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As you ve seen, the XmlReader is an abstract class. That means you cannot instantiate it directly in your applications; you must inherit from it to make any use of it. Fortunately, the .NET Framework provides three implementations of the XmlReader class. These implementations are discussed briefly in this section.
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The XmlTextReader Class
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The XmlTextReader class can be used to parse XML documents. This class has very fast parsing abilities. It checks that the underlying documents are well formed but does not validate them against a DTD or schema.
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The XmlValidatingReader Class
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The XmlValidatingReader class can validate an XML document against a DTD or XML schema.
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C HAPTE R 3 READING A ND WRITIN G XML DOC UMEN TS
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The XmlNodeReader Class
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The XmlNodeReader class allows you to read XML data from the DOM tree. The constructor of XmlNodeReader takes a parameter of type XmlNode. This XmlNode can be obtained as a result of an XPath query or directly from a DOM document. In terms of properties and methods, the XmlNodeReader class closely resembles the XmlTextReader class.
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Reading Documents by Using XmlTextReader
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In this section, you will learn the following: How to open XML documents by using the XmlTextReader class How to read and access the content How to deal with white space How to work with name tables How to deal with namespaces Let s begin by opening XML documents. Throughout our examples, we will be using the same Employees.xml file that we used earlier in the book.
Opening XML Documents
To illustrate how XML documents can be opened, we will develop a Windows application like the one shown in Figure 3-1.
Figure 3-1. Reading XML documents by using XmlTextReader
CHAPTER 3 REA DING A ND WRITING XML DOCUME NTS
The application allows you to choose the location from which the document is to be opened. The possible locations are URL, stream, or string. Depending on the choice, you need to enter the URL, filename, or XML string in the text box and click the Open Document button. Clicking the Open Document button opens the document and displays a success message box. The XmlReader class can read an XML document from either a URL or a stream. The stream can by any kind of stream, such as a FileStream or MemoryStream. The XmlReader class cannot read XML strings directly. First, you need to read the string into a MemoryStream and then feed this MemoryStream to the XmlReader class. The code from Listing 3-1 shows these three techniques. Listing 3-1. Loading an XML Document in XmlTextReader private void button1_Click(object sender, EventArgs e) { XmlTextReader reader; if (radioButton1.Checked) { reader = new XmlTextReader(textBox1.Text); } if (radioButton2.Checked) { FileStream stream=File.OpenRead(textBox1.Text); reader = new XmlTextReader(stream); //some processing code stream.Close(); reader.Close(); } if (radioButton3.Checked) { MemoryStream ms=new MemoryStream(); byte[] data=ASCIIEncoding.ASCII.GetBytes(textBox1.Text); ms.Write(data,0,data.Length); reader = new XmlTextReader(ms); //some processing code ms.Close(); reader.Close(); } MessageBox.Show("XML Document Opened Successfully!"); }
C HAPTE R 3 READING A ND WRITIN G XML DOC UMEN TS
Note Make sure to import the System.Xml and System.IO namespaces before writing the preceding
code. The XmlTextReader class resides in the System.Xml namespace, and the MemoryStream class resides in the System.IO namespace.
The code declares a variable of type XmlTextReader. It then checks to see which radio button has been selected. If the user wants to use a URL, a new instance of XmlTextReader is created by passing the URL in the constructor. If the user decides to read the file from disk, it is first read into a stream. This is done by using the OpenRead() method of the File class. The OpenRead() method opens the specified file in read-only mode. The resulting FileStream is then passed in the constructor of the XmlTextReader class. You cannot directly pass an XML string to the XmlTextReader class. Hence the third condition reads the string into MemoryStream. Note the use of the GetBytes() method to convert a string into a byte array. The resulting byte array is written to the MemoryStream object. Finally, this MemoryStream instance is supplied to the constructor of the XmlTextReader class.
Reading Attributes, Elements, and Values
In this section, we are going to develop a Windows application that will display a tree of various elements and their values. In the process, you will learn how to read attributes, elements, and text nodes from an XML document by using the XmlTextReader class. The application is shown in Figure 3-2.
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