How do I setup mutually exclusive custom config elements?

Question

I am writing a custom configuration section using the .NET configuration api. I want to define a section that can have exactly one of two sub-elements,

e.g.

<MyCustomSection>
    <myItems>
        <myItemType name="1">
            <firstSubTypeConfig />
        </myItemType>
        <myItemType name="2">
            <secondSubTypeConfig />
        </myItemType>
    </myItems>
</MyCustomSection>

Currently, I have the sections defined like:

public class MyCustomSection : ConfigurationSection
{
    public override bool IsReadOnly()
    {
        return false;
    }

    [ConfigurationProperty("myItems")]
    public MyItemsCollection MyItems 
    { 
        get { return (MyItemsCollection)base["myItems"]; }
        set { base["myItems"] = value; }
    }
}

[ConfigurationCollection(typeof(MyItemType), AddItemName="myItemType",
    CollectionType=ConfigurationElementCollectionType.BasicMap)]
public class MyItemsCollection : ConfigurationElementCollection
{
    public override bool IsReadOnly()
    {
        return false;
    }

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

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

    protected override ConfigurationElement CreateNewElement()
    {
        return new MyItemType();
    }

    protected override object GetElementKey(ConfigurationElement element)
    {
        return (element as MyItemType).Name;
    }
}


public class MyItemType : ConfigurationElement
{
    public override bool IsReadOnly()
    {
        return false;
    }

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

    [ConfigurationProperty("firstSubTypeConfig")]
    public FirstSubTypeConfig FirstSubTypeConfig 
    { 
        get { return (FirstSubTypeConfig)base["firstSubTypeConfig"]; }
        set { base["firstSubTypeConfig"] = value; }

    }

    [ConfigurationProperty("secondSubTypeConfig")]
    public SecondSubTypeConfig SecondSubTypeConfig 
    { 
        get { return (SecondSubTypeConfig)base["secondSubTypeConfig"]; }
        set { base["secondSubTypeConfig"] = value; }
    }
}

public class FirstSubTypeConfig : ConfigurationElement
{
    public override bool IsReadOnly()
    {
        return false;
    }
}

public class SecondSubTypeConfig : ConfigurationElement
{
    public override bool IsReadOnly()
    {
        return false;
    }
}

The configuration is also modified and saved programmatically, and currently, saving the configuration section will add the secondSubTypeConfig element even if I only specify the firstSubTypeConfig element.

My first thought was to introduce a common base class for FirstSubTypeConfig and SecondSubTypeConfig, but I'm not clear on if or how the configuration api would handle that.

How do I setup mutually exclusive custom config elements?

Answer 1

I'm not sure this is the "correct" approach, but this works.

The code as written in the question will load the config file fine. You can check the ElementInformation.IsPresent property to validate that exactly one element is included.

e.g.

var custom = (MyCustomSection)ConfigurationManager.GetSection("MyCustomSection");
foreach (MyItemType itemType in custom.MyItems)
{
    if (itemType.FirstSubTypeConfig.ElementInformation.IsPresent 
        && itemType.SecondSubTypeConfig.ElementInformation.IsPresent)
    {
        throw new ConfigurationErrorsException("At most one of firstSubTypeConfig or secondSubTypeConfig can be specified in a myItemType element");
    }
    else if (!itemType.FirstSubTypeConfig.ElementInformation.IsPresent
        && !itemType.SecondSubTypeConfig.ElementInformation.Ispresent)
    {
        throw new ConfigurationErrorsException("Either a firstSubTypeConfig or a secondSubTypeConfig element must be specified in a myItemType element");
    }
}

As for saving the config, it seems that checking for ElementInformation.IsPresent and explicitly setting it to null will prevent the element from being written to the config file. e.g.

var config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
var custom = (MyCustomSection)config.GetSection("MyCustomSection");

//make modifications against the custom variable ...

foreach (MyItemType itemType in custom.MyItems)
{
    if (!itemType.FirstSubTypeConfig.ElementInformation.IsPresent)
        itemType.FirstSubTypeConfig = null;
    if (!itemType.SecondSubTypeConfig.ElementInformation.IsPresent)
        itemType.SecondSubTypeConfig = null;  
}

config.Save();