本教程展示如何创建自定义属性类，如何在代码中使用它们，以及如何通过反射查询它们。
教程
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：
除一个完整的示例外，本教程还包括以下主题：
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：
using System;
[AttributeUsage(AttributeTargets.All)]
public class HelpAttribute : System.Attribute
{
public readonly string Url;
public string Topic // Topic is a named parameter
{
get
{
return topic;
}
set
{
topic = value;
}
}
public HelpAttribute(string url) // url is a positional parameter
{
this.Url = url;
}
private string topic;
}
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：
- 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：
[HelpAttribute("http://localhost/MyClassInfo")]
class MyClass
{
}
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：
[Help("http://localhost/MyClassInfo")] // [Help] == [HelpAttribute]class MyClass {}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：
class MainClass
{
public static void Main()
{
System.Reflection.MemberInfo info = typeof(MyClass);
object[] attributes = info.GetCustomAttributes(true);
for (int i = 0; i < attributes.Length; i ++)
{
System.Console.WriteLine(attributes[i]);
}
}
} 示例
下面是集合所有部分的完整示例。
// AttributesTutorial.cs // This example shows the use of class and method attributes. using System; using System.Reflection; using System.Collections; // The IsTested class is a user-defined custom attribute class. // It can be applied to any declaration including // - types (struct, class, enum, delegate) // - members (methods, fields, events, properties, indexers) // It is used with no arguments. public class IsTestedAttribute : Attribute { public override string ToString() { return "Is Tested"; } } // The AuthorAttribute class is a user-defined attribute class. // It can be applied to classes and struct declarations only. // It takes one unnamed string argument (the author's name). // It has one optional named argument Version, which is of type int. [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct)] public class AuthorAttribute : Attribute { // This constructor specifies the unnamed arguments to the attribute class. public AuthorAttribute(string name) { this.name = name; this.version = 0; } // This property is readonly (it has no set accessor) // so it cannot be used as a named argument to this attribute. public string Name { get { return name; } } // This property is read-write (it has a set accessor) // so it can be used as a named argument when using this // class as an attribute class. public int Version { get { return version; } set { version = value; } } public override string ToString() { string value = "Author : " + Name; if (version != 0) { value += " Version : " + Version.ToString(); } return value; } private string name; private int version; } // Here you attach the AuthorAttribute user-defined custom attribute to // the Account class. The unnamed string argument is passed to the // AuthorAttribute class's constructor when creating the attributes. [Author("Joe Programmer")] class Account { // Attach the IsTestedAttribute custom attribute to this method. [IsTested] public void AddOrder(Order orderToAdd) { orders.Add(orderToAdd); } private ArrayList orders = new ArrayList(); } // Attach the AuthorAttribute and IsTestedAttribute custom attributes // to this class. // Note the use of the 'Version' named argument to the AuthorAttribute. [Author("Jane Programmer", Version = 2), IsTested()] class Order { // add stuff here ... } class MainClass { private static bool IsMemberTested(MemberInfo member) { foreach (object attribute in member.GetCustomAttributes(true)) { if (attribute is IsTestedAttribute) { return true; } } return false; } private static void DumpAttributes(MemberInfo member) { Console.WriteLine("Attributes for : " + member.Name); foreach (object attribute in member.GetCustomAttributes(true)) { Console.WriteLine(attribute); } } public static void Main() { // display attributes for Account class DumpAttributes(typeof(Account)); // display list of tested members foreach (MethodInfo method in (typeof(Account)).GetMethods()) { if (IsMemberTested(method)) { Console.WriteLine("Member {0} is tested!", method.Name); } else { Console.WriteLine("Member {0} is NOT tested!", method.Name); } } Console.WriteLine(); // display attributes for Order class DumpAttributes(typeof(Order)); // display attributes for methods on the Order class foreach (MethodInfo method in (typeof(Order)).GetMethods()) { if (IsMemberTested(method)) { Console.WriteLine("Member {0} is tested!", method.Name); } else { Console.WriteLine("Member {0} is NOT tested!", method.Name); } } Console.WriteLine(); } }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!
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