Polymorphic custom configuration section

Question

Lets say I have the following configuration section....

<yak>
    <whaa hello="world" />
</yak>
<yak>
   <whaa hello="world" blot="1" />
</yak>

Because the second <whaa> element has the extra attribute on it I want to map it to a sub type of the the type that's mapped to the first <whaa> element.

So how do I get polymorphic binding?

Answer 1

Here's one approach by using a programmatic solution which overrides the OnDeserializeUnrecognizedElement method of the ConfigurationSection class. The base and child configuration element classes:

public class Whaa : ConfigurationElement
{
    [ConfigurationProperty("hello", IsRequired = true)]
    public string Hello
    {
        get
        {
            return base["hello"] as string;
        }
        set
        {
            base["hello"] = value;
        }
    }
}

public class WhaaChild : Whaa
{
    [ConfigurationProperty("blot", IsRequired = true)]
    public string Blot
    {
        get
        {
            return base["blot"] as string;
        }
        set
        {
            base["blot"] = value;
        }
    }
}

The configuration section class:

public class Yak : ConfigurationSection
{
    public Whaa MyProperty;

    protected override bool
        OnDeserializeUnrecognizedElement(string elementName, XmlReader reader)
    {
        if (elementName == "whaa")
        {
            try
            {
                var hello = reader.GetAttribute("hello");
                if (hello != null)
                {
                    var blot = reader.GetAttribute("blot");
                    if (blot != null)
                    {
                        MyProperty = new WhaaChild()
                        {
                            Blot = blot,
                            Hello = hello
                        };
                    }
                    else
                    {
                        MyProperty = new Whaa()
                        {
                            Hello = hello
                        };
                    }
                }
            }
            catch (Exception)
            {
                // TODO: add exception handling
            }
        }
        return true;
    }
}

The sample usage:

var yakSectionSettings = (Yak)ConfigurationManager.GetSection("yak");

And the configuration markup:

<configSections>
    <section name="yak" type="Yak, MyApplication"/>
</configSections>

You may use now either:

<yak>
    <whaa hello="world"/>
</yak>

for getting a Whaa object, or:

<yak>
    <whaa hello="world" blot="1"/>
</yak>

for getting a WhaaChild object at runtime.