blocks|key|2700002|text|这不管用吗？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2700003|foreach+(var+item+in+result)
{
++++CrawlerBase+crawler+=+null;

++++switch+(item.Type)
++++{
++++++++case+"Trials":
++++++++++++crawler+=+new+Trials();+
++++++++++++break;
++++++++case+"Coverage":
++++++++++++crawler+=+new+Coverage();
++++++++++++break;
++++++++default:
++++++++++++break;
++++}

++++if(crawler+!=+null)
++++++++crawler.Process(item.URL);
}|code-block|syntax|javascript|2700004|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Does this not work?

<pre><code>foreach (var item in result)
{
 CrawlerBase crawler = null;

 switch (item.Type)
 {
 case "Trials":
 crawler = new Trials(); 
 break;
 case "Coverage":
 crawler = new Coverage();
 break;
 default:
 break;
 }

 if(crawler != null)
 crawler.Process(item.URL);
}
</code></pre>

blocks|key|1062229|text|使用工厂方法创建字典：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1062230|var+factories+=+new+Dictionary<string,+Func<CrawlerBase>>
{
++{"Trials",+()+=>+new+Trials()},
++{"Coverage",+()+=>+new+Coverage()},
//+and+so+on
}|code-block|syntax|javascript|1062231|...and丢弃switch|offset|length|style|CODE|1062232|foreach+(var+item+in+result)
{
++factories[item.Type]().Process(item.URL);
}|1062233|entityMap^0|0|0|8|6|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@$I|T|J|U|K|L]]|9|@]|A|$]]|$1|M|3|N|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

Make dictionary with factory methods:

<pre><code>var factories = new Dictionary&lt;string, Func&lt;CrawlerBase&gt;&gt;
{
 {"Trials", () =&gt; new Trials()},
 {"Coverage", () =&gt; new Coverage()},
// and so on
}
</code></pre>

...and throw away <code>switch</code>:

<pre><code>foreach (var item in result)
{
 factories[item.Type]().Process(item.URL);
}
</code></pre>

blocks|key|2786012|text|你不能这么做？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2786013|+{
+++CrawlerBase+crawler+=+null;

+++switch+(item.Type)
+++{
++++++++case+"Trials":
++++++++++++crawler+=+new+Trials();+++
++++++++++++break;

++++++++case+"Coverage":
++++++++++++crawler+=+new+Coverage();
++++++++++++break;
++++++++default:
++++++++++++break;
++++}
+++crawler.Process(item.URL);|code-block|syntax|javascript|2786014|}|2786015|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

you can't do this? 

<pre><code> {
 CrawlerBase crawler = null;

 switch (item.Type)
 {
 case "Trials":
 crawler = new Trials(); 
 break;

 case "Coverage":
 crawler = new Coverage();
 break;
 default:
 break;
 }
 crawler.Process(item.URL);
</code></pre>

}

blocks|key|2786056|text|有许多方法可以从字符串创建类型。反射可以是其中之一，它是慢(但如果你正在处理web请求，你可能不会关心这一点)，但你不需要保留一长串硬编码的字符串和/或有一个丑陋的长开关/用例语句。|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|2786057|如果速度不是问题，那么让我们从一些简单的东西开始：|2786058|CrawlerBase+crawler+=+(CrawlerBase)Activator.CreateInstance(
++++Type.GetType("MyNamespace."+%2B+item.Type));

crawler.Process(item.URL);|code-block|syntax|javascript|2786059|使用反射，你不需要开关/大小写，并且，不管你有多少类型，你不会改变你的工厂代码来适应CrawlerBase的新实现。只需添加一个新的实现即可。|CODE|2786060|它是如何工作的？您可以从类的Type定义(使用Activator.CreateInstance)创建一个类的实例，所以问题在于获取Type。Type类提供了一个静态GetType方法，可用于从(完整)名称创建一个Type。在这种情况下，我们提供了一个名称空间，必须将其更改为真实的名称空间，如果您将所有类都放在同一个名称空间中，您可以编写如下内容：|2786061|string+rootNamespace+=+typeof(CrawlerBase).Namespace;|2786062|然后是虚构的版本：|2786063|CrawlerBase+crawler+=+(CrawlerBase)Activator.CreateInstance(
++++Type.GetType(rootNamespace+%2B+"."+%2B+item.Type));

crawler.Process(item.URL);|2786064|改进1|2786065|这是一个非常幼稚的实现，一个更好的版本应该在字典中缓存Type：|2786066|private+static+Dictionary<string,+Type>+_knownTypes+=
++++new+Dictionary<string,+Type>();

private+static+GetType(string+name)
{
++++string+fullName+=+typeof(CrawlerBase).Namespace
++++++++%2B+"."+%2B+name;

++++if+(_knownTypes.ContainsKey(fullName))
++++++++return+_knownTypes[fullName];

++++Type+type+=+Type.GetType(fullName);
++++_knownTypes.Add(fullName,+type);

++++return+type;
}|2786067|现在你已经很好了，你不需要管理一个(可能的)很长的字符串列表，如果你添加/删除/更改一个类型，你不需要改变无尽的开关/大小写，并且添加一个新的Crawler，你所要做的就是派生一个新的类。请注意，此函数不是线程安全的，因此，如果您有多个线程，则必须添加一个锁来保护字典访问(或ReadWriterLockSlim，这取决于您的使用模式)。|2786068|改进2|2786069|下一步是什么？如果您的要求更严格(例如，关于安全性)，则可能需要添加更多内容(我认为这不是您的情况，因此您甚至可能不需要类型字典)。还能做些什么？首先，您可以检查创建的类型是否派生自CrawlerBase+(使用IsAssignableFrom并提供更有意义的异常，而不是InvalidCastException)；其次，您可以使用属性来修饰您的类。只有具有该属性的类才会被创建(因此您可以保留私有实现，即使标记为public也不能从您的用户创建)。不仅如此，一个简单的实现通常就足以解决这类问题(当输入不是来自用户，而是来自健壮的配置文件或程序本身时)。|2786070|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/wccyzw83.aspx|1|http://msdn.microsoft.com/en-us/library/w3f99sx1.aspx^0|S|1|0|0|0|16|B|0|E|4|N|O|1T|4|1Y|4|2A|7|2Y|4|N|O|0|2A|7|1|0|0|0|0|0|R|4|0|0|3U|I|0|0|2J|B|2Y|G|3S|K|0^^$0|@$1|2|3|4|5|6|7|1J|8|@$9|1K|A|1L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|1M|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|1N|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|1O|8|@$9|1P|A|1Q|B|O]]|D|@]|E|$]]|$1|P|3|Q|5|6|7|1R|8|@$9|1S|A|1T|B|O]|$9|1U|A|1V|B|O]|$9|1W|A|1X|B|O]|$9|1Y|A|1Z|B|O]|$9|20|A|21|B|O]|$9|22|A|23|B|O]]|D|@$9|24|A|25|1|26]|$9|27|A|28|1|29]]|E|$]]|$1|R|3|S|5|J|7|2A|8|@]|D|@]|E|$K|L]]|$1|T|3|U|5|6|7|2B|8|@]|D|@]|E|$]]|$1|V|3|W|5|J|7|2C|8|@]|D|@]|E|$K|L]]|$1|X|3|Y|5|6|7|2D|8|@]|D|@]|E|$]]|$1|Z|3|10|5|6|7|2E|8|@$9|2F|A|2G|B|O]]|D|@]|E|$]]|$1|11|3|12|5|J|7|2H|8|@]|D|@]|E|$K|L]]|$1|13|3|14|5|6|7|2I|8|@$9|2J|A|2K|B|O]]|D|@]|E|$]]|$1|15|3|16|5|6|7|2L|8|@]|D|@]|E|$]]|$1|17|3|18|5|6|7|2M|8|@$9|2N|A|2O|B|O]|$9|2P|A|2Q|B|O]|$9|2R|A|2S|B|O]]|D|@]|E|$]]|$1|19|3|-4|5|6|7|2T|8|@]|D|@]|E|$]]]|1A|$1B|$5|1C|1D|1E|E|$1F|1G]]|1H|$5|1C|1D|1E|E|$1F|1I]]]]

There are many way to create a type from a string. Reflection can be one of them, it's slow (but if you're processing web requests probably you won't care of this) but you won't need to keep a long list of hard-coded strings and/or to have an ugly long switch/case statement.

If speed is not a problem then let's start with something easy:

<pre><code>CrawlerBase crawler = (CrawlerBase)Activator.CreateInstance(
 Type.GetType("MyNamespace." + item.Type));

crawler.Process(item.URL);
</code></pre>

Using reflection you do not need switch/case and, it doesn't matter how many types you have, you won't change your factory code to accomodate new implementations of <code>CrawlerBase</code>. Simply add a new implementation et-voila.

How it works? Well you can create an instance of a class from its <code>Type</code> definition (using <a href="http://msdn.microsoft.com/en-us/library/wccyzw83.aspx" rel="nofollow"><code>Activator.CreateInstance</code></a>) so the problem is to get the <code>Type</code>. <code>Type</code> class provides a static <a href="http://msdn.microsoft.com/en-us/library/w3f99sx1.aspx" rel="nofollow"><code>GetType</code></a> method that can be used to create a <code>Type</code> from its (full) name. In this case we supply a namespace, it must be changed to the real one, if you keep all your classes in the same namespace you may write something like this:

<pre><code>string rootNamespace = typeof(CrawlerBase).Namespace;
</code></pre>

And then the maked-up version:

<pre><code>CrawlerBase crawler = (CrawlerBase)Activator.CreateInstance(
 Type.GetType(rootNamespace + "." + item.Type));

crawler.Process(item.URL);
</code></pre>

<h2>Improvements 1</h2>

This is a very naive implementation, a better version should cache <code>Type</code> in a dictionary:

<pre><code>private static Dictionary&lt;string, Type&gt; _knownTypes =
 new Dictionary&lt;string, Type&gt;();

private static GetType(string name)
{
 string fullName = typeof(CrawlerBase).Namespace
 + "." + name;

 if (_knownTypes.ContainsKey(fullName))
 return _knownTypes[fullName];

 Type type = Type.GetType(fullName);
 _knownTypes.Add(fullName, type);

 return type;
}
</code></pre>

Now you're OK, you do not need to manage a (possibly) long list of strings, you do not need to change an endless switch/case if you add/remove/change a type and to add a new Crawler all you have to do is to derive a new class. Please note that this function isn't thread-safe so, if you'll have multiple threads, you have to add a lock to protect dictionary access (or a <code>ReadWriterLockSlim</code>, it depends on your usage pattern).

<h2>Improvements 2</h2>

What's next? If your're requirements are more strict (about security, for example) you may need to add something more (I think it's not your case so you may not even need the type dictionary). What to do more? First you may check that the created type derives from <code>CrawlerBase</code> (using <code>IsAssignableFrom</code> and to provide a more meaningful exception instead of <code>InvalidCastException</code>), second you may decorate your classes with an attribute. Only classes with that attributes will be created (so you can keep private implementations that even if marked as public cannot be created from your users). There is much more than this but a simple implementation usually is enough to solve this kind of problems (when the input doesn't come from user but from robust configuration file or from the program itself).

blocks|key|2786104|text|正如Redi所指出的，你可以在任何实例上调用过程，因为所有的调用都派生自同一个基类。在您的switch用例中仍然有一些重复的代码，如果您从switch+(+if+/else)转移到映射构造，则可以简化这些代码。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2786105|Dictionary<string,+Type>+Types+=+new+Dictionary<string,+Type>
{
++++{+"Trial",+typeof(Trials)+},
++++{+"Coverage",+typeof(Coverage)+},
};

void+Crawl()
{
++++string+itemType+=+"Trial";
++++CrawlerBase+crawler+=+(CrawlerBase)Activator.CreateInstance(Types[itemType]);
++++crawler.Process("url");
}|code-block|syntax|javascript|2786106|如果您将工厂方法作为值放入您的字典中，您也可以对像Dennis这样的委托执行此操作。|2786107|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

As Redi pointed out you do call call Process on any instance since all derive from the same base class. You still have some repetitive code in your switch case which can be simplified if you move away from a switch (if/else) to a mapping construct. 

<pre><code>Dictionary&lt;string, Type&gt; Types = new Dictionary&lt;string, Type&gt;
{
 { "Trial", typeof(Trials) },
 { "Coverage", typeof(Coverage) },
};

void Crawl()
{
 string itemType = "Trial";
 CrawlerBase crawler = (CrawlerBase)Activator.CreateInstance(Types[itemType]);
 crawler.Process("url");
}
</code></pre>

You can do it also with delegates like Dennis if you put a factory method as value into your dictionary.

I have an abstract class that a lot of child classes inherit:

<pre><code> public abstract class CrawlerBase
 {
 public abstract void Process(string url);
 }
</code></pre>

I'm working on this loop:

<pre><code>foreach (var item in result)
 {
 object crawler = null;

 switch (item.Type)
 {
 case "Trials":
 var t = new Trials(); 
 ct.Process(item.URL); //repetitive code. 
 break;

 case "Coverage":
 var c = new Coverage();
 c.Process(item.URL); //repetitive code.
 break;
 default:
 break;
 }

 // crawler.Process(item.URL);
 }
</code></pre>

Now the item.type string will depend on what child class needs to be instantiated. Since all my child classes inherit my base class, it would be very repetitive to call .Process() in every case statement. I want to cast the object "crawler" to the child class being instantiated, and call the crawler.Process() at the end of the switch statement, as shown in the comments. How can I do this?

Creating an object from from an ID (or name)

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我有一个很多子类都继承的抽象类：  public abstract class CrawlerBase    {        public abstract void Process(string url);    }我正在处理这个循环：foreach (var item in result)            ...

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