Fun with Castle.DynamicProxy

November 14, 2012·orientman·2 min read·Posts In English (Wpisy po angielsku)

In our application we have input data validation implemented in business layer (separate dll). Because we need to report results to user (our UI layer is ASP.NET MVC application) I came up with following interfaces (simplified version):

namespace MyCompany.Infrastructure.Validation
{
    // implemented in UI layer
    public interface IValidationListener
    {
        void OnError(string error);
        void OnWarning(string warning);
        bool IsIgnored(string warning);
    }
 
    // stops on first error but reports all warnings
    public interface IValidator
    {
        /// <returns>False if any error or warning</returns>
        bool Validate(IValidationListener listener);
    }
}

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It works well until you need to "chain" validators:

public bool Validate(IValidationListener listener)
{
    return firstValidator.Validate(listener) &&
        secondValidator.Validate(listener);
}

Validator stops on first error as it should by it doesn't collect all warnings because of "short-circuit" evaluation of "&&". I couldn't use "&" operator because it would change semantics (i.e. evaluate all validators regardless of possible errors). Instead I wanted something like this:

public bool Validate(IValidationListener listener)
{
    // stops on error but if there are only warnings reports all of them
    return listener.ListenAll(firstValidator, secondValidator);
}

How to do this without changing all implementations of IValidationListener? With extension method and dynamic proxy of course:

using Castle.DynamicProxy;
 
namespace MyCompany.Infrastructure.Validation
{
    public static class ValidationListenerExtensions
    {
        public static bool ListenAll(
            this IValidationListener listener,
            params IValidator[] validators)
        {
            var interceptor = new ErrorCatcherInterceptor();
            var proxy = new ProxyGenerator()
                .CreateInterfaceProxyWithTargetInterface(listener, interceptor);
            var success = true;
 
            foreach (var v in validators)
            {
                success &= v.Validate(proxy);
                if (interceptor.ErrorOccurred)
                    return false;
            }
 
            return success;
        }
 
        class ErrorCatcherInterceptor : IInterceptor
        {
            public bool ErrorOccurred { get; private set; }
 
            public void Intercept(IInvocation invocation)
            {
                invocation.Proceed();
                if (invocation.Method.Name == "OnError")
                    ErrorOccurred = true;
            }
        }
    }
}

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