Store Dictionary in application settings

You can use this class derived from StringDictionary. To be useful for application settings it implements IXmlSerializable.
Or you can use similar approach to implement your own XmlSerializable class.

public class SerializableStringDictionary : System.Collections.Specialized.StringDictionary, System.Xml.Serialization.IXmlSerializable
{
    public System.Xml.Schema.XmlSchema GetSchema()
    {
        return null;
    }

    public void ReadXml(System.Xml.XmlReader reader)
    {
        while (reader.Read() &&
            !(reader.NodeType == System.Xml.XmlNodeType.EndElement && reader.LocalName == this.GetType().Name))
        {
            var name = reader["Name"];
            if (name == null)
                throw new FormatException();

            var value = reader["Value"];
            this[name] = value;
        }
    }

    public void WriteXml(System.Xml.XmlWriter writer)
    {
        foreach (System.Collections.DictionaryEntry entry in this)
        {
            writer.WriteStartElement("Pair");
            writer.WriteAttributeString("Name", (string)entry.Key);
            writer.WriteAttributeString("Value", (string)entry.Value);
            writer.WriteEndElement();
        }
    }
}

Resulting XML fragment will look similar to:

...
<setting name="PluginSettings" serializeAs="Xml">
    <value>
        <SerializableStringDictionary>
            <Pair Name="property1" Value="True" />
            <Pair Name="property2" Value="05/01/2011 0:00:00" />
        </SerializableStringDictionary>
    </value>
</setting>
...