XmlSerializer define default value

Question

We have an xml document with some user settings. We just added a new setting (which is not found in legacy xml documents) and the XmlSerializer automatically sets it to false. I tried DefaultValueAttribute but it doesn't work. Any idea on how I can get the default value to be true? This is the code:

private bool _property = true;
[DefaultValueAttribute(true)]
public bool Property 
{
    get { return _property; }
    set
    {
        if (_property != value)
        {
            _property = value;
            this.IsModified = true;
        }
    }
}

Thanks!

Answer 1

DefaultValue affects the serialization insofar as if at runtime the property has a value that matches what the DefaultValue says, then the XmlSerializer won't actually write that element out (since it's the default).

I wasn't sure whether it would then affect the default value on read, but it doesn't appear to do so in a quick test. In your scenario, I'd likely just make it a property with a backing field with a field initializer that makes it 'true'. IMHO I like that better than ctor approach since it decouples it from the ctors you do or don't have defined for the class, but it's the same goal.

using System;
using System.ComponentModel;
using System.Xml.Serialization;
using System.IO;

class Program
{
    static void Main(string[] args)
    {
        var serializer = new XmlSerializer(typeof(Testing));

        string serializedString;
        Testing instance = new Testing();
        using (StringWriter writer = new StringWriter())
        {
            instance.SomeProperty = true;
            serializer.Serialize(writer, instance);
            serializedString = writer.ToString();
        }
        Console.WriteLine("Serialized instance with SomeProperty={0} out as {1}", instance.SomeProperty, serializedString);
        using (StringReader reader = new StringReader(serializedString))
        {
            instance = (Testing)serializer.Deserialize(reader);
            Console.WriteLine("Deserialized string {0} into instance with SomeProperty={1}", serializedString, instance.SomeProperty);
        }
        Console.ReadLine();
    }
}

public class Testing
{
    [DefaultValue(true)]
    public bool SomeProperty { get; set; }
}

As I mentioned in a comment, the page on the xml serialization attributes (http://msdn.microsoft.com/en-us/library/83y7df3e.aspx) claims that the DefaultValue will indeed make the serializer set the value when it's missing, but it doesn't do so in this test code (similar to above, but just deserializes 3 inputs).

using System;
using System.ComponentModel;
using System.Xml.Serialization;
using System.IO;

class Program
{
    private static string[] s_inputs = new[]
    {
        @"<?xml version=""1.0"" encoding=""utf-16""?>
          <Testing xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema"" />",

        @"<?xml version=""1.0"" encoding=""utf-16""?>
          <Testing xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema"">
              <SomeProperty />
          </Testing>",

        @"<?xml version=""1.0"" encoding=""utf-16""?>
          <Testing xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema"">
              <SomeProperty>true</SomeProperty>
          </Testing>",
    };

    static void Main(string[] args)
    {
        var serializer = new XmlSerializer(typeof(Testing));

        foreach (var input in s_inputs)
        {
            using (StringReader reader = new StringReader(input))
            {
                Testing instance = (Testing)serializer.Deserialize(reader);
                Console.WriteLine("Deserialized string \n{0}\n into instance with SomeProperty={1}", input, instance.SomeProperty);
            }
        }
        Console.ReadLine();
    }
}

public class Testing
{
    [DefaultValue(true)]
    public bool SomeProperty { get; set; }
}

Answer 2

Regardless of how it is documented to work, I also do not see DefaultValue being assigned when deserializing. The solution for me was simply set the default value within the class constructor and give up on DefaultValueAttribute.