Abstract
在(原創) 我的Design Pattern之旅：使用template改進Strategy Pattern (OO) (Design Pattern) (C/C++) (template)中，使用了C++的template改進strategy pattern，本文使用C#的generic改進strategy pattern。
Introduction
C# 2.0加入了generic對泛型的支援，所以想將原來C++的template程式一行一行的改成C# generic。
在strategy pattern中，通常為了讓strategy能完全存取物件的public method/property，我們會利用傳this將整個物件傳給strategy
public void drawShape() {
this.shape.draw(this);
}
為了達成此需求，我們的interface須如此定義
interface IShape {
void draw(Grapher grapher);
}
完整的程式碼如下
1/**//*
2(C) OOMusou 2007 http://oomusou.cnblogs.com
3
4Filename : DP_StrategyPattern3_polymorphism_this.cs
5Compiler : Visual Studio 2005 / C# 2.0
6Description : Demo how to use Strategy Pattern with this
7Release : 04/07/2007 1.0
8*/
9using System;
10
11interface IShape {
12 void draw(Grapher grapher);
13}
14
15class Grapher {
16 private IShape shape;
17 private string text;
18
19 public Grapher() { }
20 public Grapher(IShape shape) : this(shape, "Hello Shape!!") { }
21 public Grapher(IShape shape, string text) {
22 this.shape = shape;
23 this.text = text;
24 }
25
26 public void drawShape() {
27 this.shape.draw(this);
28 }
29
30 public void setShape(IShape shape, string text) {
31 this.text = text;
32 this.shape = shape;
33 }
34
35 public string getText () {
36 return this.text;
37 }
38}
39
40
41class Triangle : IShape {
42 public void draw(Grapher grapher) {
43 Console.WriteLine("Draw {0:s} in Triangle", grapher.getText());
44 }
45}
46
47class Circle: IShape {
48 public void draw(Grapher grapher) {
49 Console.WriteLine("Draw {0:s} in Circle", grapher.getText());
50 }
51}
52
53class Square : IShape {
54 public void draw(Grapher grapher) {
55 Console.WriteLine("Draw {0:s} in Square", grapher.getText());
56 }
57}
58
59class main {
60 public static void Main() {
61 Grapher theGrapher = new Grapher(new Square());
62 theGrapher.drawShape();
63
64 theGrapher.setShape(new Circle(), "Hello C#!!");
65 theGrapher.drawShape();
66 }
67}
執行結果
Draw Hello Shape!! in Square
Draw Hello C#!! in Circle
這樣的設計看似完美，但淺在一個問題
interface IShape {
void draw(Grapher grapher);
}
規定了draw()一定要傳Grapher型別進去，若將來因為需求改變，又多了一個Painter class，且和Grapher毫無相關，既非繼承亦非多型，但又想使用這些strategy，但因為IShape已規定只能傳Grapher型別，所以Painter無法繼續使用IShape interface。
在(原創) 我的Design Pattern之旅[3]：使用template改進Strategy Pattern (OO) (Design Pattern) (C/C++) (template)中，我們已成功使用C++的template解決這種型別被限制的問題，所以我們試著也使用C#的Generic來解決。
首些我們先將IShape改用Generic
interface IShape<T> {
void draw(T grapher);
}
接著其他實做IShape<T>的程式亦需改寫成
class Triangle<T> : IShape<T> where T : IGrapher {
public void draw(T grapher) {
Console.WriteLine("Draw {0:s} in Triangle", grapher.getText());
}
}
C#多了where這個keyword，這是我卡最久的地方，也是C# generic和C++ template不同之處，C++ template並沒有限定泛型的型別，但C# generic的泛型是『有限的泛型』，或稱『以interface為基礎的泛型』、『強型別泛型』，也就是說，C#泛型不能像C++泛型那樣天馬行空的泛型，C#泛型必須『限制』在interface下，所以where稱為『constraint』
where T : IGrapher
表示泛型T需限制在IGrapher的interface下
也因此，因為各strategy會用到物件的getText()，所以我們定義IGrapher interface一定要有getText()
interface IGrapher {
string getText();
}
日後若有Painter class也想使用IShape的strategy，只要也實做IGrapher即可。
完整程式碼如下
1/**//*
2(C) OOMusou 2007 http://oomusou.cnblogs.com
3
4Filename : DP_StrategyPattern3_polymorphism_this.cs
5Compiler : Visual Studio 2005 / C# 2.0
6Description : Demo how to use Strategy Pattern with this by Generic
7Release : 04/07/2007 1.0
8*/
9using System;
10
11interface IGrapher {
12 string getText();
13}
14
15interface IShape<T> {
16 void draw(T grapher);
17}
18
19class Grapher : IGrapher {
20 private IShape<Grapher> shape;
21
22 private string text;
23
24 public Grapher() { }
25 public Grapher(IShape<Grapher> shape) : this(shape, "Hello Shape!!") { }
26 public Grapher(IShape<Grapher> shape, string text) {
27 this.shape = shape;
28 this.text = text;
29 }
30
31 public void drawShape() {
32 this.shape.draw(this);
33 }
34
35 public void setShape(IShape<Grapher> shape, string text) {
36 this.text = text;
37 this.shape = shape;
38 }
39
40 public string getText () {
41 return this.text;
42 }
43}
44
45class Triangle<T> : IShape<T> where T : IGrapher {
46 public void draw(T grapher) {
47 Console.WriteLine("Draw {0:s} in Triangle", grapher.getText());
48 }
49}
50
51class Circle<T> : IShape<T> where T : IGrapher{
52 public void draw(T grapher) {
53 Console.WriteLine("Draw {0:s} in Circle", grapher.getText());
54 }
55}
56
57class Square<T> : IShape<T> where T : IGrapher {
58 public void draw(T grapher) {
59 Console.WriteLine("Draw {0:s} in Square", grapher.getText());
60 }
61}
62
63class main {
64 public static void Main() {
65 Grapher theGrapher = new Grapher(new Square<Grapher>());
66 theGrapher.drawShape();
67
68 theGrapher.setShape(new Circle<Grapher>(), "Hello C#!!");
69 theGrapher.drawShape();
70 }
71}
執行結果
Draw Hello Shape!! in Square
Draw Hello C#!! in Circle
Conclusion
C#泛型有什麽好處呢?
where T : IGrapher, IPainter
where後面的限制，並不是只能接一個interface或class而已，還能繼續接，若以後要擴充泛型T的使用，只要繼續接下去即可，如此的寫法已經比物件導向的多型還強，多型限定只能在一個interface或abstract class的多型物件體系，但泛型可以是多個interface或多個abstract class的物件體系，所以泛型可視為『更強的多型』。
假如你同時懂C++和C#，或許會覺得用C#泛型解決strategy pattern『不是那麽漂亮』!!因為若還要用interface，我大可這樣寫
1/**//*
2(C) OOMusou 2007 http://oomusou.cnblogs.com
3
4Filename : DP_StrategyPattern3_polymorphism_this_interface.cs
5Compiler : Visual Studio 2005 / C# 2.0
6Description : Demo how to use Strategy Pattern with this by interface
7Release : 04/07/2007 1.0
8*/
9using System;
10
11interface IGrapher {
12 string getText();
13}
14
15interface IShape {
16 void draw(IGrapher grapher);
17}
18
19class Grapher : IGrapher {
20 private IShape shape;
21
22 private string text;
23
24 public Grapher() { }
25 public Grapher(IShape shape) : this(shape, "Hello Shape!!") { }
26 public Grapher(IShape shape, string text) {
27 this.shape = shape;
28 this.text = text;
29 }
30
31 public void drawShape() {
32 this.shape.draw(this);
33 }
34
35 public void setShape(IShape shape, string text) {
36 this.text = text;
37 this.shape = shape;
38 }
39
40 public string getText() {
41 return this.text;
42 }
43}
44
45class Triangle : IShape {
46 public void draw(IGrapher grapher) {
47 Console.WriteLine("Draw {0:s} in Triangle", grapher.getText());
48 }
49}
50
51class Circle : IShape {
52 public void draw(IGrapher grapher) {
53 Console.WriteLine("Draw {0:s} in Circle", grapher.getText());
54 }
55}
56
57class Square : IShape {
58 public void draw(IGrapher grapher) {
59 Console.WriteLine("Draw {0:s} in Square", grapher.getText());
60 }
61}
62
63class main {
64 public static void Main() {
65 Grapher theGrapher = new Grapher(new Square());
66 theGrapher.drawShape();
67
68 theGrapher.setShape(new Circle(), "Hello C#!!");
69 theGrapher.drawShape();
70 }
71}
沒錯，這也是C#泛型的一個限制，『一個基於interface或class的泛型』，這種『強型別泛型』，好處是讓泛型也可有intelliSense，且在compile-time就能發現錯誤，不像C++屬於『弱型別泛型』，常得在run-time才能發現錯誤，所以C#泛型較C++穩定，不過也由於限制過多，所以比較沒有泛型的味道，以我個人來說，我比較喜歡C++泛型，因為既然要泛型，就是放諸四海皆准，擺脫型別的限制，擺脫interface的枷鎖，而不是到最後還是得靠interface。
雖然如此，仍就可將C#泛型看成『更強的多型』，很多原本多型設計，可以考慮用泛型讓設計應用更廣。
