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10.1.1 编写定制特性
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10.1.1 编写定制特性
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<div style='float:left;width:49%'>·<a href='100165362.htm'>9.1.3 字典(2)</a></div><div style='float:right;width:49%'>·<a href='100165363.htm'>9.2 小结</a></div><div style='float:left;width:49%'>·<a href='100165364.htm'>10.1 定制特性</a></div><div style='float:right;width:49%'>·<a href='100165366.htm'>10.1.2 定制特性示例WhatsNewAttributes</a></div><div style='float:left;width:49%'>·<a href='100165367.htm'>10.2 反射</a></div><div style='float:right;width:49%'>·<a href='100165368.htm'>10.2.1 System.Type类</a></div></div>
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<div id="text"> <link href="css.css" rel="stylesheet" type="text/css" /><h3 style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 0cm; MARGIN-RIGHT: 0cm; FTEL: 8.15pt"><a ftel="_Toc507815077"><span lang="EN-US">10.1.1 </span></a><span style="FONT-FAMILY: 黑体">编写定制特性</span></h3>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">为了理解编写定制特性的方式，应了解一下在编译器遇到代码中某个应用了定制特性的元素时，该如何处理。以数据库为例，假定有一个</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">属性声明，如下所示。</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">[FieldName("SocialSecurityNumber")]</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">public string SocialSecurityNumber</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">{</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> get {</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> // etc.</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">当</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">编译器发现这个属性有一个特性</span><span lang="EN-US">FieldName</span><span style="FONT-FAMILY: 宋体">时，首先会把字符串</span><span lang="EN-US">Attribute</span><span style="FONT-FAMILY: 宋体">添加到这个名称的后面，形成一个组合名称</span><span lang="EN-US">FieldNameAttribute</span><span style="FONT-FAMILY: 宋体">，然后在其搜索路径的所有命名空间</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">即在</span><span lang="EN-US">using</span><span style="FONT-FAMILY: 宋体">语句中提及的命名空间</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">中搜索有相同名称的类。但要注意，如果用一个特性标记数据项，而该特性的名称总是以字符串</span><span lang="EN-US">"Attribute"</span><span style="FONT-FAMILY: 宋体">结尾，编译器就不会把该字符串加到名称中，而是不修改该特性名。因此，上面的代码实际上等价于：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">[FieldNameAttribute("SocialSecurityNumber")]</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">public string SocialSecurityNumber</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">{</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> get {</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> // etc.</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">编译器会找到含有该名称的类，且这个类派生自</span><span lang="EN-US">System.Attribute</span><span style="FONT-FAMILY: 宋体">。编译器还认为这个类包含控制特性用法的信息。特别是属性类需要指定：</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span style="FONT-FAMILY: 宋体">特性可以应用到哪些程序元素上</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">类、结构、属性和方法等</span><span lang="EN-US">)</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span style="FONT-FAMILY: 宋体">它是否可以多次应用到同一个程序元素上</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span style="FONT-FAMILY: 宋体">特性在应用到类或结构上时，是否由派生类和接口继承</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span style="FONT-FAMILY: 宋体">这个特性有哪些必选和可选参数</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">如果编译器找不到对应的特性类，或者找到一个这样的特性类，但使用特性的方式与特性类中的信息不匹配<span style="LETTER-SPACING: 0.1pt">，编译器就会产生一个编译错误。</span>例如，如果特性类指定该特性只能应用于字段，但我们把它应用到结构定义<span style="LETTER-SPACING: 0.1pt">上，就会产生一个编译错误。</span></span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">继续上面的示例，假定定义了一个</span><span lang="EN-US">FieldName</span><span style="FONT-FAMILY: 宋体">特性：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">[AttributeUsage(AttributeTargets.Property,</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> AllowMultiple=<span class="codeintext-PRODUCTION"><span style="FONT-FAMILY: 'Times New Roman'">false,</span></span></span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span class="codeintext-PRODUCTION"><span lang="EN-US" style="FONT-FAMILY: 'Times New Roman'"> Inherited=false)]</span></span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span class="codeintext-PRODUCTION"><span lang="EN-US" style="FONT-FAMILY: 'Times New Roman'">public class FieldNameAttribute : Attribute</span></span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span class="codeintext-PRODUCTION"><span lang="EN-US" style="FONT-FAMILY: 'Times New Roman'">{</span></span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> private string name;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> public FieldNameAttribute(string name)</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> {</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> this.name = name;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> }</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">}</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">下面几节讨论这个定义中的每个元素。</span></p>
<h4 style="FTEL: 21.45pt"><span lang="EN-US">1. AttributeUsage</span><span style="FONT-FAMILY: 黑体">特性</span></h4>
<p class="MsoNormal"><a ftel="AttributeUsage"><span style="FONT-FAMILY: 宋体">要注意的第一个问题是特性</span><span lang="EN-US">(attribute)</span></a><span style="FONT-FAMILY: 宋体">类本身用一个特性</span><span lang="EN-US">System.ttributeUsage</span><span style="FONT-FAMILY: 宋体">来标记。这是</span><span lang="EN-US">Microsoft</span><span style="FONT-FAMILY: 宋体">定义的一个特性，</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">编译器为它提供了特殊的支持</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">您可能认为</span><span lang="EN-US">AttributeUsage</span><span style="FONT-FAMILY: 宋体">根本不是一个特性，它更像一个元特性，因为它可以应用到其他特性上，而不仅仅应用到其他类上</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">。</span><span lang="EN-US">AttributeUsage</span><span style="FONT-FAMILY: 宋体">主要用于表示定制特性可以应用到哪些类型的程序元素上。这些信息由它的第一个参数给出，该参数是必选的，其类型是枚举类型</span><span lang="EN-US">AttributeTargets</span><span style="FONT-FAMILY: 宋体">。在上面的示例中</span><span style="FONT-FAMILY: 宋体">，</span><span style="FONT-FAMILY: 宋体">指定</span><span lang="EN-US">FieldName</span><span style="FONT-FAMILY: 宋体">特性只能应用到属性</span><span lang="EN-US">(property)</span><span style="FONT-FAMILY: 宋体">上<span style="LETTER-SPACING: -1pt">——</span></span><span style="LETTER-SPACING: -1pt"> </span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">这是因为我们在前面的代码段中把它应用到属性上。</span><span lang="EN-US" style="LETTER-SPACING: 0.1pt">AttributeTargets</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">枚举的成员如下</span><span style="FONT-FAMILY: 宋体">：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">public enum AttributeTargets </span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">{</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> All = 0x00003FFF, </span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> Assembly = 0x00000001, </span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> Class = 0x00000004, </span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> Constructor = 0x00000020, </span></p>
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