I am writing a custom configuration section, and I would like to validate a configuration property with a callback, like in this example:

using System;
using System.Configuration;

class CustomSection : ConfigurationSection {

    [ConfigurationProperty("stringValue", IsRequired = false)]
    [CallbackValidator(Type = typeof(CustomSection), CallbackMethodName = "ValidateString")]
    public string StringValue {
        get { return (string)this["stringValue"]; }
        set { this["stringValue"] = value; }
    }

    public static void ValidateString(object value) {
        if (string.IsNullOrEmpty((string)value)) {
            throw new ArgumentException("string must not be empty.");
        }
    }
}

class Program {
    static void Main(string[] args) {
        CustomSection cfg = (CustomSection)ConfigurationManager.GetSection("customSection");
        Console.WriteLine(cfg.StringValue);
    }
}

And my App.config file looks like this:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <configSections>
    <section name="customSection" type="CustomSection, config-section"/>
  </configSections>
  <customSection stringValue="lorem ipsum"/>
</configuration>

My problem is that when the ValidateString function is called, the value parameter is always an empty string, and therefore the validation fails. If i just remove the validator, the string value is correctly initialized to the value in the configuration file.

What am I missing?