How do I use .NET custom ConfigurationElement properties on descendent elements?

How can I get and use an attribute set in the parent ConfigurationSection in the descendent CustomSetting element? I need this attribute when the CustomSetting element is returning the Value property.

I want to format the App.config like this:

<CustomSettings someProperty="foo">
    <CustomSetting key="bar" value="fermeneba" />
    <CustomSetting key="laa" value="jubaduba" />
</CustomSettings>

I have the code working, except that I cannot find a way to access the someProperty attribute from the CustomSetting class. The only way that I've found, so far, is to format the configuration like this, which is messy:

<CustomSettings>
    <CustomSetting someProperty="foo" key="bar" value="fermeneba" />
    <CustomSetting someProperty="foo" key="laa" value="jubaduba" />
</CustomSettings>

Achieving this is more difficult than it should be since the System.Configuration API doesn't allow you to navigate from a ConfigurationElement to its parent. Hence, if you want to access some information that on a parent element you need to create that relationship manually. I've put together a sample implementation that does that for the config snippet in your question:

public class CustomSettingsSection : ConfigurationSection
{
    [ConfigurationProperty("someProperty", DefaultValue="")]
    public string SomeProperty
    {
        get { return (string)base["someProperty"]; }
        set { base["someProperty"] = value; }
    }

    [ConfigurationProperty("", IsDefaultCollection = true)]
    public CustomSettingElementCollection Elements
    {
        get 
        {
            var elements = base[""] as CustomSettingElementCollection;
            if (elements != null && elements.Section == null)
                elements.Section = this;
            return elements;
        }
    }
}

public class CustomSettingElementCollection : ConfigurationElementCollection
{

    internal CustomSettingsSection Section { get; set; }

    public override ConfigurationElementCollectionType CollectionType
    {
        get { return ConfigurationElementCollectionType.BasicMap; }
    }

    public CustomSettingElement this[string key]
    {
        get { return BaseGet(key) as CustomSettingElement; }
    }

    protected override ConfigurationElement CreateNewElement()
    {
        return new CustomSettingElement { Parent = this };
    }

    protected override object GetElementKey(ConfigurationElement element)
    {
        return (element as CustomSettingElement).Key;
    }

    protected override string ElementName
    {
        get { return "customSetting"; }
    }
}

public class CustomSettingElement : ConfigurationElement
{

    internal CustomSettingElementCollection Parent { get; set; }

    public string SomeProperty
    {
        get
        {
            if (Parent != null && Parent.Section != null)
                return Parent.Section.SomeProperty;
            return default(string);
        }
    }




    [ConfigurationProperty("key", IsKey = true, IsRequired = true)]
    public string Key
    {
        get { return (string)base["key"]; }
        set { base["key"] = value; }
    }

    [ConfigurationProperty("value", DefaultValue = "")]
    public string Value
    {
        get { return (string)base["value"]; }
        set { base["value"] = value; }
    }

}

You can see that the CustomSettingElementCollection has a Section property which gets set in the section's Elements getter. The CustomSettingElement, in turn, has a Parent property which gets set in the collection's CreateNewElement() method.

That then makes it possible to walk up the relationship tree and to add a SomeProperty property to the element even though this one doesn't correspond to an actual ConfigurationProperty on that element.

Hope that gives you an idea how to solve your problem!