How to tell XmlSerializer to serialize properties with [DefautValue(…)] always?

Question

I am using DefaultValue attribute for the proper PropertyGrid behavior (it shows values different from default in bold). Now if I want to serialize

Answer 1

This behaviour of the XmlSerializer can can be overwritten with XmlAttributeOverrides

I borrowed the idea from here:

static public XmlAttributeOverrides GetDefaultValuesOverrides(Type type)
{
    XmlAttributeOverrides explicitOverrides = new XmlAttributeOverrides();

    PropertyDescriptorCollection c = TypeDescriptor.GetProperties(type);
    foreach (PropertyDescriptor p in c)
    {
        AttributeCollection attributes = p.Attributes;
        DefaultValueAttribute defaultValue = (DefaultValueAttribute)attributes[typeof(DefaultValueAttribute)];
        XmlIgnoreAttribute noXML = (XmlIgnoreAttribute)attributes[typeof(XmlIgnoreAttribute)];
        XmlAttributeAttribute attribute = (XmlAttributeAttribute)attributes[typeof(XmlAttributeAttribute)];

        if ( defaultValue != null && noXML == null )
        {
            XmlAttributeAttribute xmlAttribute = new XmlAttributeAttribute(attribute.AttributeName);
            XmlAttributes xmlAttributes = new XmlAttributes();
            xmlAttributes.XmlAttribute = xmlAttribute;
            explicitOverrides.Add(userType, attribute.AttributeName, xmlAttributes);
        }
    }
    return explicitOverrides;
}

And made my self an an Attribute to decorate the classes which should emit the default values. If you want do this for all classes, I'm sure you can adapt the whole concept.

Public Class EmitDefaultValuesAttribute
    Inherits Attribute
    Private Shared mCache As New Dictionary(Of Assembly, XmlAttributeOverrides)

    Public Shared Function GetOverrides(assembly As Assembly) As XmlAttributeOverrides
        If mCache.ContainsKey(assembly) Then Return mCache(assembly)
        Dim xmlOverrides As New XmlAttributeOverrides
        For Each t In assembly.GetTypes()
            If t.GetCustomAttributes(GetType(EmitDefaultValuesAttribute), True).Count > 0 Then
                AddOverride(t, xmlOverrides)
            End If
        Next
        mCache.Add(assembly, xmlOverrides)
        Return xmlOverrides
    End Function

    Private Shared Sub AddOverride(t As Type, xmlOverrides As XmlAttributeOverrides)
        For Each prop In t.GetProperties()
            Dim defaultAttr = prop.GetCustomAttributes(GetType(DefaultValueAttribute), True).FirstOrDefault()
            Dim xmlAttr As XmlAttributeAttribute = prop.GetCustomAttributes(GetType(XmlAttributeAttribute), True).FirstOrDefault()
            If defaultAttr IsNot Nothing AndAlso xmlAttr IsNot Nothing Then
                Dim attrs As New XmlAttributes '= {New XmlAttributeAttribute}
                attrs.XmlAttribute = xmlAttr
                ''overide.Add(t, xmlAttr.AttributeName, attrs)
                xmlOverrides.Add(t, prop.Name, attrs)
            End If
        Next
    End Sub

Because xsd.exe produces partial classes you can add this EmitDefaultValuesAttribute in seperate a file:

Public MyClass
    Public Property SubClass() As MySubClass
End Class


Public MySubClass
End Class

Usage is as follows:

Dim serializer As New XmlSerializer(GetType(MyClass), EmitDefaultValuesAttribute.GetOverrides(GetType(MyClass).Assembly))