XmlWriter

This class writes XML data from objects in memory. XML files are excellent for interoperability with other systems and Internet sites.

An overview

We use XmlWriter in a C# program, reviewing each step required. The XmlWriter can give the best performance, but it is not the simplest way to write XML data.

Example code

We see an Employee class, which contains 4 fields, 4 public getters, and a constructor. All the fields are private, not public.

using System.Xml;

class Program
{
    class Employee
    {
        int _id;
        string _firstName;
        string _lastName;
        int _salary;

        public Employee(int id, string firstName, string lastName, int salary)
        {
            this._id = id;
            this._firstName = firstName;
            this._lastName = lastName;
            this._salary = salary;
        }

        public int Id { get { return _id; } }
        public string FirstName { get { return _firstName; } }
        public string LastName { get { return _lastName; } }
        public int Salary { get { return _salary; } }
    }

    static void Main()
    {
        Employee[] employees = new Employee[4];
        employees[0] = new Employee(1, "David", "Smith", 10000);
        employees[1] = new Employee(3, "Mark", "Drinkwater", 30000);
        employees[2] = new Employee(4, "Norah", "Miller", 20000);
        employees[3] = new Employee(12, "Cecil", "Walker", 120000);

        using (XmlWriter writer = XmlWriter.Create("employees.xml"))
        {
            writer.WriteStartDocument();
            writer.WriteStartElement("Employees");

            foreach (Employee employee in employees)
            {
                writer.WriteStartElement("Employee");

                writer.WriteElementString("ID", employee.Id.ToString());
                writer.WriteElementString("FirstName", employee.FirstName);
                writer.WriteElementString("LastName", employee.LastName);
                writer.WriteElementString("Salary", employee.Salary.ToString());

                writer.WriteEndElement();
            }

            writer.WriteEndElement();
            writer.WriteEndDocument();
        }
    }
}

Write root

Next, we need to add a root element to the XML. It is important to add a root element with XmlWriter that contains all the other elements.

using (XmlWriter writer = XmlWriter.Create("employees.xml"))
{
    writer.WriteStartDocument();
    writer.WriteStartElement("Employees"); // <-- Important root element
    writer.WriteEndElement();              // <-- Closes it
    writer.WriteEndDocument();
}

Write array

Next, we can loop through the elements in our Employee array. The array we use here is shown in full in the first block of code above.

using (XmlWriter writer = XmlWriter.Create("employees.xml"))
{
    writer.WriteStartDocument();
    writer.WriteStartElement("Employees");

    foreach (Employee employee in employees)  // <-- This is new
    {
        writer.WriteStartElement("Employee"); // <-- Write employee element
        writer.WriteEndElement();
    }

    writer.WriteEndElement();
    writer.WriteEndDocument();
}

Write object

We take the values from each Employee and access the property getters. The 4 fields from the Employee object are inserted into special tags in the XML.

using (XmlWriter writer = XmlWriter.Create("employees.xml"))
{
    writer.WriteStartDocument();
    writer.WriteStartElement("Employees");

    foreach (Employee employee in employees)
    {
        writer.WriteStartElement("Employee");

        writer.WriteElementString("ID", employee.Id.ToString());   // <-- These are new
        writer.WriteElementString("FirstName", employee.FirstName);
        writer.WriteElementString("LastName", employee.LastName);
        writer.WriteElementString("Salary", employee.Salary.ToString());

        writer.WriteEndElement();
    }

    writer.WriteEndElement();
    writer.WriteEndDocument();
}

An error

I have often encountered this InvalidOperationException. The problem here is that you always have to put the root element into your document.

Token StartElement in state EndRoot Element would result in an invalid XML document.
Make sure that the ConformanceLevel setting is set to ConformanceLevel.Fragment or
ConformanceLevel.Auto if you want to write an XML fragment.

Output

We see that there are 4 employee elements with 4 sub-elements in each. It does not have attributes on the elements, but is a properly-formed document.

<?xml version="1.0" encoding="utf-8"?>
<Employees>
    <Employee>
        <ID>1</ID>
        <FirstName>David</FirstName>
        <LastName>Smith</LastName>
        <Salary>10000</Salary>
    </Employee>
    <Employee>
        <ID>3</ID>
        <FirstName>Mark</FirstName>
        <LastName>Drinkwater</LastName>
        <Salary>30000</Salary>
    </Employee>
    <Employee>
        <ID>4</ID>
        <FirstName>Norah</FirstName>
        <LastName>Miller</LastName>
        <Salary>20000</Salary>
    </Employee>
    <Employee>
        <ID>12</ID>
        <FirstName>Cecil</FirstName>
        <LastName>Walker</LastName>
        <Salary>120000</Salary>
    </Employee>
</Employees>

Dispose

If you use the using-statement on an XmlWriter, the Dispose method will be called. In the Dispose method, we see that the Close method is called.

.method family hidebysig newslot virtual
        instance void  Dispose(bool disposing) cil managed
{
  // Code size       19 (0x13)
  .maxstack  8
  ...
  IL_000c:  ldarg.0
  IL_000d:  callvirt   instance void System.Xml.XmlWriter::Close()
  IL_0012:  ret
} // end of method XmlWriter::Dispose

Summary

This tutorial used XmlWriter with the using-statement. And it invoked several methods on the type. We looked at some problems, and checked XML Notepad and the output XML.