how to have custom attribute in ConfigurationElementCollection?

Question

for configuration as following

<MyCollection default="one">
  <entry name="one" ... other attrubutes />
  ... other entries
</MyCollection>

when implement a MyCollection, what should i do for the "default" attribute?

Answer 1

Let's suppose you have this .config file:

<configuration>
    <configSections>
        <section name="mySection" type="ConsoleApplication1.MySection, ConsoleApplication1" /> // update type  & assembly names accordingly
    </configSections>

    <mySection>
        <MyCollection default="one">
            <entry name="one" />
            <entry name="two" />
        </MyCollection>
    </mySection>
</configuration>

Then, with this code:

public class MySection : ConfigurationSection
{
    [ConfigurationProperty("MyCollection", Options = ConfigurationPropertyOptions.IsRequired)]
    public MyCollection MyCollection
    {
        get
        {
            return (MyCollection)this["MyCollection"];
        }
    }
}

[ConfigurationCollection(typeof(EntryElement), AddItemName = "entry", CollectionType = ConfigurationElementCollectionType.BasicMap)]
public class MyCollection : ConfigurationElementCollection
{
    protected override ConfigurationElement CreateNewElement()
    {
        return new EntryElement();
    }

    protected override object GetElementKey(ConfigurationElement element)
    {
        if (element == null)
            throw new ArgumentNullException("element");

        return ((EntryElement)element).Name;
    }

    [ConfigurationProperty("default", IsRequired = false)]
    public string Default
    {
        get
        {
            return (string)base["default"];
        }
    }
}

public class EntryElement : ConfigurationElement
{
    [ConfigurationProperty("name", IsRequired = true, IsKey = true)]
    public string Name
    {
        get
        {
            return (string)base["name"];
        }
    }
}

you can read the configuration with the 'default' attribute, like this:

    MySection section = (MySection)ConfigurationManager.GetSection("mySection");
    Console.WriteLine(section.MyCollection.Default);

This will output "one"

Answer 2

I don't know if it's possible to have a default value in a ConfigurationElementCollection. (it doesn't seen to have any property for default value).

I guess you have to implement this by yourself. Look at the example below.

public class Repository : ConfigurationElement
{
    [ConfigurationProperty("key", IsRequired = true)]
    public string Key
    {
        get { return (string)this["key"]; }
    }

    [ConfigurationProperty("value", IsRequired = true)]
    public string Value
    {
        get { return (string)this["value"]; }
    }
}

public class RepositoryCollection : ConfigurationElementCollection
{
    protected override ConfigurationElement CreateNewElement()
    {
        return new Repository();
    }

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

    public Repository this[int index]
    {
        get { return base.BaseGet(index) as Repository; }
    }

    public new Repository this[string key]
    {
        get { return base.BaseGet(key) as Repository; }
    }

}

public class MyConfig : ConfigurationSection
{
    [ConfigurationProperty("currentRepository", IsRequired = true)]
    private string InternalCurrentRepository
    {
        get { return (string)this["currentRepository"]; }
    }

    [ConfigurationProperty("repositories", IsRequired = true)]
    private RepositoryCollection InternalRepositories
    {
        get { return this["repositories"] as RepositoryCollection; }
    }
}

Here's the XML config:

  <myConfig currentRepository="SQL2008">
    <repositories>
      <add key="SQL2008" value="abc"/>
      <add key="Oracle" value="xyz"/>
    </repositories>
  </myConfig>

And then, at your code, you access the default item using the following:

MyConfig conf = (MyConfig)ConfigurationManager.GetSection("myConfig");
string myValue = conf.Repositories[conf.CurrentRepository].Value;

Of course, the MyConfig class can hide the details of accessing the Repositories and CurrentRepository properties. You can have a property called DefaultRepository (of type Repository) in MyConfig class to return this.

Answer 3

This may be a bit late but may be helpful to others.

It is possible but with some modification.

• ConfigurationElementCollection inherits ConfigurationElement as such "this[string]" is available in ConfigurationElement.

• Usually when ConfigurationElementCollection is inherited and implemented in another class, the "this[string]" is hidden with "new this[string]".

• One way to get around it is to create another implementation of this[] such as "this[string, string]"

See example below.

public class CustomCollection : ConfigurationElementCollection
{
    protected override ConfigurationElement CreateNewElement()
    {
        return new CustomElement();
    }

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

    public CustomElement this[int index]
    {
        get { return (CustomElement)base.BaseGet(index); }
        set
        {
            if (BaseGet(index) != null)
                BaseRemoveAt(index);

            BaseAdd(index, value);
        }
    }

    // ConfigurationElement this[string] now becomes hidden in child class
    public new CustomElement this[string name]
    {
        get { return (CustomElement)BaseGet(name); }
    }

    // ConfigurationElement this[string] is now exposed
    // however, a value must be entered in second argument for property to be access
    // otherwise "this[string]" will be called and a CustomElement returned instead
    public object this[string name, string str = null]
    {
        get { return base[name]; }
        set { base[name] = value; }
    }
}

Answer 4

If you want to genericize it, this should help:

using System.Configuration;

namespace Abcd
{
  // Generic implementation of ConfigurationElementCollection.
  [ConfigurationCollection(typeof(ConfigurationElement))]
  public class ConfigurationElementCollection<T> : ConfigurationElementCollection
                                         where T : ConfigurationElement, IConfigurationElement, new()
  {
    protected override ConfigurationElement CreateNewElement()
    {
      return new T();
    }

    protected override object GetElementKey(ConfigurationElement element)
    {
      return ((IConfigurationElement)element).GetElementKey();
    }

    public T this[int index]
    {
      get { return (T)BaseGet(index); }
    }

    public T GetElement(object key)
    {
      return (T)BaseGet(key);
    }
  }
}

Here's the interface referenced above:

namespace Abcd
{
  public interface IConfigurationElement
  {
    object GetElementKey();
  }
}