Action Precondition Filter

'I often find the need to check for the existence of a parameter passed to a web page, such as an ID, and also often need to check if the ID is valid as well (e.g.: a valid primary key).

In the spirit of Design By Contract, I would consider this check to be a sort of "Request By Contract", at least insofar as the concept of a "precondition".'

What is MVCContrib?

Latest Changes to Action Precondition Filter

• Additionally supports HttpRequest parameters (i.e.: HttpRequest.QueryString, HttpRequest.Form, HttpRequest.ServerVariables, & HttpRequest.Cookies), specified via the ParamType enum.
• Similarities between PredicatePreconditionFilter & RegExPreconditionFilter have been refactored into an abstract base class to make development of other types of precondition filters easier and more consistent.
• The ErrorUrl argument has been removed. The same basic result can be accomplished using the RescueAttribute which is also part of MVCContrib. Just add the attribute to the action along with the PreconditionFilter attribute, for example, including [Rescue("Error")] will render the rescue view "Error" for any Exceptions thrown due to an invalid parameter caught by the PreconditionFilter.
• Exception message is now more descriptive.
• Verified against ASP.NET Preview 3 release.

Show Me the (latest) Code!

Example Controller (to illustrate usage):

using System;

using System.Web.Mvc;

namespace MvcApplication1.Controllers

{

    public class HomeController : Controller

    {

        public void Index()

        {

            RenderView("Index");

        }

        [RegExPreconditionFilter("id", PreconditionFilter.ParamType.RouteData, "^[1-9][0-9]*$", 
typeof(ArgumentOutOfRangeException))]

        public void Company(int id)

        {

            RenderView("Company");

        }

        [PredicatePreconditionFilter("id", PreconditionFilter.ParamType.RouteData, 

"IsGreaterThanZero", 
typeof(ArgumentOutOfRangeException))]

        public void Employee(int id)

        {

            RenderView("Employee");

        }

        public void Error()

        {

            RenderView("Error");

        }

        protected bool IsGreaterThanZero(object value)

        {

            try

            {

                int id = Convert.ToInt32(value);

                return id > 0;

            }

            catch

            {

                return false;

            }

        }

    }

}

Runnable Usage Example