How do I dynamically implement the Proxy Pattern?

I'm revisiting my class tracking (dirty logic), which I wrote last year. Currently I have an uber base class that deals with all the state tracking, but each property whos values I need to track needs to stick to the standard get { return _x; } set { _isDirty = true; _x = value; } way of working.

After playing with Entity Framework and reading up on the Proxy Pattern, I was hoping there was a nicer way to implement my IsDIrty Logic whilst being able to make use of auto implemented properties?

To be completely honest, I haven't a clue of what I'm talking about. Is there a way I can do something like the following:

public class Customer : ITrackable
{
    [TrackState(true)] // My own attribute
    public virtual string Name { get;set;}

    [TrackState(true)]
    public virtual  int Age { get;set;}

    // From ITrackable
    public bool IsDirty { get; protected set; }

}

And then implement a dynamic proxy which will use reflection (or another magical solution) to call another method first before setting the values on the properties with the TrackState attribute.

Obviously I could easily do this by creating a phyiscal proxy class and use IoC:

public class CustomerProxy : Customer
{
    Customer _customer;

    public override string Name 
    {
        get { return _customer.Name; }
        set { IsDirty = true; return _customer.Name; }
    }

    // Other properties
}

But I don't fancy having to do this for every object, otherwise there's no benefit from my existing solution. Hope someone can satisfy my curiosi开发者_如何学Pythonty, or at least tell me how EF achieves it.

---

Castle's DynamicProxy does exactly this: http://www.castleproject.org/dynamicproxy/index.html

Allows you to provide an interceptor:

public void Intercept(IInvocation invocation)
{
    // Call your other method first...  then proceed
    invocation.Proceed();
}

You get access to the MethodInfo object via invocation.Method. You may override the return value by setting invocation.ReturnValue. And you can access (and override) the arguments.

---

PostSharp could help.

Or if you're feeling like it, you can write your own IL-rewriter for this. Mono.Cecil is a great library that'll make it a breeze. Here is quick concoction:

class Program {

  static ModuleDefinition _module;

  static void Main(string[] args) {
    // the argument is the assembly path
    _module = ModuleDefinition.ReadModule(args[0]);
    var trackables = _module.Types.
      Where(type => type.Interfaces.Any(tr => tr.Name == "ITrackable"));
    var properties = trackables.SelectMany(type => type.Properties);
    var trackableProperties = properties.
      Where(property => property.CustomAttributes.
        Any(ca => ca.Constructor.DeclaringType.Name == "TrackStateAttribute"));
    trackableProperties.
      Where(property => property.SetMethod != null).
      ToList().
      ForEach(property => CallIsDirty(property.SetMethod));
    _module.Write(args[0]);
  }

  private static void CallIsDirty(MethodDefinition setter) {
    Console.WriteLine(setter.Name);

    var isDirty = setter.DeclaringType.Methods.
      Single(method => method.Name == "set_IsDirty");
    var reference = new MethodReference(isDirty.Name,
      _module.Import(typeof(void))) {
        DeclaringType = setter.DeclaringType,  
        HasThis = true,
        CallingConvention = MethodCallingConvention.Default
      };
    reference.Parameters.Add(new ParameterDefinition(
      _module.Import(typeof(bool))));
    var IL = setter.Body.GetILProcessor();
    var param0 = IL.Create(OpCodes.Ldarg_0);
    var param1 = IL.Create(OpCodes.Ldc_I4_1);
    var call = IL.Create(OpCodes.Call, reference);
    IL.InsertBefore(setter.Body.Instructions[0], call);
    IL.InsertBefore(setter.Body.Instructions[0], param1);
    IL.InsertBefore(setter.Body.Instructions[0], param0);
  }
}

It uses these helpers:

public class TrackStateAttribute : Attribute { }

public interface ITrackable { bool IsDirty { get; } }

Example code:

public class Customer : ITrackable {
  [TrackState] public string Name { get; set; }
  [TrackState] public int Age { get; set; }
  public bool IsDirty { get; protected set; }
}

It's assumed that the IsDirty property will also have a setter.