Performance advice for using Castle DynamicProxy in .NET web-apps

Question

I am starting out with Castle DynamicProxy and I have this sample to track changes to properties of an object.

Questions:

• Should I cache (in a static field) the ProxyGenerator() instance I use in AsTrackable()? I going to use in an ASP.NET web-app, and I was not sure if the class is thread-safe? Is it expensive to create?
• If I leave the code as is, will the generated proxy types be re-used by the different ProxyGenerator instances. I read the caching tutorial, but not sure what "module scope" means.
• Is there any other advice from a performance standpoint to improve the code?

Code:

class Program
{
    static void Main(string[] args)
    {
        var p = new Person { Name = "Jay" }.AsTrackable();

        //here's changed properties list should be empty.
        var changedProperties = p.GetChangedProperties();

        p.Name = "May";

        //here's changed properties list should have one item.
        changedProperties = p.GetChangedProperties();
    }
}

public static class Ext
{
    public static T AsTrackable<T>(this T instance) where T : class
    {
        return new ProxyGenerator().CreateClassProxyWithTarget
        (
          instance, 
          new PropertyChangeTrackingInterceptor()
        );
    }

    public static HashSet<string> GetChangedProperties<T>(this T instance) 
    where T : class
    {
        var proxy = instance as IProxyTargetAccessor;

        if (proxy != null)
        {
            var interceptor = proxy.GetInterceptors()
                                   .Select(i => i as IChangedProperties)
                                   .First();

            if (interceptor != null)
            {
                return interceptor.Properties;
            }
        }

        return new HashSet<string>();
    }
}

interface IChangedProperties
{
    HashSet<string> Properties { get; }
}

public class PropertyChangeTrackingInterceptor : IInterceptor, IChangedProperties
{
    public void Intercept(IInvocation invocation)
    {
        invocation.Proceed();

        this.Properties.Add(invocation.Method.Name);
    }

    private HashSet<string> properties = new HashSet<string>();

    public HashSet<string> Properties
    {
        get { return this.properties; }
        private set { this.properties = value; }
    }
}

public class Person
{
    public virtual string Name { get; set; }
    public virtual int Age { get; set; }
}

}

Answer 1

It is thread-safe to cache a static copy of your proxy generator and you absolutely should do this. That is considered best-practice with this API and not doing so will result in extra types defined in new dynamic assemblies for no reason.