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.

Answer 3

About Scott's last reply.

The default constructor sets any array property to "null" when no elements are found. I've set the property to a new empty array (so an instance of an empty array), but the XmlSerializer bypasses this when doing its deserialization and sets it to "null".

Example of the class:

[XmlElement("Dtc")]
public DtcMarketMapping[] DtcMarketMappingInstances
{
    get { return dtcMarketMappingInstances; }
    set { dtcMarketMappingInstances = value; }
}

It might however be something specific to arrays and your answer is still valid about setting default values to simple properties in the class' constructor.

Anyways, thanks for the answers everyone.

Answer 4

Set the default value in the constructor. The value will only be changed during deserialize if it exists in the file.

public class MySettingsClass
{
    public MySettingsClass()
    {
        _property = true;
    }

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

Answer 5

The DefaultValueAttribute doesn't actually do anything to your running code. It is (mostly) used by property browsers (such as when you bind your object to a PropertyGrid) to know what the default value should be so they can do something "special" when the value is different than the default. That's how the property grid in Visual Studio knows when to make a value bold (showing that it's different than the default value.

The XmlSerializer is setting it your _property field to false because that is the .NET Framework default value for a bool.

The simplest way to accomplish this would probably be to use a default class constructor and set the value there.