Strongly typed configuration using Castle DictionaryAdapter requires castle.core nuget package

app.settings.xml

<?xml version="1.0" encoding="utf-8" ?>
<appSettings>
    <add key="smtp:name" value="value" />
    <add key="smtp:port" value="25"/>
    <add key="stuff:something" value="bla"/>
    <!--  other stuff -->
</appSettings>
</configuration>

AppSettingsAttribute.cs

[AttributeUsage(AttributeTargets.Interface,AllowMultiple = false)]
public class AppSettingsAttribute : KeyPrefixAttribute, IDictionaryPropertyGetter, IPropertyDescriptorInitializer
{
    public AppSettingsAttribute(string keyPrefix) : base(keyPrefix)
    {
    }

    public object GetPropertyValue(IDictionaryAdapter dictionaryAdapter, string key, object storedValue,
        PropertyDescriptor property, bool ifExists)
    {
        if (storedValue == null && IsRequired(ifExists))
        {
            throw new ArgumentException("No valid value for '" + key + "' found");
        }
        return storedValue;
    }

    public void Initialize(PropertyDescriptor propertyDescriptor, object[] behaviors)
    {
        propertyDescriptor.Fetch = true;
    }

    private static bool IsRequired(bool ifExists)
    {
        return ifExists == false;
    }
}

Program.cs

internal class Program
{
    private static void Main(string[] args)
    {
        var factory = new DictionaryAdapterFactory();
        var smtp = factory.GetAdapter<SmtpConfiguration>(ConfigurationManager.AppSettings);
        Console.WriteLine(smtp.Port);
        Console.ReadKey(true);
    }
}

SmtpConfiguration.cs

[KeyPrefix("smtp:")]
public interface SmtpConfiguration
{
    string Name { get; set; }
    int Port { get; set; }
}