.Nettuce

 Create the validation behavior

    public class ValidationBehaviour<T> : BasicBehavior where T : class
    {
        private readonly IContinuationDirector continuationDirector;
        private readonly BehaviorGraph behaviorGraph;
        private readonly IFubuRequest fubuRequest;
        private readonly IValidator<T> validator;
 
        public ValidationBehaviour(IContinuationDirector continuationDirector, BehaviorGraph behaviorGraph, IFubuRequest fubuRequest, IValidator<T> validator)
            : base(PartialBehavior.Ignored)
        {
            this.continuationDirector = continuationDirector;
            this.behaviorGraph = behaviorGraph;
            this.fubuRequest = fubuRequest;
            this.validator = validator;
        }
 
        protected override DoNext performInvoke()
        {
            var inputModel = fubuRequest.Get<T>();
            var validationResult = validator.Validate(inputModel);
            if (validationResult.IsValid)
            {
                return DoNext.Continue;
            }
            fubuRequest.Set(validationResult);
            var actionCall = GetActionCallFromBehaviorGraph();
            continuationDirector.TransferToCall(actionCall);
            return DoNext.Stop;
        }
 
        private ActionCall GetActionCallFromBehaviorGraph()
        {
            return behaviorGraph
                .Behaviors
                .Where(chain => chain.FirstCall().HandlerType.Namespace == typeof(T).Namespace && chain.Route.AllowedHttpMethods.Contains(WebRequestMethods.Http.Get))
                .Select(chain => chain.FirstCall())
                .First();
        }
    }

Tell FubuMvc when to use the validation behavior

    public class ValidationConfiguration : IConfigurationAction
    {
        public void Configure(BehaviorGraph graph)
        {
            graph.Actions()
            .Where(x => x.HasInput && ObjectFactory.Model.HasDefaultImplementationFor(typeof(IValidator<>).MakeGenericType(x.InputType())))
            .Each(x => x.AddBefore(new Wrapper(typeof(ValidationBehaviour<>).MakeGenericType(x.InputType()))));
        }
    }

Add an HtmlConvention to display the error messages

            HtmlConvention(x => x.Editors.Always.Modify((request, tag) =>
                   {
                       var fubuRequest = request.Get<IFubuRequest>();
                       var validationResult = fubuRequest.Get<ValidationResult>();
                       if (validationResult.IsValid) return;
                       var ul = new HtmlTag("ul");
                       var liTags = validationResult.Errors.Where(error => error.PropertyName == request.Accessor.InnerProperty.Name).Select(vf => new HtmlTag("li", li => li.Text(vf.ErrorMessage)));
                       ul.Append(liTags);
                       tag.Append(ul);
                   }));

Apply the Validation Configuration

ApplyConvention<ValidationConfiguration>();

Wire up the IContinuationDirector and Validators

            FubuApplication.For<ConfigureFubuMvc>().StructureMapObjectFactory(container =>
                                           {
                                               container.Scan(scanner =>
                                                      {
                                                          scanner.TheCallingAssembly();
                                                          scanner.WithDefaultConventions();
                                                          scanner.ConnectImplementationsToTypesClosing(typeof(IValidator<>));
                                                      });
                                               container.For<IContinuationDirector>().Use<ContinuationHandler>();
                                           })
                .Bootstrap(RouteTable.Routes);