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XmlWriter.WriteProcessingInstruction Method

When overridden in a derived class, writes out a processing instruction with a space between the name and text as follows: <?name text?>.

Namespace:  System.Xml
Assemblies:   System.Xml (in System.Xml.dll)
  System.Xml.ReaderWriter (in System.Xml.ReaderWriter.dll)

public abstract void WriteProcessingInstruction(
	string name,
	string text
)

Parameters

name
Type: System.String

The name of the processing instruction.

text
Type: System.String

The text to include in the processing instruction.

ExceptionCondition
ArgumentException

The text would result in a non-well formed XML document.

name is either null or String.Empty.

This method is being used to create an XML declaration after WriteStartDocument has already been called.

InvalidOperationException

An XmlWriter method was called before a previous asynchronous operation finished. In this case, InvalidOperationException is thrown with the message “An asynchronous operation is already in progress.”

This method can be used to write the XML declaration (rather than WriteStartDocument). This could result in the encoding attribute being incorrectly written. For example, the following C# code would result in an invalid XML document because the default encoding is UTF-8.

XmlWriter writer = XmlWriter.Create("output.xml");
writer.WriteProcessingInstruction("xml", "version='1.0' encoding='UTF-16'");
writer.WriteStartElement("root");
writer.Close();

If text is either null or String.Empty, this method writes a ProcessingInstruction with no data content, for example <?name?>.

If text contains an invalid sequence of "?>", the XmlWriter can either throw an ArgumentException (XmlTextWriter objects) or insert a space "? >" to avoid writing invalid XML (XmlWriter objects created by the Create method).

For the asynchronous version of this method, see WriteProcessingInstructionAsync.

The following example writes an XML file representing a book.

using System;
using System.IO;
using System.Xml;

public class Sample {

  private const string filename = "sampledata.xml";

  public static void Main() {

     XmlWriterSettings settings = new XmlWriterSettings();
     settings.Indent = true;
     XmlWriter writer = XmlWriter.Create(filename, settings);

     // Write the Processing Instruction node.
     String PItext="type=\"text/xsl\" href=\"book.xsl\"";
     writer.WriteProcessingInstruction("xml-stylesheet", PItext);

     // Write the DocumentType node.
     writer.WriteDocType("book", null , null, "<!ENTITY h \"hardcover\">");

     // Write a Comment node.
     writer.WriteComment("sample XML");

     // Write the root element.
     writer.WriteStartElement("book");

     // Write the genre attribute.
     writer.WriteAttributeString("genre", "novel");

     // Write the ISBN attribute.
     writer.WriteAttributeString("ISBN", "1-8630-014");

     // Write the title.
     writer.WriteElementString("title", "The Handmaid's Tale");

     // Write the style element.
     writer.WriteStartElement("style");
     writer.WriteEntityRef("h");
     writer.WriteEndElement(); 

     // Write the price.
     writer.WriteElementString("price", "19.95");

     // Write CDATA.
     writer.WriteCData("Prices 15% off!!");

     // Write the close tag for the root element.
     writer.WriteEndElement();

     writer.WriteEndDocument();

     // Write the XML to file and close the writer.
     writer.Flush();
     writer.Close();  
  }

}

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0, 2.0, 1.0

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8
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