         /// <param name="headers">The headers</param>
29         /// <param name="items">A group of item</param>
30         public static void List(string[] headers, IListable[] items)
31         {
32             int[] columWidths = DisplayHeader(headers);
33 
34             for (int count = 0; count < items.Length; count++)
35             {
36                 string[] values = items[count].Columnvalues;
37 
38                 DisplayItemsRow(columWidths, values);
39             } // for
40         } // List
41 
42         /// <summary>
43         /// out put headers to console
44         /// </summary>
45         /// <param name="headers">The header of one entity</param>
46         /// <returns type="int[]">The columWidth</returns>
47         private static int[] DisplayHeader(string[] headers)
48         {
49             int[] columWidth = new int[headers.Length];
50             string head = string.Empty;
51 
52             for (int count = 0; count < headers.Length; count++)
53             {
54                 head += headers[count];
55                 columWidth[count] = headers[count].Length;
56             } // for
57 
58             Console.WriteLine(head);
59 
60             return columWidth;
61         } // DisplayHeader
62 
63         /// <summary>
64         /// out put items' value to console
65         /// </summary>
66         /// <param name="columWidth">The columWidth</param>
67         /// <param name="values">Item values</param>
68         private static void DisplayItemsRow(int[] columWidth, string[] values)
69         {
70             string value = string.Empty;
71 
72             for (int count = 0; count < values.Length; count++)
73             {
74                 int strLen = values[count].Length;
75                 values[count] = values[count].PadRight(columWidth[count]);
76                 value += values[count];
77             } // for
78 
79             Console.WriteLine(value);
80         } // DisplayItemsRow
81     } // class
82 }

View Code

5、主函数中：

 1 // <copyright file="Program.cs" company="TNT">
 2 // Copyright (c) TNT. All rights reserved Program.cs.
 3 // </copyright>
 4 // <author>Li Mingjian</author>
 5 // <date> 2014-11-14 </date>
 6 // <summary>联系人抽象类</summary>
 7 namespace InterfaceMultiState
 8 {
 9     using System;
10     using System.Collections.Generic;
11     using System.Linq;
12     using System.Text;
13 
14     /// <summary>
15     /// A access of the whole project
16     /// </summary>
17     /// <see cref=""></see>
18     /// <remarks>
19     /// <c>创建人：Li 
20     /// 创建日期：2014-11-14
21     /// 版本：1.0</c>
22     /// </remarks>
23     public class Program
24     {
25         /// <summary>
26         /// entry of process
27         /// </summary>
28         /// <param name="args">The args</param>
29         private static void Main(string[] args)
30         {
31             Contact[] contacts = new Contact[6];
32             contacts[0] = new Contact("Dick", "Traci", "123. Main St.", "123-123-1234");
33             contacts[1] = new Contact("Aick", "Traci", "123. Main St.", "123-123-1234");
34             contacts[2] = new Contact("Bick", "Traci", "123. Main St.", "123-123-1234");
35             contacts[3] = new Contact("Cick", "Traci", "123. Main St.", "123-123-1234");
36             contacts[4] = new Contact("Eick", "Traci", "123. Main St.", "123-123-1234");
37             contacts[5] = new Contact("Fick", "Traci", "123. Main St.", "123-123-1234");
38 
39            // 将类的实例当作接口类型传入
40             ConsoleListControl.List(Contact.Headers, contacts);
41 
42             Publication[] publicatons = new Publication[3];
43             publicatons[0] = new Publication("HerryPotter", "Luolin", 1998);
44             publicatons[1] = new Publication("XiYou", "WuChengEn", 1326);
45             publicatons[2] = new Publication("DaXue", "Unkonwn", 239);
46 
47             // 将类的实例当作接口类型传入，包含了一次隐式转型
48             ConsoleListControl.List(Publication.Headers, publicatons);
49         } // Main
50     } // class
51 } // namespace InterfaceMultiState

View Code

注意到里面的：

1 // 将类的实例当作接口类型传入
2  ConsoleListControl.List(Contact.Headers, contacts);

第二个参数是接口类型，显然存在一次转型，而输出的时候确实是按照contact的形式输出的。

输出结果如下：