我们知道校验XML文件的方式有XSD 和XDR两种方式，这两种方式是利用XML Schema来实现的，现在我们介绍另外的一种校验：DTD。 DTD(Document type definition) 校验是使用W3C中有效性约束来实现的。DTD使用一种正式的文法来描述相适应的XML文档的结构和语法。完成一个DTD校验，是利用XmlValidatingReader类。XmlValidatingReader类将DTD定义在DOCTYPE中。这个DOCTYPE定义的可以包括在线的DTD或是指向一个DTD文件。

在什么情况下使用DTD校验呢？使用DTD校验，可以使一个用户组中独立的不同的用户使用共同的DTD来交换数据。比如，你可以使用共同的DTD校验你所接受到的XML数据是不是有效，你也可以用DTD来校验你自己的数据。

下面我们用一个例子来说明DTD是如何使用的。

首先写一个HeadCount.dtd:

<!ELEMENT HeadCount (Name)*>//ELEMENT说明元素是HeadCount

<!ELEMENT Name (Name)*> //ELEMENT说明元素是Name

<!ATTLIST Name First CDATA #REQUIRED>//ATTLIST说明元素Name后面是属性First

<!ATTLIST Name Last CDATA #REQUIRED>//ATTLIST说明元素Name后面还有一个是属性Last

<!ATTLIST Name Relation (self | spouse | child) "self">

<!ENTITY MyFirst "Jeff">

<!ENTITY MyLast "Smith">

具体的DTD规范可以参考W3C规范。

还有一个待校验的XML文件HeadCount.xml：

<!DOCTYPE HeadCount SYSTEM "HeadCount.dtd">

<HeadCount>

<Name First="Waldo" Last="Pepper">

<Name First="Salt" Last="Pepper" Relation="spouse"/>

<Name First="Red" Last="Pepper" Relation="child"/>

</Name>

<Name First="&MyFirst;" Last="&MyLast;">

<Name First="Sharon" Last="&MyLast;" Relation="spouse"/>

<Name First="Morgan" Last="&MyLast;" Relation="child"/>

<Name First="Shelby" Last="&MyLast;" Relation="child"/>

</Name>

</HeadCount>

现在我们写一个cs文件来实现校验：

testDtd.cs

using System;

using System.IO;

using System.Xml;

using System.Xml.Schema;

class testDtd

{

public static void Main()

{

XmlTextReader tr = new XmlTextReader("HeadCount.xml");

XmlValidatingReader vr = new XmlValidatingReader(tr);

vr.ValidationType = ValidationType.DTD;

vr.ValidationEventHandler += new ValidationEventHandler (ValidationHandler);

while(vr.Read());

Console.WriteLine("Validation finished");

}

public static void ValidationHandler(object sender, ValidationEventArgs args)

{

Console.WriteLine("***Validation error");

Console.WriteLine("\tSeverity:{0}", args.Severity);

Console.WriteLine("\tMessage :{0}", args.Message);

}

}

运行这个程序，可以发现校验正确。

但如果我们把它改下呢?

我们将HeadCount.dtd改成：

<!ELEMENT HeadCount1 (Name)*>

<!ELEMENT Name (Name)*>

<!ATTLIST Name First CDATA #REQUIRED>

<!ATTLIST Name Last CDATA #REQUIRED>

<!ATTLIST Name Relation (self | spouse | child) "self">

<!ENTITY MyFirst "Jeff">

<!ENTITY MyLast "Smith">

既将第一个元素改成HeadCount1,再运行这个程序，结果如下：

***Validation error

Severity:Error

Message :The 'HeadCount' element is not declared. An error occurred at

file:///F:/writer/validReader/testDtd/HeadCount.xml, (2, 2).

Validation finished

可以发现HeadCount.xml文件中第一个元素不是所期待的HeadCount1,所以报错。

恢复HeadCount.dtd文件，将HeadCount.xml改一下：

<!DOCTYPE HeadCount SYSTEM "HeadCount.dtd">

<HeadCount>

<Name First1="Waldo" Last1="Pepper">

<Name First="Salt" Last="Pepper" Relation="spouse"/>

<Name First="Red" Last="Pepper" Relation="child"/>

</Name>

<Name First1="&MyFirst;" Last1="&MyLast;">

<Name First="Sharon" Last="&MyLast;" Relation="spouse"/>

<Name First="Morgan" Last="&MyLast;" Relation="child"/>

<Name First="Shelby" Last="&MyLast;" Relation="child"/>

</Name>

</HeadCount>

既将First改成First1，Last改成Last1，再运行这个程序，结果如下：

***Validation error

Severity:Error

Message :The 'HeadCount' element is not declared. An error occurred at

file:///F:/writer/validReader/testDtd/HeadCount.xml, (2, 2).

Validation finished

F:\writer\validReader\testDtd>testDtd

***Validation error

Severity:Error

Message :The 'First1' attribute is not declared. An error occurred at f

ile:///F:/writer/validReader/testDtd/HeadCount.xml, (3, 9).

***Validation error

Severity:Error

Message :The 'Last1' attribute is not declared. An error occurred at fi

le:///F:/writer/validReader/testDtd/HeadCount.xml, (3, 24).

***Validation error

Severity:Error

Message :The required attribute 'Last' is missing. An error occurred at

file:///F:/writer/validReader/testDtd/HeadCount.xml, (3, 4).

***Validation error

Severity:Error

Message :The 'First1' attribute is not declared. An error occurred at f

ile:///F:/writer/validReader/testDtd/HeadCount.xml, (7, 9).

***Validation error

Severity:Error

Message :The 'Last1' attribute is not declared. An error occurred at fi

le:///F:/writer/validReader/testDtd/HeadCount.xml, (7, 28).

***Validation error

Severity:Error

Message :The required attribute 'Last' is missing. An error occurred at

file:///F:/writer/validReader/testDtd/HeadCount.xml, (7, 4).

Validation finished

通过报出来的错误信息，我们可以得知，First1和Last1属性在DTD文件中没有定义，所以出错。

通过以上的测试，可以得知DTD校验的一些简单的规律，它同Xml Schema校验相辅相成，共同构成了XML 校验，保证XML数据的正确性，是XML成为数据承载和传输的有效而可靠的载体。