在c#中使用属性(Attribute)--转载自100down.com
属性提供功能强大的方法以将声明信息与 C# 代码（类型、方法、属性等）相关联。与程序实体关联后，属性可在运行时查询，并可以以任意多种方式使用。
属性的用法示例包括：
- 将帮助文档与程序实体关联（通过 Help 属性）。
- 将值编辑器关联到 GUI 框架中的特定类型（通过 ValueEditor 属性）。
除一个完整的示例外，本教程还包括以下主题：
- 声明属性类
第一件需要做的事情是声明属性。
- 使用属性类
创建属性后，随即将属性与特定的程序元素关联。
- 通过反射访问属性
属性与程序元素关联后，即可使用反射查询属性存在及其值。
声明属性类
在 C# 中声明属性很简单：它采取从 System.Attribute 继承的类声明的形式，并已用 AttributeUsage 属性标记，如下所示：
1
using System;
2[AttributeUsage(AttributeTargets.All)]
3public class HelpAttribute : System.Attribute
4
{
5
public readonly string Url;
6
7 public string Topic // Topic is a named parameter
8
{
9 get
10 {
11 return topic;
12
}
13 set
14 {
16 topic = value;
17 }
18 }
19
20 public HelpAttribute(string url) // url is a positional parameter
21 {
22 this.Url = url;
23 }
24
25 private string topic;
26
}
using System;2
[AttributeUsage(AttributeTargets.All)]3
public class HelpAttribute : System.Attribute 4
{
5
public readonly string Url;6
7
public string Topic // Topic is a named parameter8
{
9
get 10
{ 11
return topic; 12
}13
set 14
{ 16
topic = value; 17
}18
}19
20
public HelpAttribute(string url) // url is a positional parameter21
{22
this.Url = url;23
}24
25
private string topic;26
}代码讨论
- 属性 AttributeUsage 指定该属性可以应用于的语言元素。
- 属性类是从 System.Attribute 派生的公共类，至少有一个公共构造函数。
- 属性类有两种类型的参数：
- “定位参数”，每次使用属性时都必须指定这些参数。定位参数被指定为属性类的构造函数参数。在上面的示例中，url 便是一个定位参数。
- “命名参数”，可选。如果使用属性时指定了命名参数，则必须使用参数的名称。通过包含非静态字段或属性来定义命名参数。在上面的示例中，Topic 便是一个命名参数。
- 属性参数限制为下列类型的常数值：
- 简单类型（bool、byte、char、short、int、long、float 和 double）
- string
- System.Type
- enums
- 对象（对象类型的属性参数的参数必须是属于上述类型之一的常数值。）
- 以上任意类型的一维数组
AttributeUsage 属性的参数
属性 AttributeUsage 提供声明属性的基础机制。
AttributeUsage 具有一个定位参数：
- AllowOn 指定可以将属性赋给的程序元素（类、方法、属性、参数等）。该参数的有效值可以在 .NET Framework 中的 System.Attributes.AttributeTargets 枚举中找到。该参数的默认值是所有程序元素 (AttributeElements.All)。
AttributeUsage 有一个命名参数：
- AllowMultiple，一个布尔值，指示是否可以为一个程序元素指定多个属性。该参数的默认值为 false。
使用属性类
以下是使用上一节中声明的属性的简单示例：
1[HelpAttribute("http://localhost/MyClassInfo")]
2class MyClass
3{
4}
[HelpAttribute("http://localhost/MyClassInfo")]2
class MyClass 3
{4
}本示例中，HelpAttribute 属性与 MyClass 关联。
注意 根据约定，所有属性名称都以单词“Attribute”结束，以便将它们与 .NET Framework 中的其他项区分。但是，在代码中使用属性时不需要指定属性后缀。例如，可以如下指定 HelpAttribute：
1[Help("http://localhost/MyClassInfo")] // [Help] == [HelpAttribute]
2class MyClass
3{
4}
[Help("http://localhost/MyClassInfo")] // [Help] == [HelpAttribute]2
class MyClass 3
{4
}通过反射访问属性
属性与程序元素关联后，可以使用反射查询属性存在及其值。查询属性的主要反射方法包含在 System.Reflection.MemberInfo 类（GetCustomAttributes 方法族）中。下面的示例演示使用反射获取对属性的访问的基本方法：
1class MainClass
2{
3 public static void Main()
4 {
5 System.Reflection.MemberInfo info = typeof(MyClass);
6 object[] attributes = info.GetCustomAttributes(true);
7 for (int i = 0; i < attributes.Length; i ++)
8 {
9 System.Console.WriteLine(attributes[i]);
10 }
11 }
12}
class MainClass 2
{3
public static void Main() 4
{5
System.Reflection.MemberInfo info = typeof(MyClass);6
object[] attributes = info.GetCustomAttributes(true);7
for (int i = 0; i < attributes.Length; i ++)8
{9
System.Console.WriteLine(attributes[i]);10
}11
} 12
} 示例
下面是集合所有部分的完整示例。
1// AttributesTutorial.cs
2// This example shows the use of class and method attributes.
3
4using System;
5using System.Reflection;
6using System.Collections;
7
8// The IsTested class is a user-defined custom attribute class.
9// It can be applied to any declaration including
10// - types (struct, class, enum, delegate)
11// - members (methods, fields, events, properties, indexers)
12// It is used with no arguments.
13public class IsTestedAttribute : Attribute
14{
15 public override string ToString()
16 {
17 return "Is Tested";
18 }
19}
20
21// The AuthorAttribute class is a user-defined attribute class.
22// It can be applied to classes and struct declarations only.
23// It takes one unnamed string argument (the author's name).
24// It has one optional named argument Version, which is of type int.
25[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct)]
26public class AuthorAttribute : Attribute
27{
28 // This constructor specifies the unnamed arguments to the attribute class.
29 public AuthorAttribute(string name)
30 {
31 this.name = name;
32 this.version = 0;
33 }
34
35 // This property is readonly (it has no set accessor)
36 // so it cannot be used as a named argument to this attribute.
37 public string Name
38 {
39 get
40 {
41 return name;
42 }
43 }
44
45 // This property is read-write (it has a set accessor)
46 // so it can be used as a named argument when using this
47 // class as an attribute class.
48 public int Version
49 {
50 get
51 {
52 return version;
53 }
54 set
55 {
56 version = value;
57 }
58 }
59
60 public override string ToString()
61 {
62 string value = "Author : " + Name;
63 if (version != 0)
64 {
65 value += " Version : " + Version.ToString();
66 }
67 return value;
68 }
69
70 private string name;
71 private int version;
72}
73
74// Here you attach the AuthorAttribute user-defined custom attribute to
75// the Account class. The unnamed string argument is passed to the
76// AuthorAttribute class's constructor when creating the attributes.
77[Author("Joe Programmer")]
78class Account
79{
80 // Attach the IsTestedAttribute custom attribute to this method.
81 [IsTested]
82 public void AddOrder(Order orderToAdd)
83 {
84 orders.Add(orderToAdd);
85 }
86
87 private ArrayList orders = new ArrayList();
88}
89
90// Attach the AuthorAttribute and IsTestedAttribute custom attributes
91// to this class.
92// Note the use of the 'Version' named argument to the AuthorAttribute.
93[Author("Jane Programmer", Version = 2), IsTested()]
94class Order
95{
96 // add stuff here 
97}
98
99class MainClass
100{
101 private static bool IsMemberTested(MemberInfo member)
102 {
103 foreach (object attribute in member.GetCustomAttributes(true))
104 {
105 if (attribute is IsTestedAttribute)
106 {
107 return true;
108 }
109 }
110 return false;
111 }
112
113 private static void DumpAttributes(MemberInfo member)
114 {
115 Console.WriteLine("Attributes for : " + member.Name);
116 foreach (object attribute in member.GetCustomAttributes(true))
117 {
118 Console.WriteLine(attribute);
119 }
120 }
121
122 public static void Main()
123 {
124 // display attributes for Account class
125 DumpAttributes(typeof(Account));
126
127 // display list of tested members
128 foreach (MethodInfo method in (typeof(Account)).GetMethods())
129 {
130 if (IsMemberTested(method))
131 {
132 Console.WriteLine("Member {0} is tested!", method.Name);
133 }
134 else
135 {
136 Console.WriteLine("Member {0} is NOT tested!", method.Name);
137 }
138 }
139 Console.WriteLine();
140
141 // display attributes for Order class
142 DumpAttributes(typeof(Order));
143
144 // display attributes for methods on the Order class
145 foreach (MethodInfo method in (typeof(Order)).GetMethods())
146 {
147 if (IsMemberTested(method))
148 {
149 Console.WriteLine("Member {0} is tested!", method.Name);
150 }
151 else
152 {
153 Console.WriteLine("Member {0} is NOT tested!", method.Name);
154 }
155 }
156 Console.WriteLine();
157 }
158}
// AttributesTutorial.cs2
// This example shows the use of class and method attributes.3
4
using System;5
using System.Reflection;6
using System.Collections;7
8
// The IsTested class is a user-defined custom attribute class.9
// It can be applied to any declaration including10
// - types (struct, class, enum, delegate)11
// - members (methods, fields, events, properties, indexers)12
// It is used with no arguments.13
public class IsTestedAttribute : Attribute14
{15
public override string ToString()16
{17
return "Is Tested";18
}19
}20
21
// The AuthorAttribute class is a user-defined attribute class.22
// It can be applied to classes and struct declarations only.23
// It takes one unnamed string argument (the author's name).24
// It has one optional named argument Version, which is of type int.25
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct)]26
public class AuthorAttribute : Attribute27
{28
// This constructor specifies the unnamed arguments to the attribute class.29
public AuthorAttribute(string name)30
{31
this.name = name;32
this.version = 0;33
}34
35
// This property is readonly (it has no set accessor)36
// so it cannot be used as a named argument to this attribute.37
public string Name 38
{39
get 40
{41
return name;42
}43
}44
45
// This property is read-write (it has a set accessor)46
// so it can be used as a named argument when using this47
// class as an attribute class.48
public int Version49
{50
get 51
{52
return version;53
}54
set 55
{56
version = value;57
}58
}59
60
public override string ToString()61
{62
string value = "Author : " + Name;63
if (version != 0)64
{65
value += " Version : " + Version.ToString();66
}67
return value;68
}69
70
private string name;71
private int version;72
}73
74
// Here you attach the AuthorAttribute user-defined custom attribute to 75
// the Account class. The unnamed string argument is passed to the 76
// AuthorAttribute class's constructor when creating the attributes.77
[Author("Joe Programmer")]78
class Account79
{80
// Attach the IsTestedAttribute custom attribute to this method.81
[IsTested]82
public void AddOrder(Order orderToAdd)83
{84
orders.Add(orderToAdd);85
}86
87
private ArrayList orders = new ArrayList();88
}89
90
// Attach the AuthorAttribute and IsTestedAttribute custom attributes 91
// to this class.92
// Note the use of the 'Version' named argument to the AuthorAttribute.93
[Author("Jane Programmer", Version = 2), IsTested()]94
class Order95
{96
// add stuff here 97
}98
99
class MainClass100
{101
private static bool IsMemberTested(MemberInfo member)102
{103
foreach (object attribute in member.GetCustomAttributes(true))104
{105
if (attribute is IsTestedAttribute)106
{107
return true;108
}109
}110
return false;111
}112
113
private static void DumpAttributes(MemberInfo member)114
{115
Console.WriteLine("Attributes for : " + member.Name);116
foreach (object attribute in member.GetCustomAttributes(true))117
{118
Console.WriteLine(attribute);119
}120
}121
122
public static void Main()123
{124
// display attributes for Account class125
DumpAttributes(typeof(Account));126
127
// display list of tested members128
foreach (MethodInfo method in (typeof(Account)).GetMethods())129
{130
if (IsMemberTested(method))131
{132
Console.WriteLine("Member {0} is tested!", method.Name);133
}134
else135
{136
Console.WriteLine("Member {0} is NOT tested!", method.Name);137
}138
}139
Console.WriteLine();140
141
// display attributes for Order class142
DumpAttributes(typeof(Order));143
144
// display attributes for methods on the Order class145
foreach (MethodInfo method in (typeof(Order)).GetMethods())146
{147
if (IsMemberTested(method))148
{149
Console.WriteLine("Member {0} is tested!", method.Name);150
}151
else152
{153
Console.WriteLine("Member {0} is NOT tested!", method.Name);154
}155
}156
Console.WriteLine();157
}158
}输出
Attributes for : Account
Author : Joe Programmer
Member GetHashCode is NOT tested!
Member Equals is NOT tested!
Member ToString is NOT tested!
Member AddOrder is tested!
Member GetType is NOT tested!
Attributes for : Order
Author : Jane Programmer Version : 2
Is Tested
Member GetHashCode is NOT tested!
Member Equals is NOT tested!
Member ToString is NOT tested!
Member GetType is NOT tested!
